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Aerobe or facultative anaerobe organism that metabolizes acetate?

Aerobe or facultative anaerobe organism that metabolizes acetate?


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I'm assuming bacteria, but will take any suggestions. Organism must survive primarily on the acetate (plus trace elements), but I can give/take electrons, if necessary. Not interested in strict anaerobes. I've tried Shewanella oneidensis, but have not succeeded in getting it to eat any acetate (just lactate). Feel free to give journal refs. Self-promotion won't be frowned upon if it's helpful. :-) Opportunity for joint pub, if successful.


You could try Paracoccus denitrificans.

Here is a study where acetate is used as the growth-limiting substrate: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.320.1692&rep=rep1&type=pdf

Here's the details of its version of acetate kinase: http://www.uniprot.org/uniprot/A1B9S8

I'd be happy to collaborate with you further on this.


Like other anaerobic organisms, anaerobic bacteria do not require oxygen for respiration. Rather various organic and even inorganic materials may be used as electron acceptors during this process.

Here, it's worth noting that some of the anaerobes can tolerate oxygen and even use it for respiration. For others, however, oxygen cannot be tolerated and is even poisonous to the bacteria.

Here are the major classifications of anaerobic bacteria:

Based on metabolic characteristics, anaerobic bacteria are divided into the following groups:

Aerobically different bacteria behave differently when grown in liquid culture:

  1. Obligate aerobic bacteria gather at the top of the test tube in order to absorb maximal amount of oxygen.
  2. Obligate anaerobic bacteria gather at the bottom to avoid oxygen.
  3. Facultative bacteria gather mostly at the top, since aerobic respiration is advantageous (ie, energetically favorable) but as lack of oxygen does not hurt them, they can be found all along the test tube.
  4. Microaerophiles gather at the upper part of the test tube but not at the top. They require oxygen, but at a lower concentration.
  5. Aerotolerant bacteria are not affected at all by oxygen, and they are evenly spread along the test tube.

HomeMorphological Evaluation of Bacterial Isolates

The bacteria we will examine in this lab include species in different genera Staphylocccus, Micrococcus, Streptococcus, Enterococcus, and Neisseria. At the cellular level, the one characteristic common to all of them is cellular morphology&mdashall are cocci. They differ, however, in many other characteristics.

A volunteer from your lab bench should obtain cultures from your instructor, who will provide you with the species names. Write the name and BSL for each of the cultures below:

Colonial morphology

Although there are similarities, the bacteria we will examine in this lab have notable differences, starting with appearance of their colonies. For the bacteria listed in the table below, choose one species from each of the genera below to observe, and describe the colonial morphology of the bacteria in the table below. Note that up until about a decade ago, Streptococcus and Enterococcus were considered part of the same genus and are very similar with regard to both cellular and colonial morphology.

Bacterium Colonial Morphology
Staphylococcus saprophyticus
Micrococcus luteus
Enterococcus faecalis

Gram stain and cellular morphology

On a cellular level, all of the bacteria we will look at in this lab have a similar morphology, but there are significant differences in Gram stain reaction, cell size, and cellular arrangements. These differences help to target the particular genus of a bacterial sample.

Prepare smears of the three bacteria you examined (above) and Gram stain them. Also look at a prepared Gram stained slide of Neisseria gonorrhoeae (the causative agent of the STD gonorrhea) and describe what you see in the table below.

Bacterium(write in species name)Gram stain results(reaction, morphology, and arrangement)Staphylococcus _________________(also representative of other Staphylococcus spp.) Micrococcus luteus (also representative of other Micrococcus spp.) Enterococcus faecalis (representative of both Entercoccus and Streptococcus spp.) Neisseria gonorrhoeae(There is no culture of this &ndash view the prepared slide)

Physiology and growth characteristics

Growing bacteria in culture requires consideration of their nutritional and physical needs. Food, provided in the media, is broken down by cells and used for energy and building biomass. Unlike eukaryotic cells, bacteria have options when it comes to making energy, which depend not only on the type of organic molecules in the food but also on the availability of oxygen as a final electron acceptor for respiration.

Respiration is the pathway in which organic molecules are sequentially oxidized to strip off electrons, which are then deposited with a final electron acceptor. Along the way, ATP is made. For many types of bacteria, oxygen serves as the final electron acceptor in respiration. Remarkably, oxygen is not always a requirement for respiration. For bacteria that live in environments with no air, alternative electron acceptors may take the place of oxygen.

Unlike the majority of eukaryotes, bacteria have options when it comes to making ATP. Aerobic respiration and anaerobic respiration generate ATP by chemiosmosis, and some bacteria may also ferment sugars, although the oxidation is not complete and energy is left behind. Chemical by-products and end products of these pathways are detectable and serve as the basis for many biochemical tests performed to identify bacteria.

Fermentation and anaerobic respiration are anaerobic processes&mdashmeaning that no oxygen is required for ATP production. Some bacteria have the capability (meaning they produce the appropriate enzymes) to use more than one, or even all three, of these pathways depending on growth conditions.

Based on whether oxygen is required for growth, bacteria can be considered to be either aerobes or anaerobes. However, because some bacteria may use more than one pathway, there are additional categories that describe a culture&rsquos requirement for oxygen in the atmosphere. The three major categories are:

Strict aerobe&mdashBacteria that are strict aerobes must be grown in an environment with oxygen. Typically, these bacteria rely on aerobic respiration as their sole means of making ATP, but some may also ferment sugars.

Strict anaerobe&mdashThese bacteria live only in environments lacking oxygen, using anaerobic respiration or fermentation to survive. For these types of cells, oxygen can be lethal because they lack normal cellular defenses against oxidative stress (enzymes that protect cells from oxygen free radicals).

Facultative anaerobe&mdashThe most versatile survivalists there are. These bacteria typically have access to all three ATP-forming pathways, along with the requisite enzymes to protect cells from oxidative stress.

Additionally, overlapping categories include:

Microaerophile&mdashAs the name implies, these bacteria prefer environments with oxygen, but at lower levels than normal atmospheric conditions. Often, microaerophiles also have a requirement for increased levels of carbon dioxide in the atmosphere and may also be called capnophiles. These bacteria make ATP by aerobic respiration and may also ferment sugars aerobically.

Aerotolerant anaerobe&mdashThese bacteria make ATP by anaerobic respiration and may also be fermentive. However, they are &ldquotolerant&rdquo of oxygen because they may have cellular defenses against oxygen free radicals.

Tests that detect either components or end-products of these pathways may be used to assess a culture&rsquos overall oxygen requirement category. The following tests provide the information necessary to assess this growth characteristic.

The catalase test detects the ability of bacteria to produce an enzyme called catalase which is found in cells that live where there is air. Various chemical reactions in electron transport pathways create oxygen free radicals, which are electron-scavenging chemical species that can oxidize and potentially damage biomolecules in cells. One of these is hydrogen peroxide (H2O2), the substrate of the catalase enzyme which converts hydrogen peroxide to water and oxygen. The catalase test is performed by mixing a small amount of a bacterial culture with a drop of hydrogen peroxide on a slide. If the bacteria have the catalase enzyme, the substrate will be split, forming water and oxygen which is observed as bubbling when the gas is released (see Figure 1). A positive test result indicates that the bacteria live aerobically, and are likely to produce ATP by aerobic respiration. Strict aerobes, facultative anaerobes and microaerophiles may be positive for this test. Anaerobes (strict or aerotolerant) will be negative).

The oxidase test identifies bacteria that produce cytochrome oxidase or indophenol oxidases, which are redox enzymes in the electron transport system that shuttle electrons to oxygen. The cytochrome system is usually only present in aerobic organisms that use oxygen as the final electron acceptor in respiration. There are several ways in which this test may be performed, but one of the simplest is to use a commercial test system, such as the BBL DrySlide Oxidase test, which consists of a card saturated with a chemical reagent that is colorless in its reduced state and turns dark blue when oxidized. The cytochrome oxidase enzymes donate electrons to the reagent, changing the color of the card from colorless to blue for a positive test (see Figure 2). Aerobic bacteria with a cytochrome-based electron transport system (similar to what is found in the mitochondria of eukaryotic cells) will be positive for this test.

The nitrate reduction test detects reduced forms of nitrate, which occurs when bacteria use nitrate (NO3) as a substitute for oxygen (O2) during respiration. In the biogeochemical cycle known as the nitrogen cycle, nitrate reduction is the first step in a series of reactions collectively referred to as denitrification (Figure 3).

On an ecosystem scale, denitrification decreases the levels of NO3 in soil and slows leaching of this substance into groundwater. On the other hand, denitrification may lead to an increase in N2O, a &ldquogreenhouse gas&rdquo in the atmosphere and depletes nitrate from soil, which deprives plants and other microbes of this important nutrient. On a cellular scale, some bacteria reduce nitrate as a substitute for oxygen when they are in anoxic environments, and therefore, nitrate respiration can be a useful test for discriminating among bacterial species. This test is performed by subculturing bacteria to nitrate broth, a medium containing food and a source of nitrate available to serve as a final electron acceptor (as a substitute for oxygen for anaerobically respiring bacteria).

Nitrate reduction is demonstrated by adding chemicals that react with nitrite and noting development of a red color, which will occur if the bacteria reduced nitrate to nitrite. No color change after the chemicals are added might mean either the bacteria did not reduce the nitrate at all, or it may also mean the bacteria fully reduced the nitrate to N2 (denitrification). This can be discriminated by adding zinc to cultures that do not change color when the reagents were added. Electrons donated by zinc will subsequently reduce any nitrate remaining in the broth to nitrite, and the broth will become red&mdashtherefore a negative test. If the bacteria already reduced all the nitrate to forms other than nitrite, no color change will occur, and this is considered a positive test. A positive nitrate reduction test is indication of an anaerobic lifestyle.

