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9.4B: Signaling in Bacteria - Biology

9.4B: Signaling in Bacteria - Biology


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Bacterial signaling allows bacteria to monitor cellular conditions and communicate with each other.

Learning Objectives

  • Describe how cell signaling occurs in single-celled organisms such as bacteria

Key Points

  • Gene expression in bacteria is initiated when the population density of the bacteria reaches a certain level.
  • Bacterial signaling is called quorum sensing because cell density is the determining factor for signaling.
  • Quorum sensing uses autoinducers, which are secreted by bacteria to communicate with other bacteria of the same kind, as signaling molecules.
  • Autoinducers may be small, hydrophobic molecules, or they can be larger peptide-based molecules; regardless, each type of molecule has a different mode of action.
  • Some bacteria form biofilms, which are complex colonies of bacteria that exchange chemical signals to coordinate the release of toxins that attack the host.

Key Terms

  • quorum sensing: a method of communication between bacterial cells by the release and sensing of small diffusible signal molecules
  • autoinducer: any of several compounds, synthesized by bacteria, that have signalling functions in quorum sensing
  • biofilm: a thin film of mucus created by and containing a colony of bacteria and other microorganisms

Signaling in Bacteria

Signaling in bacteria, known as quorum sensing, enables bacteria to monitor extracellular conditions, ensure sufficient amounts of nutrients are present, and avoid hazardous situations. There are circumstances, however, when bacteria communicate with each other.

The first evidence of bacterial communication was observed in a bacterium that has a symbiotic relationship with Hawaiian bobtail squid. When the population density of the bacteria reached a certain level, specific gene expression was initiated: the bacteria produced bioluminescent proteins that emitted light. Because the number of cells present in the environment (the cell density) is the determining factor for signaling, bacterial signaling was named quorum sensing. Interestingly, in politics and business, a quorum is the minimum number of members required to be present to vote on an issue.

Quorum sensing uses autoinducers as signaling molecules. Autoinducers are signaling molecules secreted by bacteria to communicate with other bacteria of the same kind. The secreted autoinducers can be small, hydrophobic molecules, such as acyl-homoserine lactone (AHL), or larger peptide-based molecules. Each type of molecule has a different mode of action. When AHL enters target bacteria, it binds to transcription factors, which then switch gene expression on or off. The peptide autoinducers stimulate more complicated signaling pathways that include bacterial kinases. The changes in bacteria following exposure to autoinducers can be quite extensive. The pathogenic bacterium Pseudomonas aeruginosa has 616 different genes that respond to autoinducers.

Some species of bacteria that use quorum sensing form biofilms, which are complex colonies of bacteria (often containing several species) that exchange chemical signals to coordinate the release of toxins that attack the host. Bacterial biofilms can sometimes be found on medical equipment. When biofilms invade implants, such as hip or knee replacements or heart pacemakers, they can cause life-threatening infections.


Chemotaxis

Chemotaxis (from chemo- + taxis) is the movement of an organism or entity in response to a chemical stimulus. [1] Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food (e.g., glucose) by swimming toward the highest concentration of food molecules, or to flee from poisons (e.g., phenol). In multicellular organisms, chemotaxis is critical to early development (e.g., movement of sperm towards the egg during fertilization) and subsequent phases of development (e.g., migration of neurons or lymphocytes) as well as in normal function and health (e.g., migration of leukocytes during injury or infection). [2] In addition, it has been recognized that mechanisms that allow chemotaxis in animals can be subverted during cancer metastasis. [3] The aberrant chemotaxis of leukocytes and lymphocytes also contribute to inflammatory diseases such as atherosclerosis, asthma, and arthritis. [4] [5] [6] [7] Sub-cellular components, such as the polarity patch generated by mating yeast, may also display chemotactic behavior. [8]

Positive chemotaxis occurs if the movement is toward a higher concentration of the chemical in question negative chemotaxis if the movement is in the opposite direction. Chemically prompted kinesis (randomly directed or nondirectional) can be called chemokinesis.


