SaPIs (Staphylococcus aureus or superantigen pathogenicity islands) are a family of mobile genetic elements resident in the genome of some strains of Staphylococcus aureus. Much like bacteriophages, SaPIs can be transferred to uninfected cells and integrate into the host chromosome. Unlike the bacterial viruses, however, integrated SaPIs are mobilized by host infection with "helper" bacteriophages (specific SaPIs may require specific helper bacteriophages for mobilization, though Staphylococcus phage 80alpha appears to mobilize all known SaPIs).
One particular SaPI, SaPI1, is encapsidated in and exits the host cell in particles assembled from proteins encoded by its helper phage.
Role in pathogenicity
SaPIs were found to carry the gene encoding toxic shock syndrome toxin. Other SaPI subtypes have been found to provide host Staphylococcus strains with the ability to coagulate animal host blood plasma by coding for different alleles of a von Willebrand factor-binding protein. This virulence factor may help S. aureus adapt to distinct animal hosts such as horses or ruminants.