Triple Sugar Iron is a slant medium with two growth environments: aerobic (on the slant) and anaerobic (in the &ldquobutt&rdquo). The medium contains three sugars in varying concentrations and a pH indicator that turns yellow at pH measurements below 6.8, and a deeper red at pH measurements above 8.2. Bacteria that ferment typically produce one or more types of acid as a byproduct, therefore, fermentation (both aerobic on the slant and anaerobic in the butt) is noted as a change in the color of the media. The medium also identifies strict aerobes that only grow on the slant surface, and also bacteria that produce H2S, either as a way to produce ATP anaerobically using sulfur or sulfate as a final electron acceptor, or as a result of the breakdown of proteins that contain high numbers of sulfur-containing amino acids (cysteine or methionine).

The results of this test are reported as appearance of the slant/appearance of the butt, using A to indicate acid reaction (yellow color), K to indicate an alkaline reaction, and NC to indicate no change in the medium. H2S (detected as a blackening in the media) and the production of gas (CO2) as a byproduct of fermentation are also reported if observed (see Figure 4).

As an example, and for practice, the interpretation and outcomes for the 4 TSI tests shown are provided in the table below. Note that many other possible reactions may also occur so proper interpretation of this test is important.

Table 1. TSI reactions shown in the cultures in Figure 4, from left to right.
Outcome Interpretation
Uninoculated control For color comparison with inoculated samples
K/NC Aerobic respiration (dark red on the slant) only. Bacteria are strict aerobes.
A/A gas Fermentation of all three sugars with CO2 produced. Bacteria are facultative anaerobes.
K/A H2S Aerobic respiration (dark red on slant), fermentation of glucose (acid only in butt), anaerobic respiration (black in butt). Bacteria are facultative anaerobes.
K/A Aerobic respiration (dark red on the slant) fermentation of glucose (acid only in butt). Bacteria are facultative anaerobes.

How would you interpret the outcome of the TSI slant, the appearance of which is described below?

Appearance Outcome and Interpretation
Slant is a dark red color, butt is yellow with noticeable cracking and bubbling.

After inoculation and test procedures have been demonstrated, perform these tests on the bacteria listed in the table, and record the outcomes below:

Bacterium Catalase Oxidase Nitrate Reduction TSI
Staphylococcus (aureus OR epidermidis)
Micrococcus luteus
Enterococcus faecalis

For each bacterium, determine if the test results provide evidence of aerobic or anaerobic respiration or fermentation, and indicate why you reached that conclusion. Then, based on your observations, state the logical growth category related to oxygen for each.

Staphylococcus ______________(write in species you tested)State what evidence from test results indicates the bacteria use this pathway. If there is no evidence, write &ldquonone.&rdquo

Aerobic respiration
Anaerobic respiration
Fermentation

Oxygen Growth Requirement Category______________________________________________

Micrococcus luteus State what evidence from test results indicates the bacteria use this pathway. If there is no evidence, write &ldquonone.&rdquo
Aerobic respiration
Anaerobic respiration
Fermentation

Oxygen Growth Requirement Category______________________________________________

Enterococcus faecalis State what evidence from test results indicates the bacteria use this pathway. If there is no evidence, write &ldquonone.&rdquo
Aerobic respiration
Anaerobic respiration
Fermentation

Oxygen Growth Requirement Category______________________________________________


Understanding aerobic/anaerobic metabolism in Caldibacillus debilis through a comparison with model organisms

Caldibacillus debilis GB1 is a facultative anaerobe isolated from a thermophilic aero-tolerant cellulolytic enrichment culture. There is a lack of representative proteomes of facultative anaerobic thermophilic Bacillaceae, exploring aerobic/anaerobic expression. The C. debilis GB1 genome was sequenced and annotated, and the proteome characterized under aerobic and anaerobic conditions while grown on cellobiose. The draft sequence of C. debilis GB1 contains a 3,340,752 bp chromosome and a 5,386 bp plasmid distributed over 49 contigs. Two-dimensional liquid chromatography mass spectrometry/mass spectrometry was used with Isobaric Tags for Relative and Absolute Quantification (iTRAQ) to compare protein expression profiles, focusing on energy production and conversion pathways. Under aerobic conditions, proteins in glycolysis and pyruvate fermentation pathways were down-regulated. Simultaneously, proteins within the tricarboxylic acid cycle, pyruvate dehydrogenase, the electron transport chain, and oxygen scavenging pathways showed increased amounts. Under anaerobic conditions, protein levels in fermentation pathways were consistent with the generated end-products: formate, acetate, ethanol, lactate, and CO2. Under aerobic conditions CO2 and acetate production was consistent with incomplete respiration. Through a direct comparison with gene expression profiles from Escherichia coli, we show that global regulation of core metabolism pathways is similar in thermophilic and mesophilic facultative anaerobes of the Phylum Proteobacteria and Firmicutes.

Keywords: Caldibacillus debilis Genomics Mixed acids fermentation Proteomics Respiration Thermophilic.


Microbiology with Diseases by Taxonomy Chapter 6 Answers

Microbiology with Diseases by Taxonomy Chapter 6 Answers 1CM
The concept map that describes culture media is shown below:

Microbiology with Diseases by Taxonomy Chapter 6 Answers 1CT
The characteristics of an organism are given by a scientist as:

  • Chemoheterotrophic:
    A chemoheterotroph is an organism, which uses organic compounds as its source of carbon and source of energy.
  • Aerotolerant:
    Aerotolerant organisms are those organisms, which prefer an atmosphere without oxygen. Since they are aerotolerant, these organisms can tolerate low levels of oxygen.
  • Mesophilic:
    A mesophilic organism is one which can grow best in a temperature range of 20 0 C to 40 0 C.
  • Facultatively halophilic bacillus:
    A facultatively halophilic organism is one which can tolerate high concentrations of salt. The cell membranes of such organisms are resistant to salt. It can also survive in normal conditions.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 1FB
Sources of carbon in different organisms are given below:

  1. In photoautotrophs – carbon dioxide.
  2. In chemoautotrophs – carbon dioxide.
  3. In photoheterotrophs – organic compounds.
  4. In chemoheterotrophs – organic compounds.

Sources of energy in different organisms are given below:

  1. In photoautotrophs – light.
  2. In chemoautotrophs – organic molecules.
  3. In photoheterotrophs – light.
  4. In chemoheterotrophs – organic compounds.

Sources of electrons in different organisms are given below:

  1. In organotrophs – organic molecules.
  2. In lithotrophs – inorganic sources.
  3. In photoheterotrophs – organic molecules.
  4. In photoautotrophs – organic molecules.
  5. In chemoautotrophs – organic molecules.

Thus, sources of carbon, energy, and electrons are needed by all cells.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 1L
The labeled thioglycollate tubes are shown below:

  1. Obligate aerobes are those organisms, which cannot live in the absence of oxygen.
  2. Obligate anaerobes are those organisms that cannot live in oxygen, since oxygen is toxic to such organisms. These organisms in the electron transport chain cannot use oxygen as terminal electron acceptor.
  3. Facultative anaerobes are those organisms, which can live in absence and presence of oxygen. These organisms can switch between aerobic respiration and fermentation.
  4. Aero tolerant anaerobes are those organisms, which can tolerate low levels of oxygen. These organisms prefer anaerobic conditions to aerobic conditions.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 1MC
(a) An anaerobe:
An anaerobe cannot grow on a Petri plate. An anaerobe can grow only under specially maintained conditions. Oxygen should not be present for the growth of an anaerobe. In a Petri plate, aerobes usually grow. Viruses cannot grow on agar in a Petri plate. Hence, the option (a) is incorrect.
(b) Viruses on an agar surface:
Viruses cannot grow on agar surfaces in Petri plates. Viruses can grow only on specialized growth medium. They can also be grown in tissues like the egg. Viruses can grow on monolayers of bacterial cells. A colony of agar surface can grow in a Petri plate, in a lab. Hence, the option (c) is incorrect
(c) Barophiles:
Barophiles cannot grow on agar in a Petri plate. Barophiles are organisms, which live in areas of high pressures. Such pressures cannot be maintained in a laboratory hence, cannot grow in a Petri plate. Organisms like viruses cannot grow on agar in the lab. Hence, the option (d) is incorrect
(d) A colony on an agar surface:
A colony on an agar surface can grow in a Petri plate in a lab. A colony usually means a bacterial or fungal colony. Only aerobes can grow in Petri plates. The growth of anaerobes can be seen only in special jars like the Gas Pak jar. Barophiles cannot grow on culture. Thus, a colony on an agar surface can grow on a Petri plate in the laboratory.
Hence, the correct option is (b) a colony on an agar surface.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 1SA
High temperature affects the life of a microbe:
Explanation
Condition of high temperature conditions affects the shape of molecules in a cell. When a cell is subjected to high temperatures many changes occur, unless it is a thermophile. In normal cells, the high temperature denatures the enzymes present in the cell. Moreover, the enzymes are deactivated and denatured. Denaturation is the process in which the shape of the enzyme is changed. The native conformation of an enzyme is important in the function of the enzyme.
Additionally, if conformation of the enzyme is destroyed, the function of the enzyme is lost. Due to the changes in the enzyme, the reactions taking part in the cell do not occur. In contrast, as a result of exposure to high temperatures, metabolism stops. Unless, the conditions are altered quickly, the cell will die.
Thus, the life of a microbe can change drastically if enzymes are denatured. Since, reactions are dependent on enzymes metabolism of a cell is also dependent on enzymes. When metabolism stops, the cell cannot continue its life processes.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 2CT
Heat of pasteurization kills microorganism:
Explanation
The process of pasteurization uses a temperature of 72 0 C to remove the pathogens in food. At a high temperature of 72 0 C, the enzymes of most of the pathogens become deactivated. When enzymes are deactivated or denatured, the metabolism of the cell grinds to a halt.
Additionally, organisms are usually divided into psychrophiles, mesophiles, and thermophiles.