Signaling in Bacteria

Signaling in bacteria enables bacteria to monitor extracellular conditions, ensure that there are sufficient amounts of nutrients, and ensure that hazardous situations are avoided. There are circumstances, however, when bacteria communicate with each other.

The first evidence of bacterial communication was observed in a bacterium that has a symbiotic relationship with Hawaiian bobtail squid. When the population density of the bacteria reaches a certain level, specific gene expression is initiated, and the bacteria produce bioluminescent proteins that emit light. Because the number of cells present in the environment (cell density) is the determining factor for signaling, bacterial signaling was named quorum sensing. In politics and business, a quorum is the minimum number of members required to be present to vote on an issue.

Quorum sensing uses autoinducers as signaling molecules. Autoinducers are signaling molecules secreted by bacteria to communicate with other bacteria of the same kind. The secreted autoinducers can be small, hydrophobic molecules such as acyl-homoserine lactone, (AHL) or larger peptide-based molecules each type of molecule has a different mode of action. When AHL enters target bacteria, it binds to transcription factors, which then switch gene expression on or off (see Figure 2). The peptide autoinducers stimulate more complicated signaling pathways that include bacterial kinases. The changes in bacteria following exposure to autoinducers can be quite extensive. The pathogenic bacterium Pseudomonas aeruginosa has 616 different genes that respond to autoinducers.

Practice Question

Figure 2. Autoinducers are small molecules or proteins produced by bacteria that regulate gene expression.

Which of the following statements about quorum sensing is false?

  1. Autoinducer must bind to receptor to turn on transcription of genes responsible for the production of more autoinducer.
  2. The receptor stays in the bacterial cell, but the autoinducer diffuses out.
  3. Autoinducer can only act on a different cell: it cannot act on the cell in which it is made.
  4. Autoinducer turns on genes that enable the bacteria to form a biofilm.

Some species of bacteria that use quorum sensing form biofilms, complex colonies of bacteria (often containing several species) that exchange chemical signals to coordinate the release of toxins that will attack the host. Bacterial biofilms (Figure 3) can sometimes be found on medical equipment when biofilms invade implants such as hip or knee replacements or heart pacemakers, they can cause life-threatening infections.

Cell-cell communication enables Staphylococcus aureus bacteria (Figure 3a) to work together to form a biofilm inside a hospital patient’s catheter, seen here via scanning electron microscopy. S. aureus is the main cause of hospital-acquired infections. Hawaiian bobtail squid (Figure 3b) have a symbiotic relationship with the bioluminescent bacteria Vibrio fischeri. The luminescence makes it difficult to see the squid from below because it effectively eliminates its shadow. In return for camouflage, the squid provides food for the bacteria. Free-living V. fischeri do not produce luciferase, the enzyme responsible for luminescence, but V. fischeri living in a symbiotic relationship with the squid do. Quorum sensing determines whether the bacteria should produce the luciferase enzyme.

Practice Question

Figure 3. (a) Staphylococcus aureus bacteria. (b) Hawaiian bobtail squid. (credit a: modifications of work by CDC/Janice Carr credit b: modifications of work by Cliff1066/Flickr)

What advantage might biofilm production confer on the S. aureus inside the catheter?

Research on the details of quorum sensing has led to advances in growing bacteria for industrial purposes. Recent discoveries suggest that it may be possible to exploit bacterial signaling pathways to control bacterial growth this process could replace or supplement antibiotics that are no longer effective in certain situations.

Watch geneticist Bonnie Bassler discuss her discovery of quorum sensing in biofilm bacteria in squid.

Watch this collection of interview clips with biofilm researchers in “What Are Bacterial Biofilms?”

Practice Questions

Quorum sensing is triggered to begin when ___________.

  1. treatment with antibiotics occurs
  2. bacteria release growth hormones
  3. bacterial protein expression is switched on
  4. a sufficient number of bacteria are present

Why is signaling in multicellular organisms more complicated than signaling in single-celled organisms?


