Staphylococcus aureus causes inflammatory, pyogenic diseases such as endocarditis, osteomyelitis, and septic arthritis, as well as nonpyogenic, exotoxin-mediated diseases such as toxic shock syndrome, scalded skin syndrome, and food poisoning. How do bacteria that belong to the same genus and species cause such widely divergent diseases? The answer is that individual bacteria produce different virulence factors that endow them with the capability to cause different diseases.
Different virulence factors are encoded on plasmids, transposons, on the genome of lysogenic phages, and on pathogenicity islands. These transferable genetic elements are not present in all bacterial strains, accounting for the ability to cause different diseases. Table 1 describes different virulence factors for three of the most important bacterial pathogens: S. aureus, S. pyogenes, and E. coli. Figure 1 describes the importance of pathogenicity islands in determining the types of diseases caused by E. coli.
Table1. Different Strains of Bacteria Can Cause Different Diseases
Fig1. Pathogenicity islands encode virulence factors that determine the type of infection. The top of the figure depicts a nonpathogenic strain of Escherichia coli that does not contain a pathogenicity island (PAI) in the genome DNA. The black oval line within the E. coli cell is the genome DNA. PAIs can be transferred, by either conjugation or transduction, from another enteric gram-negative rod into the nonpathogenic strain of E. coli. Acquisition of a PAI that encodes virulence factors endows the nonpathogenic E. coli with the ability to cause specific dis eases. In this figure, PAI-A encodes an enterotoxin, PAI-B encodes the pili that bind to urinary tract epithelium, and PAI-C encodes the enzymes that synthesize the K-1 capsular polysaccharide. This results in three different strains of E. coli capable of causing three different infections.