  1. Psychrophiles are the organisms, which prefer low temperatures close to freezing.
  2. Mesophiles are those organisms, which prefer a medium range of 20 0 C to 40 0 C.
  3. Thermophiles are those organisms, which can withstand high temperatures.

Since, the temperature used in pasteurization is, only thermophiles can withstand this process. The other organisms present are killed due to effect on enzymes and cellular metabolism.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 2FB
Singlet oxygen:
Explanation
Singlet oxygen acts as a reactive oxidizing agent and is very toxic. The singlet oxygen species are produced by the reaction of oxygen and light. Singlet oxygen species are removed by carotenoids in phototrophic microorganisms.
In humans, phagocytic cells use the singlet oxygen species to destroy pathogens that enter the body. After its use by phagocytic cells, the singlet oxygen species is removed from the cell. Humans have enzymes to remove toxic oxygen species from the body.
Singlet oxygen species is used in photodynamic therapy. In this therapy, the reactive oxygen species are used to kill cancer cells.
Singlet oxygen is a toxic form of oxygen. It is molecular oxygen in a higher energy state.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 2MC
(a) Obligate aerobe:
The term of obligate anaerobe describes an organism that cannot exist in the presence of oxygen. Obligate aerobes are those organisms, which cannot live in the absence of oxygen. They need oxygen compulsorily. An organism that cannot exist in the presence of oxygen would be an obligate anaerobe. Hence, option (a) is incorrect.
(b) Facultative aerobe:
Facultative aerobes are the organisms, which can live in the absence and presence of oxygen. These organisms can change between aerobic respiration and fermentation. An organism that cannot exist in the presence of oxygen would be an obligate anaerobe. Hence, option (b) is incorrect.
(c) Facultative anaerobe:
Facultative anaerobes are the organisms that can live either in absence or presence of oxygen. These organisms can switch between aerobic respiration and fermentation. An organism that cannot exist in the presence of oxygen would be an obligate anaerobe. Hence, option (d) is incorrect.
(d) Obligate anaerobe:
Obligate anaerobes are those organisms that cannot live in the presence of oxygen. Oxygen is toxic to such organisms. These organisms cannot use oxygen as the terminal electron acceptor in the electron transport chain.
Hence, the correct option is (c) obligate anaerobe.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 2SA
Microbes cannot tolerate low pH in human stomach:
Explanation
The above statement is supported by many scientists conducted in the research. The stomach is an organ, which has a low pH of 1.5. The low pH is maintained by the presence of HCl. Most of the microbes, which enter the digestive tract, are killed at low pH.
However, the acidic conditions of the stomach disrupt the cell membrane of the microbes. The cell membranes of most microbes are sensitive to the conditions present in the human stomach.
Additionally, few organisms are resistant to the acidic conditions of the stomach. One of them is the bacterium Helicobacter pylori. The organism causes the formation of peptic ulcers in the stomach wall. The ulcers are formed by combined action of stomach acid and bacterial action.
Thus, the statement “Microbes cannot tolerate low pH in the human stomach” is true.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 3CT
Differences between the dry weights of facultative anaerobe structures:
Explanation
When two cultures of a facultative anaerobe are grown under different conditions, they show differences in dry weights. When one culture is grown in the presence of oxygen and the other is grown in the absence of oxygen, difference in growth is observed.
In contrast, the organism is a facultative anaerobe that can grow in the presence or absence of oxygen. Moreover, it grows better in the presence of oxygen, since aerobic respiration gives higher energy yield when compared to anaerobic respiration.
Similarly, the culture in the presence of oxygen grows faster and has a high rate of metabolism. The culture in the absence of oxygen also grows, but does not have similar growth, rate as the other culture.
Thus, when the cultures are dried and weights are taken, the culture grown in the presence of oxygen show higher dry weight than the other culture.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 3FB
All amino acids possess the element of nitrogen. The basic structure of an amino acid is:

The nucleotides present in the nucleic acids contain the essential element of nitrogen. The basic structure of a nucleotide is given below:

The structures of the five nucleotides are:

Thus, the essential element of nitrogen is recycled from amino acids and nucleotides in all cells.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 3MC
(a) Causes hydrogen peroxide to become toxic:
The enzyme superoxide dismutase does not cause hydrogen peroxide to become toxic. Instead the enzyme detoxifies the toxic substance. The presence of hydrogen peroxide causes damage to cells. The enzyme detoxifies these radicals, so that the cells are not damaged. Hence, the option (a) is incorrect.
(b) The enzyme cannot neutralize singlet oxygen species in the cell. Singlet oxygen species are removed by carotenoids in phototrophic microorganisms. The enzyme superoxide dismutase detoxifies the toxic superoxide radicals generated by the processes of the cell. Hence, the option (c) is incorrect.
(c) The enzyme superoxide dismutase must be produced in aerobic organisms. The superoxide radicals are formed during electron transport and need to be removed from the cell. When these radicals are formed, they cause death of the cell. Anaerobic organisms do not have this enzyme hence, cannot live in the presence of oxygen. Hence, the option (d) is incorrect.
(d) Superoxide dismutase detoxifies superoxide radicals. The enzyme detoxifies toxic superoxide radicals generated by the processes of the cell. The presence of hydrogen peroxide causes damage to cells. The enzyme detoxifies the radicals, so that the cells are not damaged.
Hence, the correct option is (b) detoxifies superoxide radicals.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 3SA
Relation of Quorum sensing in biofilm formation:
Explanation
The ability to respond changes in a density of population is called quorum sensing. In this process, bacteria respond to changes by using signal and receptor molecules. The process is often seen in biofilms where many species are present together.
Quorum sensing is important in the formation of biofilms. Biofilms are complex relationships among different organisms. The biofilms are found attached to surfaces like the surface of teeth. Additionally, biofilms develop an extracellular matrix, which is composed of DNA, proteins, and polysaccharides of cells. The extracellular matrix is responsible for attachment of cells to one another. It is also responsible in sticking of biofilm to its substrate.
Furthermore, the cells in the biofilm secrete molecules, which act as signals. Other cells have receptors for these signals. When the number of microbes increase, the concentration of the signal molecules also increase. The binding of the signals to receptors reach a certain threshold in the genes where expression gets suppressed.
Thus, the genes that are suppressed are then expressed and allow the biofilm to have new characteristics. Moreover, gives organisms the ability to form biofilms.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 4CT
Requirement of riboflavin to synthesize FAD:
Explanation
The nutritional requirement of some organisms includes vitamin riboflavin. This vitamin is used to synthesize the co-factor FAD. The cofactor FAD is used for many purposes in the cell. Moreover, FAD (oxidized flavin adenine dinucleotide) can change to FADH2 (reduced flavin adenine dinucleotide) by accepting two protons.
The conversion is shown in the below structure:

In eukaryotes, the main role of FAD is an electron carrier in the electron transport chain. The number of ATP (adenosine triphosphate) molecules obtained from the oxidation of each molecule of FADH2 is 2.
Thus, in the process of ??oxidation, FAD behaves as a coenzyme to the enzyme acyl coA dehydrogenase.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 4FB
Most of the organisms cannot synthesize some substances that are necessary for metabolism. The organic chemical substances are known as growth factors. For instance, some of the growth factors are listed below:

Vitamins may not be a growth factor for all organisms. In humans, many vitamins are growth factors. But in organisms like E. coli, can synthesize its own vitamins, they are not growth factors. Other nutrients, which are synthesized in humans, may be growth factors for E. coli. The growth factors can be used as coenzymes or as a part of molecules like cytochromes.
Thus, small organic molecules, which are required in low amounts for metabolism, are growth factors.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 4MC
(a) The peroxide anion:
Peroxide anions are present in hydrogen peroxide. The anion makes hydrogen peroxide an antimicrobial agent. The peroxide anion is detoxified by the enzymes catalase and peroxidase. The enzyme catalase converts the toxic hydrogen peroxide to water and oxygen. The most reactive of the four toxic forms of oxygen is the hydroxyl radical. Hence, the option (b) is incorrect.
(b) The superoxide radical:
The superoxide radicals are formed during electron transport and need to be removed from the cell. When these radicals are formed, they cause the death of the cell. Hydroxyl radical is the most reactive of the four toxic forms of oxygen. Hence, the option (c) is incorrect.
(c) Singlet oxygen:
The oxygen species are produced by reaction of oxygen and light. Singlet oxygen species are removed by carotenoids in phototrophic microorganisms. Singlet oxygen species is molecular oxygen with a higher energy state. The most reactive of the four toxic forms of oxygen is the hydroxyl radical. Hence, the option (d) is incorrect.
(d) The hydroxyl radical:
The radicals result from incomplete reduction of hydrogen peroxide. They can also result from ionizing radiation. These radicals are the most reactive of the four toxic forms of oxygen. In anaerobes, the radicals cause damage. In aerobes, no danger exists due to these radicals due to the presence of enzymes like catalase and peroxidase. Thus, hydroxyl radical is the most reactive form of oxygen.
Hence, the correct option is (a) the hydroxyl radical.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 4SA
Sterilization of instruments and vessels is important in microbiological procedures:
Explanation
The media, vessels, and instruments used in microbiology must be sterilized before use. The process of sterilization is very important in the field of microbiology. Sterilization is the process where all microbes are killed and destroyed.
The media used, for the growth of microbes must be sterilized. Media is usually sterilized in an autoclave. The temperature is 121 0 C for 15 minutes. The broth and agar must be sterilized before use to prevent the growth of any contaminants.
The vessels used are mainly glassware. These are washed, wrapped, and then placed in a hot air oven for sterilization. The temperature is 350 0 C for 2-3 hours.
The instruments used are the laminar air flow chamber and inoculation loops. The laminar air flow chamber is sterilized using UV light. The instruments to be used are placed inside the chamber. Inoculation loops and glass spreaders are kept inside the chamber and are sterilized.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 5CT
Oxygen requirement to bacteria:
Explanation
A bacterium is inoculated into a slant tube containing a complex medium. After incubation, only few colonies are seen on the surface of the agar. But many colonies are seen in the depth of the agar. Moreover, the slant of the agar shows few colonies, while butt shows prolific growth. The oxygen requirements of the bacterium can be determined by observing the growth in the tube.
Since, few colonies are seen on the surface of the agar and many colonies in the depth of the agar, it indicates that the bacterium is an aerotolerant anaerobe. Aerotolerant anaerobes are those organisms, which prefer an anaerobic environment. They grow best under anaerobic conditions, but can tolerate oxygen.
Thus, the prolific number of colonies inside the agar shows that the bacterium is an anaerobe. The scant colonies seen on the surface of the agar prove that the bacterium is an aerotolerant anaerobe.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 5FB
Each organism has a growth range to function properly. Within the growth range, three types of temperatures are facilitated, which are given below:

