Troublesome strains of Staphylococcus aureus are often troublesome because they carry genes for superantigens and multiple antibiotic resistance. But don’t blame the bacteria. These genes are hitchhikers that arrived by horizontal gene transfer, embedded within mobile pathogenicity islands known as SaPIs. SaPIs are common; all S. aureus strains investigated so far carry at least one. They have also been found in other staph species and a few other Gram-positive genera. They have garnered much research attention because they rapidly move those clinically-significant genes from host to host. (They received some attention on this blog, too, several years ago.) The typical SaPI is composed of 15-17,000 base pairs of DNA that encode 18-22 proteins. Of those proteins, nine at most are required for the SaPI life cycle. This leaves plenty available to use to benefit or manipulate their host bacterium.
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