Section Summary

Yeasts and multicellular organisms have similar signaling mechanisms. Yeasts use cell-surface receptors and signaling cascades to communicate information on mating with other yeast cells. The signaling molecule secreted by yeasts is called mating factor.

Bacterial signaling is called quorum sensing. Bacteria secrete signaling molecules called autoinducers that are either small, hydrophobic molecules or peptide-based signals. The hydrophobic autoinducers, such as AHL, bind transcription factors and directly affect gene expression. The peptide-based molecules bind kinases and initiate signaling cascades in the cells.


Contents

The term "PAMP" has been criticized on the grounds that most microbes, not only pathogens, express the molecules detected the term microbe-associated molecular pattern (MAMP), [7] [8] [9] has therefore been proposed. A virulence signal capable of binding to a pathogen receptor, in combination with a MAMP, has been proposed as one way to constitute a (pathogen-specific) PAMP. [10] Plant immunology frequently treats the terms "PAMP" and "MAMP" interchangeably, considering their recognition to be the first step in plant immunity, PTI (PAMP-triggered immunity), a relatively weak immune response that occurs when the host plant does not also recognize pathogenic effectors that damage it or modulate its immune response. [11]

In mycobacteria Edit

Mycobacteria are intracellular bacteria which survive in host macrophages. The mycobacterial wall is composed of lipids and polysaccharides and also contains high amounts of mycolic acid. Purified cell wall components of mycobacteria activate mainly TLR2 and also TLR4. Lipomannan and lipoarabinomannan are strong immunomodulatory lipoglycans. [12] TLR2 with association of TLR1 can recognize cell wall lipoprotein antigens from Mycobacterium tuberculosis, which also induce production of cytokines by macrophages. [13] TLR9 can be activated by mycobaterial DNA.


Cellular Communication in YeastsThe first cellular form of life on our planet likely consisted of single-celled prokaryotic organisms that had limited interaction with each other. While some external signaling occurs between different species of single-celled organisms, the majority of signaling within bacteria and yeasts concerns only other members of the same species. The evolution of cellular communication is an absolute necessity for the development of multicellular organisms, and this innovation is thought to have required approximately 2 billion years to appear in early life forms.

Yeasts are single-celled eukaryotes and, therefore, have a nucleus and organelles characteristic of more complex life forms. Comparisons of the genomes of yeasts, nematode worms, fruit flies, and humans illustrate the evolution of increasingly complex signaling systems that allow for the efficient inner workings that keep humans and other complex life forms functioning correctly.

Kinases are a major component of cellular communication, and studies of these enzymes illustrate the evolutionary connectivity of different species. Yeasts have 130 types of kinases. More complex organisms such as nematode worms and fruit flies have 454 and 239 kinases, respectively. Of the 130 kinase types in yeast, 97 belong to the 55 subfamilies of kinases that are found in other eukaryotic organisms. The only obvious deficiency seen in yeasts is the complete absence of tyrosine kinases. It is hypothesized that phosphorylation of tyrosine residues is needed to control the more sophisticated functions of development, differentiation, and cellular communication used in multicellular organisms.

Because yeasts contain many of the same classes of signaling proteins as humans, these organisms are ideal for studying signaling cascades. Yeasts multiply quickly and are much simpler organisms than humans or other multicellular animals. Therefore, the signaling cascades are also simpler and easier to study, although they contain similar counterparts to human signaling.G. Manning, G.D. Plowman, T. Hunter, S. Sudarsanam, “Evolution of Protein Kinase Signaling from Yeast to Man,” Trends in Biochemical Sciences 27, no. 10 (2002): 514–520.