The optimum temperature is the temperature at which the organism has the highest growth rate. All its systems are function properly and metabolism is at its highest range.
The lowest temperature where a microbe can metabolize is minimum growth temperature. Once the temperature goes lesser, minimum growth temperature prevails and cell membranes become less fluid. Moreover, transport processes are very slow to support metabolism.
The maximum growth temperature is the highest temperature at which a microbe can metabolize. Once the temperature goes higher the growth temperature is higher and the enzymes are denatured for vital processes.
The lowest temperature, microbe can metabolize at minimum growth temperature. Beyond the minimum growth temperature, metabolism does not occur, due to inactivation of necessary molecules.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 5MC
(a) Aerotolerant:
Aerotolerant microbes are the organism, which prefer an anaerobic atmosphere, but can tolerate low levels of oxygen. Microaerophiles that grow best with a low level of oxygen and a high concentration of carbon dioxide are called capnophiles. Hence, the option (a) is incorrect.
(b) Facultative anaerobes:
Facultative aerobes are the organisms, which can live in the absence and presence of oxygen. The organisms cannot exist in high concentrations of carbon dioxide and less concentration of oxygen. Capnophiles are organisms, which can exist in low levels of oxygen and high concentrations of carbon dioxide. Hence, the option (c) is incorrect.
(c) Fastidious:
A fastidious organism has complex requirements for its growth. It can grow only when specific nutrients are added. The organisms cannot grow at low concentrations of oxygen and high concentrations of carbon dioxide. Capnophiles are organisms which can exist at low levels of oxygen and high concentrations of carbon dioxide. Hence, the option (c) is incorrect.
(e) Capnophiles:
Microaerophiles that grow best with a low level of oxygen and a high concentration of carbon dioxide are called capnophiles. Capnophiles are organisms, which can exist in low oxygen levels and high concentrations of carbon dioxide. The organisms are microaerophiles with slightly different requirements.
Hence, the correct option is (b) capnophiles.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 5SA
Agar is used in microbiology as a solidifying agent. Media without the addition of agar is known as broth, which is liquid in consistency. When agar is added, the medium becomes solid.
Agar is a polysaccharide, which is complex in nature and derived from the cell walls of certain red algae.
Agar is very useful in the field of microbiology because of the following reasons:

  • Agar cannot be digested by most microbes. Hence, even with the growth of microbes and fungi, the media stays solid.
  • Agar comes in powder form, which can dissolve in water at. At this temperature, most of the added nutrients remain unchanged and undamaged.
  • The solidification of agar is seen at temperatures below. This temperature allows us to add nutrients like blood without any problem.
  • The agar at such temperatures can also be poured over bacterial culture without any problem. This is seen in the pour plate technique.
  • Once agar becomes solidified, it does not melt unless heated to temperatures above. This characteristic of agar can be used while growing thermophiles.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 6CT
Nutritional and oxygen characteristics of bacteria in everyday language:
Explanation
An article gives the following characteristics of a bacterium:

An obligate microaerophilic is an organism, which need to have a concentration of 2%-10% of oxygen in the air. Higher concentrations of oxygen cannot be tolerated by the bacterium. Since, it is an obligate it indicates the absence of the oxygen concentration, the bacterium cannot survive.
Additionally, chemoorganoheterotroph is an organism, which uses organic compounds as its source of carbon. It obtains its energy from redox reactions in organic compounds. Redox reactions are reduction and oxidation reactions.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 6FB
Osmotic pressure is responsible for shrinking of cells seen in hypertonic solutions like saltwater.
Solutions can usually be described in three ways:

Hypertonic solutions have higher concentration of solutes compared to the cell. The water from the cell moves out into the surrounding solution and the cell shrivels.
Isotonic solutions have the same concentration of solutes as the cell. In this situation, the cell does not undergo any change.
Hypotonic solutions have lower concentration of solutes compared to the cell. The water from the solution moves into the cell and it bursts.
Saltwater is a hypertonic solution which results in the shrinking of cells seen. Thus, cells that shrink in hypertonic solutions such as saltwater are responding to osmotic pressure.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 6MC
(a) Cholesterol:
Some organisms might need cholesterol as a growth factor. Since, cholesterol is one of the molecules present in the cell membrane, some organisms require as a nutrient. Presence of cholesterol in the cell membrane prevents quick crystallization of the lipids in the membrane. Cholesterol helps to keep the fluidity of the membrane. Hence, the option (a) is incorrect.
(b) Vitamins:
Vitamins are one of the growth factors needed for many microorganisms. Vitamins are needed for many purposes in the cell. Since, many organisms cannot synthesize all the required, vitamins need them as a growth nutrient. Vitamins act as coenzymes in metabolism and are required in the cell. Hence, the option (c) is incorrect.
(c) Heme:
Heme is an important constituent required in a cell. The cytochromes, which take part in the electron transport chain, consist of heme. Without cytochromes, the process of aerobic respiration cannot occur. In photosynthetic organisms, photophosphorylation cannot take place in the absence of cytochromes. Hence, the option (d) is incorrect.
(d) Water:
Water is a constituent which is not required as a growth factor. Since, water can be obtained easily, it is not considered as a growth factor. Even if the cell does not take in water, the metabolic water obtained is usually enough for the needs of the cell. Hence, water need not be added as a nutrient for growth. Thus, water is not a required growth factor for various microbes.
Hence, the correct option is (b) water.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 6SA
The difference between complex media and defined media is that the exact composition of a complex medium is unknown while that of a defined medium is known.
In a defined medium, the exact chemical composition of the medium is known. Every ingredient is known and the exact amount of each ingredient is known. Some media can support the growth of many organisms. But some media are specific for the growth of some microbes only.
In a complex medium, partially digested yeast, beef extract, and casein are added. The exact composition of the medium is not known since the process of partial digestion releases different chemicals in different concentrations.
A complex medium can support the growth of many microbes. They can be used to culture microbes whose nutritional requirements are not fully known.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 7CT
The elements of phosphorous, sulfur, iron, and magnesium are required by microorganisms. These elements are used in the following way:
Phosphorous:
The phosphate ion is present in the phosphodiester linkages of DNA. The addition of a phosphate ion can activate or deactivate a molecule in the process of signal transduction.
Sulfur:
Sulfur containing compounds act as electron donors and electron acceptors. Sulfur is also present in many enzymes. Two amino acids possess the element of sulfur. Sulfur is a vital part of disulfide bonds.
Iron:
The molecules of hemoglobin and myoglobin contain the element of iron. The active sites of many enzymes also contain the element of iron. The element is also a part of cytochromes, which are vital for the process of electron transport chain.
Magnesium:
Many enzymes need magnesium ions to function properly. It plays a vital role in the manipulation of compounds like ATP, DNA, and RNA. This ion has an important interaction with the phosphate present in nucleic acids.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 7FB
Obligate halophiles:
Explanation
Normal cells cannot exist in either hypotonic or hypertonic solutions. They can exist only in isotonic solutions.
The halophiles are the organisms, which have the ability to resist high osmotic pressures. They do not behave like other cells when placed in hypertonic solutions. The halophiles burst when placed in hypotonic solutions. Obligate halophiles are those organisms, which are adapted to such high osmotic conditions.
There are also organisms called facultative halophiles, which can tolerate high concentrations of salt. They do not require them, but can exist in them. These organisms can tolerate slightly lesser salt concentrations than obligate halophiles. Unlike other organisms, which shrivel in hypertonic solutions adjust to osmotic conditions.
Those organisms, which can exist in salty conditions, due to their ability to withstand high osmotic pressures are called as obligate halophiles.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 7MC
(a) Barophiles:
Barophiles are organisms, which live in areas of high pressures. The organisms are generally found at the bottom of oceans. These organisms cannot thrive in icy waters. Organisms, which are called psychrophiles, live in icy waters preferentially. Hence, the option (a) is incorrect.
(b) Thermophiles:
Thermophiles are those organisms, which can withstand high temperatures. The organisms are seen in places like hot springs and deep sea vents. The organisms cannot thrive in icy waters. Organisms, which are called psychrophiles, live in icy waters preferentially. Hence, the option (b) is incorrect.
(c) Mesophiles:
The organisms live at moderate temperatures. They cannot live in too cold or too hot temperatures. These organisms cannot thrive in icy waters. Organisms, which are called psychrophiles, live in icy waters preferentially. Hence, the option (c) is incorrect.
(d) Psychrophiles:
The organisms can live in icy cold waters. The organisms can be found in glaciers and ice caps. Moreover, the organisms have cell membranes, which can resistant to the extreme cold conditions. The DNA of the microbes can resist low temperatures. Thus, psychrophiles are the organisms, which can happily thrive in icy waters.
Hence, the correct option is (d) psychrophiles.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 7SA
The four distinct phases seen in a bacterial growth curve are shown in the graph below:

