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15.12B: Scarlet Fever - Biology


Learning Objectives

  • Describe the bacterium Streptococcus pyogenes that causes scarlet fever

A Bacteriophage Hitchhiker

Scarlet fever is an infectious disease which most commonly affects 4-8 year-old children. Symptoms include sore throat, fever, and a characteristic red rash. It is usually spread by inhalation. There is no vaccine, but the disease is effectively treated with antibiotics. Scarlet fever is caused by an erythrogenic toxin, a substance produced by the bacterium Streptococcus pyogenes (group A strep. ) when it is infected by a certain bacteriophage.

Scarlet fever is caused by secretion of pyrogenic (fever inducing) exotoxins by the infected Streptococcus. Exotoxin A (speA) is probably the best studied of these toxins. It is carried by the bacteriophage T12, which integrates into the Streptococcal genome, from where the toxin is transcribed.

The phage itself integrates into a serine tRNA gene on the chromosome. The T12 virus itself has not been placed into a taxon by the International Committee on Taxonomy of Viruses. It has a double stranded DNA genome; on morphological grounds it appears to be a member of the Siphoviridae. The speA gene was cloned and sequenced in 1986. It is 753 base pairs in length and encodes a 29.244 kiloDalton (kDa) protein. The protein contains a putative 30 amino acid signal peptide. Removal of the signal sequence gives a predicted molecular weight of 25.787 (kDa) for the secreted protein. Both a promoter and a ribosome-binding site (Shine-Dalgarno sequence) are present upstream of the gene. A transcriptional terminator is located 69 bases downstream from the translational termination codon. The carboxy terminal portion of the protein exhibits extensive homology with the carboxy terminus of Staphylococcus aureus enterotoxins B and C1. Streptococcal phages other than T12 may also carry the speA gene.

Key Points

  • Scarlet fever usually affects children. Historically, it had devastating effects.
  • While antibiotics are effective against scarlet fever, the illness is actually caused by a bacteriophage infecting Streptococcus that has infected a person.
  • The bacteriophage T12 inserts into the genome of Streptococcus. This leads to the expression of an exotoxin, which causes scarlet fever.

Key Terms

  • scarlet fever: a streptococcal infection, mainly occurring among children, and characterized by a red skin rash, sore throat and fever
  • Shine-Dalgarno sequence: A ribosomal binding site in the mRNA of prokaryotes.
  • exotoxin: Any toxin secreted by a microorganism into the surrounding environment.
  • enterotoxin: Any of several toxins produced by intestinal bacteria

The bacteriologist is staining the slides of a client diagnosed with scarlet fever and rheumatic fever. The slide identifies S. pyogenes and stains purple by crystal violet dye. The result would indicate:

affects the water bodies: excessive use of fertilizers causes eutrophication. the fertilizers which are used on plants affect the quality of land on which it is used, further causing erosion of soil. the fertilizers used on plants contain harmful chemical fumes which affect the environment.

Gram stain color the bacteria either red or violet depending on whether they are Gram positive or Gram negative. Gram positive bacteria have thick cell wall containing peptidoglycan compared to Gram negative bacteria which have thin cell wall. Due to this Gram positive bacteria stains purple by crystal violet. So, slide containing S. pyogenes with purple violet indicates that it is a Gram positive bacteria.