Palmitoylation of NOD1 and NOD2 is required for bacterial sensing

The nucleotide oligomerization domain (NOD)-like receptors 1 and 2 (NOD1/2) are intracellular pattern-recognition proteins that activate immune signaling pathways in response to peptidoglycans associated with microorganisms. Recruitment to bacteria-containing endosomes and other intracellular membranes is required for NOD1/2 signaling, and NOD1/2 mutations that disrupt membrane localization are associated with inflammatory bowel disease and other inflammatory conditions. However, little is known about this recruitment process. We found that NOD1/2 S-palmitoylation is required for membrane recruitment and immune signaling. ZDHHC5 was identified as the palmitoyltransferase responsible for this critical posttranslational modification, and several disease-associated mutations in NOD2 were found to be associated with defective S-palmitoylation. Thus, ZDHHC5-mediated S-palmitoylation of NOD1/2 is critical for their ability to respond to peptidoglycans and to mount an effective immune response.


9.4 Signaling in Single-Celled Organisms

In this section, you will explore the following questions:

  • How do single-celled yeasts use cell signaling to communicate with each other?
  • How does quorum sensing allow some bacteria to form biofilms?

Connection for AP ® Courses

Cell signaling allows bacteria to respond to environmental cues, such as nutrient levels and quorum sensing (cell density). Yeasts are eukaryotes (fungi), and the components and processes found in yeast signals are similar to those of cell-surface receptor signals in multicellular organisms. For example, budding yeasts often release mating factors that enable them to participate in a process that is similar to sexual reproduction.

Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 3 of the AP ® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 3 Living systems store, retrieve, transmit and respond to information essential to life processes.
Enduring Understanding 3.D Cells communicate by generating, transmitting and receiving chemical signals.
Essential Knowledge 3.D.1 Cell communication processes share common features that reflect a shared evolutionary history.
Science Practice 1.5 The student can re-express key elements of natural phenomena across multiple representations in the domain.
Learning Objective 3.36 The student is able to describe a model that expresses the key elements of signal transduction pathways by which a signal is converted to a cellular response.
Essential Knowledge 3.D.1 Cell communication processes share common features that reflect a shared evolutionary history.
Science Practice 6.1 The student can justify claims with evidence.
Learning Objective 3.37 The student is able to justify claims based on scientific evidence that changes in signal transduction pathways can alter cellular response.

Teacher Support

Unicellular organisms were assumed to communicate at a very primitive level, but current research reveals the existence more complex signaling systems. Examples of these forms of communication are the formation of biofilms and quorum sensing. Biofilms have received the attention of researchers only recently for several historical and technical reasons. Since the germ theory of disease was established, the interest had been to isolate and characterize pathogens, not to study microorganisms as a community.

It is much easier to grow bacteria as pure cultures than replicate mixed populations biofilms, making the latter difficult to study in the laboratory setting. Such slime layers, previously considered haphazard assemblies of microorganisms, have been found to be highly organized ecosystems. The slime layer is made of extracellular polymers crisscrossed with channels for gases, nutrients, waste exchanges. Microbes attach to the solid substrate in a succession of populations.

Quorum sensing exists both within a same species and across species. It allows microbes to behave as multicellular populations and coordinate responses. One such example is the expression of genes encoding toxins in Staphylococcus aureus. Dr. Bonnie Bassler presents quorum sensing communication in Vibrio harveyi in this Ted Talk. Her enthusiasm and clear explanations make this video a thoroughly engaging experience. This is an opportunity to show a strong female role model in science.

Also available is this video clip: Quorum sensing molecules presented by Dr. Bonnie Bassler:

And an animation on quorum sensing in Vibrio harveyi can be found here.

Further reading: Painter, Kimberley L. et al. (2014). What role does the quorum-sensing accessory gene regulator system play during Staphylococcus aureus bacteremia? Trends in Microbiology 22:676–685

The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 3.31][APLO 3.37]

Within-cell signaling allows bacteria to respond to environmental cues, such as nutrient levels. Some single-celled organisms also release molecules to signal to each other.