In the lag phase, the cells adjust to the new surroundings where they are inoculated. The cells synthesize enzymes needed for the latter phases. The length of the lag phase differs from species to species. It also depends on the nutrients present in the medium.
In the log phase, the number of cells increases rapidly and logarithmically. The synthesis of DNA (deoxyribonucleic acid) and proteins occur rapidly as new cells are formed. The metabolic rate is at a maximum during this phase.
In the stationary phase, the rate of reproduction decreases gradually. Slowly the number of cells synthesized equals the number of cells dying. The death of cells seen is due to accumulation of by-products. This phase is called stationary phase because the exponential growth slowly stops.
In the death phase, the nutrients are depleted and wastes accumulated. This results in death of the cells at a faster rate that the synthesis of cells. Sometimes all the cells in the culture die, while in some cultures a few cells remain alive. This phase is called death phase because most of the cells die.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 8FB
Carotenoids pigments:
Explanation
Singlet oxygen is molecular oxygen in toxic form with higher energy state. This species is very toxic. Singlet oxygen acts as a reactive oxidizing agent. Additionally, the singlet oxygen species are produced by the reaction of oxygen and light. They are produced during the process of photosynthesis in chlorophyll molecules.
In phototrophic organisms, carotenoid pigments remove toxic form of oxygen. The excess energy of this species is removed by carotenoids. Another method is quenching of the species.
The photochemically produced singlet oxygen in phototrophic organisms is removed from cells by the pigment called carotenoid.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 8MC
(a) Live at normal barometric pressure:
Barophiles do not live at normal barometric pressures. Barophiles are organisms, which live in areas of high pressures. The organisms are generally found at the bottom of oceans where the pressure is quite high. Hence, the option (b) is correct.
(b) Die if put under high pressure:
Barophiles do not die under high pressures. Barophiles live in areas of high pressures. The organisms have the capability to grow at high pressure in the bottom of oceans. Hence, the option (c) is correct.
(c) Thrive in warm air:
Barophiles cannot live in warm air. Their cells are tailored to withstand high pressure conditions. Barophiles are organisms, which live in areas of high pressures like deep sea vents and bottom of oceans. Hence, the option (d) is correct.
(d) Barophiles cannot infect and cause disease in humans. Barophiles are organisms, which live in areas of high pressures. These organisms are generally found at the bottom of oceans. Since, the pressure in human cells is not very high, these organisms cannot infect humans. Thus, barophiles cannot cause disease in humans. Hence, the option (a) is correct.
Hence, the correct option is (a) cannot cause disease in humans.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 8SA
The number of cells in 1 ?l of sewage is 47.
The number of cells estimated to be in 1 liter of sewage can be calculated as follows:
1 liter = 1000000 ?l
When 1 ?l contains 47 cells, 1 liter of sewage will contain:
= 47×1000000
= 47,000,000 cells
The number of cells in one liter of sewage would be 47,000,000 cells.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 9FB
Nitrogen fixation:
Explanation
Many cyanobacteria and Rhizobium species are vital in the process of nitrogen fixation. The atmospheric nitrogen is converted into ammonia. Nitrogen fixation can occur naturally and with the help of microbes. Naturally nitrogen fixation occurs by means of lightning.
Similarly, nitrogen fixation is usually carried out by Rhizobium species. An enzyme called nitrogenase is present in the bacteria. This enzyme works best under anaerobic conditions. The roots of plants like legumes contain nodules to harbor the bacteria.
The nodules provide bacteria an anaerobic environment to work and reduce nitrogen in exchange for protection. The anaerobic environment is obtained using the pigment leghemoglobin.
Nitrogen fixation is the process by which N2(nitrogen) is converted to NH3(ammonia) by microbes.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 9MC
(a) Broth culture:
A broth culture is one in which the medium is liquid in consistency. A loopful of culture is then mixed in the broth. The medium before inoculation is usually transparent. After incubating the broth for 24-48 hours, the broth becomes turbid, due to the growth of the microbes. Hence, the option (a) is incorrect.
(b) Pour plate:
A pour plate method is a technique in which the microbial culture is mixed in liquid agar. The agar is then poured into Petri plates and left to cool. In this method some sub-surface colonies are formed. Some colonies are formed inside the agar. This is not a technique for isolated colonies. Hence, the option (b) is incorrect.
(d) Dilution plate:
The technique of dilution plate is to use the process of serial dilution. The broth with culture is diluted serially and each dilution is then streaked on the agar plate. The difference in the number of colonies on the plates helps us to know the original number of microbes. Hence, the option (d) is incorrect.
(e) Streak plate:
The streak plate method is a laboratory technique in which a sterile inoculating loop of culture is moved across an agar plate to obtain isolated colonies. Moreover, in the technique a sterilized inoculating loop is used to spread a loopful of culture across the surface of an agar plate. The repeated moving of the loop across the plate in different directions allows the isolation of single colonies.
Hence, the correct option is (c) streak plate.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 9SA
The three indirect methods of counting microbes are:
Measurement of metabolic activity:
The rate of utilization of nutrients and the rate of production of waste is dependent on the number of cells in the culture. By taking these two factors into account, the rate of metabolism can be determined. When the rate of metabolism is known, the number of cells in the culture can be known.
Dry weight:
Some organisms, which have filaments, cannot be easily counted. In the case of such organisms, filtration is followed by drying of the cells. The dry cells are then weighed.
Turbidity:
Turbidity can be measured only in liquid culture. Turbidity can be measured using a spectrophotometer. This instrument measures the amount of light transmitted through the liquid. As bacterial growth makes the liquid turbid, more light is absorbed or scattered. Very less light is transmitted if growth of culture is seen.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 10FB
Streak plate method:
Explanation
The streak plate method is one of the laboratory techniques in which a researcher streaks a plate many times by flaming the loop. This method is used to get isolated colonies. A single loopful of culture is taken and streaked in one direction on the plate. After flaming the loop, the culture on the plate is again streaked in another direction.
Similarly, the action is repeated in many directions. The repeated moving of the loop across the plate in different directions allows isolation of single colonies.
Additionally, other laboratory technique are isolated and colonies are obtained is serial dilution. The technique of dilution plate is to use the process of serial dilution. The broth with culture is diluted serially and each dilution is streaked on an agar plate. Isolated colonies are obtained in the last few plates.
Thus, the streak plate method is used to isolate micro organisms.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 10MC
(a) Agar is available for microbial nutrition:
Agar is a constituent of most microbial media, but it does not give any nutrition to the microbe. Agar is meant as a solidifying agent. A defined medium is one in which the exact composition of the medium is known. Hence, the option (b) is incorrect.
(b) Blood may be included:
Blood can be included in some media like blood agar. It is needed for the microbes, which lyse the blood cells. But the presence of blood in a medium doesn’t make it a defined medium. A defined medium is one in which the exact composition of the medium is known. Hence, the option (c) is incorrect.
(c) Organic chemicals are excluded:
The medium in which organic chemicals are excluded is not a defined medium. Some organic chemicals are added in media. A defined medium is one in which the exact composition of the medium is known. Hence, the option (d) is incorrect.
(d) The particular medium containing the chemical composition is known:
A defined medium is one in which the exact composition of the medium is known. Every constituent in the medium is known and the exact amount of each ingredient is known. The particular medium containing the chemical composition is known as a defined medium.
Hence, the correct option is (a) the exact chemical composition of the medium is known.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 10SA
The five direct methods of counting microbes are:

