The ability to produce toxins and other virulence factors is frequently transferred genetically from one strain of bacteria to another by transduction, transformation, or conjugation. A well-known example relates to the prominent capsule of the pneu-mococcus, Streptococcus pneumoniae, which protects it from phagocytosis and destruction. The ability to produce the capsule can be transferred by transformation (see Perspective 8.1).
Genes for virulence factors carried on plasmids, as well as chromosomal genes, can be transferred by conjugation. For example, a gene of E. coli that codes for a particular adherence factor, an adhesin, is carried on plasmids and can be transferred from one strain to another. While usually a benign member of the normal flora of the large intestine, E. coli can acquire a plas-mid containing genes that code for the adherence factor and for the production of toxins that affect the gastrointestinal tract. Once the cells have attached, they produce the toxins, resulting in severe diarrhea, sometimes called "traveler's diarrhea." Many other plasmid-transferable virulence factors account for the pathogenicity of various strains of E. coli. ■ adhesin, p. 65
Genes for toxins and other virulence factors are found in many different locations on the bacterial chromosome. In Gram-negative bacteria, however, many of the genes required for pathogenicity are located close together on the chromosome in large regions called pathogenicity islands. For example, some strains of E. coli are known to cause urinary tract infections, whereas other strains cause gastrointestinal diseases. The pathogenicity island in the strains causing urinary tract infections codes for a toxin that can destroy cells in the urinary tract, whereas the pathogenicity island in the strains that cause gastrointestinal disease codes for proteins that lead to intestinal cell damage. Usually, a pathogenicity island contains genes for a number of different virulence factors, and a single bacterial strain may contain several different pathogenicity islands. The entire set of genes in a pathogenicity island can be transferred as a unit between bacteria usually by conjugation. It is clear that the virulence of microbial strains can rapidly change as a result of gaining one or more genes coding for virulence factors.
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