Signaling in Yeast

Yeasts are eukaryotes (fungi), and the components and processes found in yeast signals are similar to those of cell-surface receptor signals in multicellular organisms. Budding yeasts (Figure 9.16) are able to participate in a process that is similar to sexual reproduction that entails two haploid cells (cells with one-half the normal number of chromosomes) combining to form a diploid cell (a cell with two sets of each chromosome, which is what normal body cells contain). In order to find another haploid yeast cell that is prepared to mate, budding yeasts secrete a signaling molecule called mating factor . When mating factor binds to cell-surface receptors in other yeast cells that are nearby, they stop their normal growth cycles and initiate a cell signaling cascade that includes protein kinases and GTP-binding proteins that are similar to G-proteins.

Signaling in Bacteria

Signaling in bacteria enables bacteria to monitor extracellular conditions, ensure that there are sufficient amounts of nutrients, and ensure that hazardous situations are avoided. There are circumstances, however, when bacteria communicate with each other.

The first evidence of bacterial communication was observed in a bacterium that has a symbiotic relationship with Hawaiian bobtail squid. When the population density of the bacteria reaches a certain level, specific gene expression is initiated, and the bacteria produce bioluminescent proteins that emit light. Because the number of cells present in the environment (cell density) is the determining factor for signaling, bacterial signaling was named quorum sensing . In politics and business, a quorum is the minimum number of members required to be present to vote on an issue.

Quorum sensing uses autoinducers as signaling molecules. Autoinducers are signaling molecules secreted by bacteria to communicate with other bacteria of the same kind. The secreted autoinducers can be small, hydrophobic molecules such as acyl-homoserine lactone (AHL) or larger peptide-based molecules each type of molecule has a different mode of action. When AHL enters target bacteria, it binds to transcription factors, which then switch gene expression on or off (Figure 9.17). The peptide autoinducers stimulate more complicated signaling pathways that include bacterial kinases. The changes in bacteria following exposure to autoinducers can be quite extensive. The pathogenic bacterium Pseudomonas aeruginosa has 616 different genes that respond to autoinducers.

Visual Connection

  1. Autoinducers must bind to receptors to turn on transcription of genes responsible for the production of more autoinducers.
  2. Autoinducers can only act on a different cell. It cannot act on the cell in which it is made.
  3. Autoinducers turn on genes that enable the bacteria to form a biofilm.
  4. The receptor stays in the bacterial cell, but the autoinducers diffuse out.

Some species of bacteria that use quorum sensing form biofilms, complex colonies of bacteria (often containing several species) that exchange chemical signals to coordinate the release of toxins that will attack the host. Bacterial biofilms (Figure 9.18) can sometimes be found on medical equipment when biofilms invade implants such as hip or knee replacements or heart pacemakers, they can cause life-threatening infections.

The ability of certain bacteria to form biofilms has evolved because of a selection of genes that enable cell-cell communication confers an evolutionary advantage. When bacterial colonies form biofilms, they create barriers that prevent toxins and antibacterial drugs from affecting the population living in the biofilm. As a result, these populations are more likely to survive, even in the presence of antibacterial agents. This often means that bacteria living in biofilms have higher fitness than bacteria living on their own.

Science Practice Connection for AP® Courses

Think About It

Why is signaling in multicellular organisms more complicated than signaling in single-celled organisms such as microbes?

Teacher Support

This question is an application of LO 3.36 and Science Practice 1.5 because students are describing and comparing models of signaling pathways in different types of organisms.

Everyday Connection

  1. The squid provides certain nutrients that allow the bacteria to luminesce.
  2. The squid produces the luminescent luciferase enzyme, so bacteria living outside the squid do not luminesce.
  3. The ability to luminesce does not benefit free-living bacteria, so free-living bacteria do not produce luciferase.
  4. Luciferase is toxic to free-living bacteria, so free-living bacteria do not produce this enzyme.

Research on the details of quorum sensing has led to advances in growing bacteria for industrial purposes. Recent discoveries suggest that it may be possible to exploit bacterial signaling pathways to control bacterial growth this process could replace or supplement antibiotics that are no longer effective in certain situations.