  • Serial dilution and viable plate counts:
    The process of serial dilution is usually used when the number of microbes is very high in a small sample. The sample is serially diluted. Each tube of the dilution is then plated and incubated. The colonies of the last dilutions can be easily counted. The number of microbes in the original sample can then be determined by multiplying the dilution factor with the number of colonies found on the plate.
  • Membrane filtration:
    The process of membrane filtration is done by pouring a large sample of fluid through a membrane filter. The filter traps the cells in the liquid. The membrane is then transferred to a solid medium. The cells are then allowed to grow. The number of colonies indicates the number of organisms in the original sample.
  • Most probable number:
    This is a statistical technique, which uses MPN (most probable number) tables. The sample is diluted in sets of tubes. Each dilution has five tubes. After incubation, the number of tubes in each dilution which shows turbidity is counted. This number is then compared with the numbers in the MPN table. The MPN table then gives the estimate of the number of bacteria in the sample.
  • Microscopic counts:
    Counting chambers like the Petroff-Hausser counting chamber is used in this method. The microbiologist can count the number of cells in the grid. The number of cells can be used to determine the number of bacteria in 1 ml of the sample.
  • Electronic counters:
    Electronic counters like the Coulter counter are used to count cells. Mainly large cells are counted using this method. An electric current is used in this counter. When cells interrupt the flow of the current, they are counted.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 11MC
(a) Logarithmic reproduction of the growth curve:
The best method to represent bacterial growth as a growth curve is to use a semilogarithmic graph. A logarithmic graph does not show the bacterial growth curve properly. So, it is not used for plotting bacterial growth. Hence, the option (a) is incorrect.
(b) An arithmetic graph of the lag phase followed by a logarithmic section for the log, stationary, and death phases:
By showing the lag phase in an arithmetic graph and the rest of the phases in a logarithmic graph, the exact growth curve is not obtained. It is also confusing when plotting on two graphs. A semilogarithmic graph is the best method to represent bacterial growth, as a growth curve. Hence, the option (c) is incorrect.
(c) None of the above would best represent a population growth curve:
A semilogarithmic graph is the best method to represent bacterial growth, as a growth curve. The four phases of bacterial growth can be easily seen when using a semilogarithmic graph. Hence, the option (d) is incorrect.
(d) A semilogarithmic graph using a log scale on the y axis:
A semilogarithmic graph is the best method to represent bacterial growth as a growth curve. Using a log scale on the y axis, when bacterial growth is plotted, four phases are seen. The four phases are – lag, log, stationary, and death phase. A semilogarithmic graph using a log scale on the y axis is the most useful, while showing population growth on a graph.
Hence, the correct option is (b) a semilogarithmic graph using a log scale on the y axis.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 11SA
The differences between the different groups are shown according to the source of carbon, energy, and electrons.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 12MC
(a) Dry weight:
When filamentous organisms are present, direct counting methods cannot be used. The filamentous organisms are filtered, dried, and weighed. The method is known as dry weight method and is an indirect method. It is not the best method for counting fecal bacteria in a stream. Hence, the option (a) is incorrect.
(b) Turbidity:
Turbidity can be measured in liquid culture. Turbidity can be measured using a spectrophotometer. The instrument measures the amount of light transmitted through the liquid. As bacterial growth makes the liquid turbid, more light is absorbed or scattered. Very less light is transmitted. It is not the best method for counting fecal bacteria in a stream. Hence, the option (b) is incorrect.
(c) Viable plate counts:
Viable plate counts can be done after the process of serial dilution. After the process of serial dilution, each dilution is then plated on an agar plate. After incubation, the cells are counted on a culture plate. The original number of microbes in the original sample can be determined by multiplying the number of cells into the dilution factor. It is not the best method for counting fecal bacteria in a stream. Hence, the option (c) is incorrect.
(d) Membrane filtration:
The process of membrane filtration is done by pouring a large sample of fluid through a membrane filter. The filter traps the cells in the liquid. The membrane is then transferred to a solid medium. The cells are then allowed to grow. The number of colonies indicates the number of organisms in the original sample. Thus, the best method for counting fecal bacteria in a stream to determine the portability of the water would be membrane filtration.
Hence, the correct option is (d) membrane filtration.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 12SA
The composition of the Escherichia coli medium is given below:

  • Glucose – 1 g
  • Na2HPO4 – 16.4 g
  • KH2PO4 – 1.5 g
  • (NH4)3PO4– 2 g
  • MgSO4.7H2O – 0.2 g
  • CaCl2 – 0.01 g
  • FeSO4.7H2O -0.005 g
  • Distilled water – bring volume to 1 liter

The compositions of the blood agar and MacConkey agar are shown below:
Blood agar:

  • Agar – 15 g
  • Pancreatic digest of casein – 15 g
  • Papaic digest of soybean mean – 5 g
  • NaCl – 5 g
  • Sterile blood – 50 ml
  • Distilled water – to bring volume to 950 ml

MacConkey agar:

  • Peptone – 20 g
  • Agar – 12 g
  • Lactose – 10 g
  • Bile salts – 5 g
  • NaCl – 5 g
  • Neutral red – 0.075 g
  • Crystal violet – 0.001 g
  • Distilled water – to bring volume to 1 liter

The E. coli medium is described as defined while blood agar and MacConkey agar are described as complex.
The E. coli medium is described as defined because all the components of the medium are known. The exact amount of each ingredient is also known. When all the components are known in a medium along with the amount of each ingredient, it is known as a defined medium.
The blood agar is described as a complex medium because of the presence of blood and other substances like casein, and soybean. The presence of digested proteins also makes the medium complex. A medium is defined as complex when the exact chemical composition is not known. Since partial digestion releases different chemicals in different concentrations, the exact chemical composition is not known.
The MacConkey agar is described as a complex medium because it is used as a differential medium to differentiate between coliforms and non-coliforms. This medium inhibits the growth of other organisms and only allows the growth of coliforms.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 13MC
(a) A statistical estimation using 15 dilution tubes and a table of numbers to estimate the number of bacteria per milliliter:
A Coulter counter is not a statistical estimation using tubes and a table of numbers. It is a device that directly counts microbes when they pass in front of an electronic detector. Hence, the option (a) is incorrect.
(b) An indirect method of counting microorganisms:
The indirect methods of counting microorganisms are dry weight, measurement of metabolic activity, turbidity, and analysis of kinds of genetic sequences. A Coulter counter is a device that directly counts microbes when they pass in front of an electronic detector. Hence, the option (b) is incorrect.
(c) A device that directly counts microbes that are differentially stained with fluorescent dyes:
The Coulter counter does not use fluorescent dyes, but electricity to detect the number of cells. It is a device that directly counts microbes when they pass in front of an electronic detector. The detector detects the number of times the flow of electric current is interrupted. It does not depend on the principle of fluorescence. Hence, the option (c) is incorrect.
(d) A device that directly counts microbes as they pass through a tube in front of an electronic detector:
The Coulter counter is a device that directly counts microbes when they pass in front of an electronic detector. The cells are counted when they interrupt the flow of electric current. This counter is mainly used for large cells like yeast, algae, and protozoa. A device that directly counts microbes when they pass in front of an electronic detector is known as a Coulter counter.
Hence, the correct option is (c) a device that directly counts microbes as they pass through a tube in front of an electronic detector.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 14MC
(a) Deep-freezing:
The process of deep-freezing is placing the culture at temperatures between -50 0 C to -95 0 C. When cultures are frozen using this method they need to be thawed and then placed in a suitable medium. Deep-freezing is not freeze-drying. The process of lyophilization is the removal of water molecules from a culture using a vacuum. Hence, the option (b) is incorrect.
(b) Refrigeration:
The process of refrigeration is the best technique for short-term storage of culture. Cultures stored in this way cannot be stored for many years. The process of refrigeration does not indicate the process of freeze-drying or lyophilization. The process of lyophilization is the removal of water molecules from a culture using a vacuum. Hence, the option (c) is incorrect.
(c) Pickling is the method in which cultures are preserved in brine solutions. This type of preservation is usually seen in food preservation. Pickling does not involve low temperatures at all. It is not the process of freeze-drying or lyophilization. The process of lyophilization is the removal of water molecules from a culture using a vacuum. Hence, the option (d) is incorrect.
(d) Freeze-drying is called as lyophilization. In the process of lyophilization the removal of water molecules from a culture is by using a vacuum. The culture needs to be frozen for this process. The ice in the culture directly undergoes the process of sublimation to become a gas. The culture does not sustain any damage of any sort and can be preserved for many years.
Hence, the correct option is (a) freeze-drying.

Microbiology with Diseases by Taxonomy Chapter 6 Answers 15MC
(a) A characteristic of most bacteria:
The process of quorum sensing is not present in most bacteria. The process of quorum sensing is the ability to react to variations in the density of a population. It is mainly seen in the organisms of a biofilm and helps the organisms to get new characteristics. Hence, the option (b) is incorrect.
(b) Dependent on direct contact among cells:
The process of quorum sensing is not dependent on direct contact among cells. It depends on the signals released by microbes. When many species of microbes are present in a biofilm, they secrete molecules into the matrix which act as signals. Direct contact among cells is not needed. Hence, the option (c) is incorrect.
(c) The process of quorum sensing is not associated with colonies present on an agar plate. The colonies on an agar plate mostly do not show the process of quorum sensing. It is mainly seen in the organisms of a biofilm and helps the organisms to get new characteristics. Hence, the option (d) is incorrect.
(d) The ability to alter the changes in the density of a population is quorum sensing. In this process, bacteria can respond to these changes by using signal and receptor molecules. This process is often seen in biofilms where many species are present together.
Hence, the correct option is (a) the ability to respond to changes in population density.


Engineering Cooperation in an Anaerobic Coculture

Over the past century, microbiologists have studied organisms in pure culture, yet it is becoming increasingly apparent that the majority of biological processes rely on multispecies cooperation and interaction. While little is known about how such interactions permit cooperation, even less is known about how cooperation arises. To study the emergence of cooperation in the laboratory, we constructed both a commensal community and an obligate mutualism using the previously noninteracting bacteria Shewanella oneidensis and Geobacter sulfurreducens Incorporation of a glycerol utilization plasmid (pGUT2) enabled S. oneidensis to metabolize glycerol and produce acetate as a carbon source for G. sulfurreducens, establishing a cross-feeding, commensal coculture. In the commensal coculture, both species coupled oxidative metabolism to the respiration of fumarate as the terminal electron acceptor. Deletion of the gene encoding fumarate reductase in the S. oneidensis/pGUT2 strain shifted the coculture with G. sulfurreducens to an obligate mutualism where neither species could grow in the absence of the other. A shift in metabolic strategy from glycerol catabolism to malate metabolism was associated with obligate coculture growth. Further targeted deletions in malate uptake and acetate generation pathways in S. oneidensis significantly inhibited coculture growth with G. sulfurreducens The engineered coculture between S. oneidensis and G. sulfurreducens provides a model laboratory system to study the emergence of cooperation in bacterial communities, and the shift in metabolic strategy observed in the obligate coculture highlights the importance of genetic change in shaping microbial interactions in the environment.IMPORTANCE Microbes seldom live alone in the environment, yet this scenario is approximated in the vast majority of pure-culture laboratory experiments. Here, we develop an anaerobic coculture system to begin understanding microbial physiology in a more complex setting but also to determine how anaerobic microbial communities can form. Using synthetic biology, we generated a coculture system where the facultative anaerobe Shewanella oneidensis consumes glycerol and provides acetate to the strict anaerobe Geobacter sulfurreducens In the commensal system, growth of G. sulfurreducens is dependent on the presence of S. oneidensis To generate an obligate coculture, where each organism requires the other, we eliminated the ability of S. oneidensis to respire fumarate. An unexpected shift in metabolic strategy from glycerol catabolism to malate metabolism was observed in the obligate coculture. Our work highlights how metabolic landscapes can be expanded in multispecies communities and provides a system to evaluate the evolution of cooperation under anaerobic conditions.