Link to Learning

Watch geneticist Bonnie Bassler discuss her discovery of quorum sensing in biofilm bacteria in squid.

  1. Bacteria interact by physical signals among a colony.
  2. Bacterium interact by chemical signals when it is alone.
  3. Bacterium interact by physical signals when it is alone.
  4. Bacteria interact by chemical signals among a colony.

Evolution Connection

The first life on our planet consisted of single-celled prokaryotic organisms that had limited interaction with each other. While some external signaling occurs between different species of single-celled organisms, the majority of signaling within bacteria and yeasts concerns only other members of the same species. The evolution of cellular communication is an absolute necessity for the development of multicellular organisms, and this innovation is thought to have required approximately 2.5 billion years to appear in early life forms.

Yeasts are single-celled eukaryotes, and therefore have a nucleus and organelles characteristic of more complex life forms. Comparisons of the genomes of yeasts, nematode worms, fruit flies, and humans illustrate the evolution of increasingly complex signaling systems that allow for the efficient inner workings that keep humans and other complex life forms functioning correctly.

Kinases are a major component of cellular communication, and studies of these enzymes illustrate the evolutionary connectivity of different species. Yeasts have 130 types of kinases. More complex organisms such as nematode worms and fruit flies have 454 and 239 kinases, respectively. Of the 130 kinase types in yeast, 97 belong to the 55 subfamilies of kinases that are found in other eukaryotic organisms. The only obvious deficiency seen in yeasts is the complete absence of tyrosine kinases. It is hypothesized that phosphorylation of tyrosine residues is needed to control the more sophisticated functions of development, differentiation, and cellular communication used in multicellular organisms.

Because yeasts contain many of the same classes of signaling proteins as humans, these organisms are ideal for studying signaling cascades. Yeasts multiply quickly and are much simpler organisms than humans or other multicellular animals. Therefore, the signaling cascades are also simpler and easier to study, although they contain similar counterparts to human signaling. 2

  1. The tyrosine kinases evolved before yeast diverged from other eukaryotes, but the other fifty-five subfamilies of kinases evolved after yeast diverged.
  2. Fifty-five subfamilies of kinases evolved before yeast diverged from other eukaryotes, but the tyrosine kinases evolved after yeast diverged.
  3. All kinases evolved in yeast, but yeast later lost the tyrosine kinases because they do not need them.
  4. The evolution of tyrosine kinases involved in cellular communication occurred about 2.5 billion years ago.

Link to Learning

Watch this collection of interview clips with biofilm researchers in “What Are Bacterial Biofilms?”

  1. Bacteria often form biofilms in recurrent infections and these may be more antibiotic resistant.
  2. Bacteria rarely form biofilms in recurrent infections, making them more resistant to antibiotics than if they were not in a biofilm.
  3. Bacteria produce biofilms, which behave like a unicellular organism.
  4. Bacteria don't produce biofilms in recurrent infections but become resistant due to repeated exposure to antibiotics.

Footnotes

    G. Manning, G.D. Plowman, T. Hunter, S. Sudarsanam, “Evolution of Protein Kinase Signaling from Yeast to Man,” Trends in Biochemical Sciences 27, no. 10 (2002): 514–520.

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    Author information

    These authors contributed equally: Tianmin Wang, Xiang Zheng.

    Affiliations

    Key Laboratory of Industrial Biocatalysis, Ministry of Education Institute of Biochemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing, China

    Tianmin Wang, Xiang Zheng, Haonan Ji, Xin-Hui Xing & Chong Zhang

    Tsinghua–Peking Center for Life Sciences, School of Medicine, Tsinghua University, Beijing, China

    School of Medicine, Tsinghua University, Beijing, China

    Center for Synthetic and Systems Biology, Tsinghua University, Beijing, China

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    Contributions

    T.W. conceived the project and contributed to all computational analysis, validation experiments, data analysis, model design and preparation of the manuscript. X.Z. conceived the project, performed FACS–seq, validation experiments and data analysis, and helped write the manuscript. H.J. helped in performing validation experiments. T.L.W. helped in analysis of the TnaC–nascent peptide structure. C.Z. conceived and supervised the project, and edited the manuscript. X.-H.X. supervised the project.