Keywords: Geobacter Shewanella anaerobic respiration metabolism.


Aerobic Bacteria

Aerobic bacteria require oxygen to perform cellular respiration and derive energy to survive. In short, aerobic bacteria grows and multiplies only in the presence of oxygen. To know more about aerobic bacteria, read on.

Aerobic bacteria require oxygen to perform cellular respiration and derive energy to survive. In short, aerobic bacteria grows and multiplies only in the presence of oxygen. To know more about aerobic bacteria, read on.

Mention bacteria and nearly all of us assume them to be disease-causing microbes. However, not many of us know that bacteria play a major role in the overall functioning of our ecosystem. Believe it or not, a major biomass of the earth is contributed by minute bacterial cells. Consequently, the variability of bacteria in terms of shape, size, dwelling place, feeding habit, and surviving requirements is extremely vast.

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Bacteria are omnipresent, meaning that they are found in any type of environmental conditions. Some of the species are isolated from the least hospitable areas like hot springs, below the earth’s crust, and in radioactive wastes. So, you can imagine the adaptability of bacteria in comparison to other living beings. Based on whether oxygen is required for survival or not, bacteria are classified as aerobic and anaerobic. This article introduces you to aerobic bacteria with examples and tells you how they differ from the anaerobic ones.
Types of Aerobic Bacteria Aerobic bacteria require oxygen for survival. Various types of aerobic bacteria are enlisted here.

Obligate Aerobes: These bacterial strains compulsorily require oxygen for deriving energy, growth, reproduction, and cellular respiration.

Facultative and Microaerophiles: Contrary to obligate aerobes, there are anaerobic bacteria, which live in a non-oxygenated environment throughout their life. Intermediary to these two groups are facultative bacteria (e.g., E. coli, Staphylococcus) and microaerophilic bacteria (e.g., Campylobacter, Helicobacter pylori). Facultative bacteria behave both aerobically and anaerobically, according to the prevailing conditions. Those of the microaerophilic type require oxygen, but in very low concentrations.

How to distinguish between Aerobic and Anaerobic Bacteria In microbiology and biology experiments, obligate aerobic bacteria can be isolated easily by culturing a mass of bacterial strains in a liquid medium. Since they are oxygen-needing organisms, they tend to collect in the top surface of the liquid medium, so as to absorb the maximum oxygen available to them.

Examples of Aerobic Bacteria
Studying the characteristic features and importance of bacteria is a major part of bacteriology. Mentioned below are some examples of aerobic bacteria and their characteristic features:

Bacillus
The genus Bacillus encompasses both obligate and facultative types of bacterial species. They include free living or pathogenic strains. For example, B. subtilis is a free-living soil bacterium, while B. anthrax infection causes anthrax disease. Ubiquitous in habit and having a large-sized genome, various species of Bacillus are commercially used for enzyme production and genetic researches.

Mycobacterium tuberculosis
As the name suggests, it is a species of pathogenic bacteria that cause tuberculosis. It is a rod shaped, obligate aerobic bacteria, characterized by the presence of a waxy layer on the wall. Being an oxygen-needing species, M. tuberculosis infects the lungs of mammalians, where oxygen is present in very high amounts. It divides at a very slow rate, after about 15 hours of infection.

Nocardia
Rod-shaped and gram positive type, the genus nocardia comprises more than 80 species. Out of these, some are capable of causing health conditions, while others are non-pathogenic. The disease caused by infection of nocardia is called nocardiosis, affecting only the lungs or the whole body. Usually, nocardia thrives in the oral cavity, mostly in the gums and periodontal pockets.

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Lactobacillus
Lactobacillus is not a true aerobic bacteria, but it is included in the facultative type. You might have already heard about the application of this bacterium in curdling and fermentation of food items. It is normally found in the oral cavity and intestines without causing any symptoms. Rather, some Lactobacillus species are beneficial for health and classified as probiotic flora.

Pseudomonas Aeruginosa
Pseudomonas Aeruginosa is an gram-negative, rod shaped, obligate bacterium that causes diseases in humans and animals. It attacks people with a weak immune system. It is found everywhere in the environment. Infections caused by this bacterium are characterized by inflammations. If this infection occurs in the lungs or other vital organs, it can prove to be fatal. Diseases caused by it are pneumonia, urinary tract infections, and gastrointestinal infections. In addition to the above-mentioned strains, the list of aerobic bacteria includes Staphylococcus (facultative) and Enterobacteriacae species (facultative) among others. The major roles of aerobic bacteria include recycling of nutrients, decomposing waste products, and assisting in plant nutrient absorption. As they play a crucial role in the efficient working of septic systems, aerobic bacteria generators used in tanks. Bacteria from the generator aid in digesting harmful gases, foul odor, and help with other waste digesting problems.

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What is a Facultative Anaerobe?

Facultative anaerobe is an organism that can make energy by aerobic respiration when the oxygen is present and switches to anaerobic respiration or fermentation to produce energy when the oxygen is absent. Facultative anaerobes do not necessarily need oxygen for respiration.

Figure 02: Facultative Anaerobic E. coli

Some bacteria belonging to facultative anaerobes are Staphylococcus spp, Streptococcus spp, Escherichia coli, Salmonella, Listeria, Corynebacterium and Shewanella oneidensis. Some fungi such as yeast etc are also facultative anaerobes.


Materials and Methods

Bacterial isolates

Twenty five E. coli and 25 Salmonella enterica isolates were selected through systematic random sampling from existing isolate collections at the Department of Diagnostic Medicine/Pathobiology of Kansas State University. The isolates were obtained from cattle feces by earlier field studies in the U.S. beef feedlots. All available generic E. coli isolates were included in one sampling frame for the random selection of 25 isolates. A separate frame was implemented for the random selection of isolates from the collection of each available serovar of Salmonella enterica so as to select a total of 25 isolates of diverse serovars. After the antimicrobial susceptibility testing, all the isolates of E. coli and Salmonella which would be interpreted as susceptible to the tested drug based on the clinical breakpoint interpretation for human infection [7] were included in the statistical analyses (the isolates which would be interpreted as resistant were excluded.) This design ensured that n≥30 isolates would be available for the statistical analysis of the difference in the isolates’ susceptibility between the two conditions for each drug tested. The analyzed set of isolates contained E. coli of serovars O26, O45, O103, O111, O145, and Salmonella enterica of serovars Agona, Anatum, Give, Infantis, Kentucky, Montevideo, Muenchen, Oranienburg, and Typhimurium.

Antimicrobials

For each bacterial isolate, its MIC value in anaerobic conditions and its MIC value in aerobic conditions were determined using E-Test ® as described below for the following drugs as representatives of their antimicrobial classes: ampicillin for older β-lactams aminopenicillins and ceftriaxone for newer β-lactams cephalosporins gentamicin and kanamycin for aminoglycosides enrofloxacin for fluoroquinolones erythromycin for classic macrolides azithromycin for azalides sulfamethoxazole/trimethoprim for sulfonamides and tetracycline for tetracyclines.

Testing antimicrobial susceptibility of bacterial isolates

The E-Test ® (bioMerieux, Durham, NC, USA) assay was chosen to test antimicrobial susceptibility of the isolates for two reasons. Firstly, it is suitable for measuring MIC in both aerobic and anaerobic conditions [23–27]. Secondly, the assay evaluates the exact MIC value on continuous scale (rather than MIC ranges) this was particularly useful for comparing the MIC values for individual isolates between the two conditions. Escherichia coli ATCC 25922 strain was included in the experiments for each drug as a reference strain the MIC for this strain in aerobic conditions was checked against the expected ranges, as per E-Test ® manufacturer's recommendations.

Each bacterial isolate used was purified at the time of original isolation and stored at -80°C. From the thawed freezer tube, the isolate was plated using an inoculation loop on a plate of the selected agar and incubated at 37°C for 24 hours. For the aerobic test, two to three colonies were picked from the agar plate using an inoculation loop and suspended in 9 mL Mueller-Hinton broth (MHB) (BD Diagnostic, Sparks, MD, USA) until the optical density of the suspension was equivalent to a 0.5 McFarland turbidity standard. A sterile cotton swab applicator was used to plate the suspension (approximately 100 μL, as per E-Test ® manufacturer’s recommendations) on a new plate of the selected agar. The E-Test ® strip containing the antimicrobial drug was placed in the center of this plate, and the plate was incubated upside down at 37°C for 24 hours, after which the result was read and recorded. For reading the result, an ellipse-shaped zone of bacterial growth inhibition centered along the strip was evaluated visually the MIC value was read as the drug concentration on the strip where the ellipse’s edge intersected the strip, as per E-Test ® manufacturer's recommendations. For the anaerobic test, all these procedures were performed in an anaerobic chamber (Thermo Fisher Scientific Inc., Waltham, MA, USA) a sufficient number of colonies were picked up from the first agar plate for the optical density of the suspension in MHB to be equivalent to a 0.5 McFarland turbidity standard. With this experimental design, the bacterial culture was exposed to the aerobic or anaerobic conditions for 24 hours (while on the first agar plate) prior to the test of its antimicrobial susceptibility, and for further 24 hours during the test (while on the second agar plate with the test strip). The aerobic and anaerobic tests were performed separately on Mueller-Hinton (MH) agar (Becton, Dickinson and Company, Sparks, MD, USA) and on a more complex tryptic soy agar with 5% sheep blood (BAP) agar (Remel, Lenexa, KS, USA).