    Corresponding authors


    Signaling in Bacteria

    Bacterial signaling allows bacteria to monitor cellular conditions and communicate with each other.

    Learning Objectives

    Describe how cell signaling occurs in single-celled organisms such as bacteria

    Key Takeaways

    Key Points

    • Gene expression in bacteria is initiated when the population density of the bacteria reaches a certain level.
    • Bacterial signaling is called quorum sensing because cell density is the determining factor for signaling.
    • Quorum sensing uses autoinducers, which are secreted by bacteria to communicate with other bacteria of the same kind, as signaling molecules.
    • Autoinducers may be small, hydrophobic molecules, or they can be larger peptide-based molecules regardless, each type of molecule has a different mode of action.
    • Some bacteria form biofilms, which are complex colonies of bacteria that exchange chemical signals to coordinate the release of toxins that attack the host.

    Key Terms

    • quorum sensing: a method of communication between bacterial cells by the release and sensing of small diffusible signal molecules
    • autoinducer: any of several compounds, synthesized by bacteria, that have signalling functions in quorum sensing
    • biofilm: a thin film of mucus created by and containing a colony of bacteria and other microorganisms

    Signaling in Bacteria

    Signaling in bacteria, known as quorum sensing, enables bacteria to monitor extracellular conditions, ensure sufficient amounts of nutrients are present, and avoid hazardous situations. There are circumstances, however, when bacteria communicate with each other.

    The first evidence of bacterial communication was observed in a bacterium that has a symbiotic relationship with Hawaiian bobtail squid. When the population density of the bacteria reached a certain level, specific gene expression was initiated: the bacteria produced bioluminescent proteins that emitted light. Because the number of cells present in the environment (the cell density) is the determining factor for signaling, bacterial signaling was named quorum sensing. Interestingly, in politics and business, a quorum is the minimum number of members required to be present to vote on an issue.

    Quorum sensing uses autoinducers as signaling molecules. Autoinducers are signaling molecules secreted by bacteria to communicate with other bacteria of the same kind. The secreted autoinducers can be small, hydrophobic molecules, such as acyl-homoserine lactone (AHL), or larger peptide-based molecules. Each type of molecule has a different mode of action. When AHL enters target bacteria, it binds to transcription factors, which then switch gene expression on or off. The peptide autoinducers stimulate more complicated signaling pathways that include bacterial kinases. The changes in bacteria following exposure to autoinducers can be quite extensive. The pathogenic bacterium Pseudomonas aeruginosa has 616 different genes that respond to autoinducers.

    Autoinducers: Autoinducers are small molecules or proteins produced by bacteria that regulate gene expression.

    Some species of bacteria that use quorum sensing form biofilms, which are complex colonies of bacteria (often containing several species) that exchange chemical signals to coordinate the release of toxins that attack the host. Bacterial biofilms can sometimes be found on medical equipment. When biofilms invade implants, such as hip or knee replacements or heart pacemakers, they can cause life-threatening infections.

    Bacterial Biofilms: Cell-cell communication enables these (a) Staphylococcus aureus bacteria to work together to form a biofilm inside a hospital patient’s catheter, seen here via scanning electron microscopy. S. aureus is the main cause of hospital-acquired infections. (b) Hawaiian bobtail squid have a symbiotic relationship with the bioluminescent bacteria Vibrio fischeri. The luminescence makes it difficult to see the squid from below because it effectively eliminates its shadow. In return for camouflage, the squid provides food for the bacteria. Free-living V. fischeri do not produce luciferase, the enzyme responsible for luminescence, but V. fischeri living in a symbiotic relationship with the squid do. Quorum sensing determines whether the bacteria should produce the luciferase enzyme.