A satellite experiment was performed to investigate how quickly from the start of bacterial exposure to anaerobic conditions the physiological changes in the bacteria lead to a detectable change in the bacterial susceptibility to antimicrobials. The experiment was conducted with five isolates of E. coli and five isolates of Salmonella enterica selected at random from the isolates in the study interpreted as susceptible to all antimicrobials tested based on the clinical breakpoint interpretation for human infection [7]. The experiment was performed with the 10 isolates for ampicillin, gentamicin, kanamycin, and enrofloxacin, and with the five E. coli isolates only for azithromycin. The experiment with a given antimicrobial drug was performed on either MH or BAP agar. In the experiment, each of 10 isolates was plated in duplicate on two plates of the selected agar and incubated at 37°C for 24 hours aerobically. From the first plate, two to three colonies were picked to perform E-Test ® on a new plate aerobically. The second plate was transferred to the anaerobic chamber, and two to three colonies were picked up from the plate to perform E-Test ® anaerobically on a new plate. These tests provided measurements of the isolate’s susceptibility to the antimicrobials in aerobic conditions, and in anaerobic conditions following 0 hours of adaptation to these conditions. Each of the 10 isolates was also freshly plated in the anaerobic chamber on a third agar plate. Following each 12 hours of incubation in the chamber at 37°C, colonies were picked from this plate (sufficient number of colonies for the optical density of the suspension in MHB to be equivalent to a 0.5 McFarland turbidity standard) to perform E-Test ® anaerobically on a new plate. This provided measurements of the isolate’s susceptibility to the antimicrobials in anaerobic conditions following a 12, 24, 36, and 48 hour period of adaptation to these conditions.

Statistical analysis

Statistical significance of the difference between the MIC values for the isolates in the anaerobic and aerobic tests was evaluated for each bacterial species, antimicrobial drug tested, and agar media combination. The statistical significance was evaluated using the t-test for paired samples assuming heteroscedasticity implemented in Microsoft Excel 2013 ® software for Windows (Microsoft Corporation, Redmond, WA, USA) and using the nonparametric Wilcoxon signed-rank test implemented in SigmaPlot TM v. 13.0 software (Systat Software Inc., San Jose, CA, USA). Descriptive statistics of the distributions of the MIC values and differences between the MIC values in the two conditions for the isolates were obtained in Microsoft Excel 2013 ® software. Potential correlation between the relative magnitude of the MIC values of individual isolates in aerobic and anaerobic conditions across the isolates tested was evaluated using the nonparametric Spearman correlation coefficient implemented in SigmaPlot TM v. 13.0 software. Figures were prepared in SigmaPlot TM v. 13.0 software.


Detoxification of Reactive Oxygen Species

Aerobic respiration constantly generates reactive oxygen species (ROS), byproducts that must be detoxified. Even organisms that do not use aerobic respiration need some way to break down some of the ROS that may form from atmospheric oxygen. Three main enzymes break down those toxic byproducts: superoxide dismutase, peroxidase, and catalase. Each one catalyzes a different reaction. Reactions of type seen in Reaction 1 are catalyzed by peroxidases.

In these reactions, an electron donor (reduced compound e.g., reduced nicotinamide adenine dinucleotide [NADH]) oxidizes hydrogen peroxide, or other peroxides, to water. The enzymes play an important role by limiting the damage caused by peroxidation of membrane lipids. Reaction 2 is mediated by the enzyme superoxide dismutase (SOD) and breaks down the powerful superoxide anions generated by aerobic metabolism:

The enzyme catalase converts hydrogen peroxide to water and oxygen as shown in Reaction 3.

Figure 5. The catalase test detects the presence of the enzyme catalase by noting whether bubbles are released when hydrogen peroxide is added to a culture sample. Compare the positive result (right) with the negative result (left). (credit: Centers for Disease Control and Prevention)

Obligate anaerobes usually lack all three enzymes. Aerotolerant anaerobes do have SOD but no catalase. Reaction 3, shown occurring in Figure 5, is the basis of a useful and rapid test to distinguish streptococci, which are aerotolerant and do not possess catalase, from staphylococci, which are facultative anaerobes. A sample of culture rapidly mixed in a drop of 3% hydrogen peroxide will release bubbles if the culture is catalase positive.

Bacteria that grow best in a higher concentration of CO2 and a lower concentration of oxygen than present in the atmosphere are called capnophiles. One common approach to grow capnophiles is to use a candle jar. A candle jar consists of a jar with a tight-fitting lid that can accommodate the cultures and a candle. After the cultures are added to the jar, the candle is lit and the lid closed. As the candle burns, it consumes most of the oxygen present and releases CO2.

Think about It

  • What substance is added to a sample to detect catalase?
  • What is the function of the candle in a candle jar?

Clinical Focus: Nataliya, Part 2

This example continues Nataliya’s story that started in How Microbes Grow.

The health-care provider who saw Nataliya was concerned primarily because of her pregnancy. Her condition enhances the risk for infections and makes her more vulnerable to those infections. The immune system is downregulated during pregnancy, and pathogens that cross the placenta can be very dangerous for the fetus. A note on the provider’s order to the microbiology lab mentions a suspicion of infection by Listeria monocytogenes, based on the signs and symptoms exhibited by the patient.

Nataliya’s blood samples are streaked directly on sheep blood agar, a medium containing tryptic soy agar enriched with 5% sheep blood. (Blood is considered sterile therefore, competing microorganisms are not expected in the medium.) The inoculated plates are incubated at 37 °C for 24 to 48 hours. Small grayish colonies surrounded by a clear zone emerge. Such colonies are typical of Listeria and other pathogens such as streptococci the clear zone surrounding the colonies indicates complete lysis of blood in the medium, referred to as beta-hemolysis (Figure 6). When tested for the presence of catalase, the colonies give a positive response, eliminating Streptococcus as a possible cause. Furthermore, a Gram stain shows short gram-positive bacilli. Cells from a broth culture grown at room temperature displayed the tumbling motility characteristic of Listeria (Figure 6). All of these clues lead the lab to positively confirm the presence of Listeria in Nataliya’s blood samples.

Figure 6. (a) A sample blood agar test showing beta-hemolysis. (b) A sample motility test showing both positive and negative results. (credit a: modification of work by Centers for Disease Control and Prevention credit b: modification of work by “VeeDunn”/Flickr)

We’ll return to Nataliya’s example in later pages.

Key Concepts and Summary

  • Aerobic and anaerobic environments can be found in diverse niches throughout nature, including different sites within and on the human body.
  • Microorganisms vary in their requirements for molecular oxygen. Obligate aerobes depend on aerobic respiration and use oxygen as a terminal electron acceptor. They cannot grow without oxygen.
  • Obligate anaerobes cannot grow in the presence of oxygen. They depend on fermentation and anaerobic respiration using a final electron acceptor other than oxygen.
  • Facultative anaerobes show better growth in the presence of oxygen but will also grow without it.
  • Although aerotolerant anaerobes do not perform aerobic respiration, they can grow in the presence of oxygen. Most aerotolerant anaerobes test negative for the enzyme catalase.
  • Microaerophiles need oxygen to grow, albeit at a lower concentration than 21% oxygen in air.
  • Optimum oxygen concentration for an organism is the oxygen level that promotes the fastest growth rate. The minimum permissive oxygen concentration and the maximum permissive oxygen concentration are, respectively, the lowest and the highest oxygen levels that the organism will tolerate.
  • Peroxidase, superoxide dismutase, and catalase are the main enzymes involved in the detoxification of the reactive oxygen species. Superoxide dismutase is usually present in a cell that can tolerate oxygen. All three enzymes are usually detectable in cells that perform aerobic respiration and produce more ROS.
  • A capnophile is an organism that requires a higher than atmospheric concentration of CO2 to grow.

Multiple Choice

An inoculated thioglycolate medium culture tube shows dense growth at the surface and turbidity throughout the rest of the tube. What is your conclusion?

  1. The organisms die in the presence of oxygen
  2. The organisms are facultative anaerobes.
  3. The organisms should be grown in an anaerobic chamber.
  4. The organisms are obligate aerobes.

An inoculated thioglycolate medium culture tube is clear throughout the tube except for dense growth at the bottom of the tube. What is your conclusion?

  1. The organisms are obligate anaerobes.
  2. The organisms are facultative anaerobes.
  3. The organisms are aerotolerant.
  4. The organisms are obligate aerobes.

Pseudomonas aeruginosa is a common pathogen that infects the airways of patients with cystic fibrosis. It does not grow in the absence of oxygen. The bacterium is probably which of the following?

  1. an aerotolerant anaerobe
  2. an obligate aerobe
  3. an obligate anaerobe
  4. a facultative anaerobe

Streptococcus mutans is a major cause of cavities. It resides in the gum pockets, does not have catalase activity, and can be grown outside of an anaerobic chamber. The bacterium is probably which of the following?

  1. a facultative anaerobe
  2. an obligate aerobe
  3. an obligate anaerobe
  4. an aerotolerant anaerobe

Why do the instructions for the growth of Neisseria gonorrheae recommend a CO2-enriched atmosphere?

  1. It uses CO2 as a final electron acceptor in respiration.
  2. It is an obligate anaerobe.
  3. It is a capnophile.
  4. It fixes CO2 through photosynthesis.

Matching

Four tubes are illustrated with cultures grown in a medium that slows oxygen diffusion. Match the culture tube with the correct type of bacteria from the following list: facultative anaerobe, obligate anaerobe, microaerophile, aerotolerant anaerobe, obligate aerobe.