SaPIs are a family of mobile genetic elements resident in the genome

Genome

In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

 of some strains of Staphylococcus aureus

Staphylococcus aureus

Staphylococcus aureus is a facultative anaerobic Gram-positive coccal bacterium. It is frequently found as part of the normal skin flora on the skin and nasal passages. It is estimated that 20% of the human population are long-term carriers of S. aureus. S. aureus is the most common species of...

. 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

Capsid

A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...

 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

Toxic shock syndrome

Toxic shock syndrome is a potentially fatal illness caused by a bacterial toxin. Different bacterial toxins may cause toxic shock syndrome, depending on the situation. The causative bacteria include Staphylococcus aureus and Streptococcus pyogenes...

 toxin. Other SaPI subtypes have been found to provide host Staphylococcus strains with the ability to coagulate animal host blood plasma

Blood plasma

Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...

 by coding for different alleles of a von Willebrand factor

Von Willebrand factor

von Willebrand factor is a blood glycoprotein involved in hemostasis. It is deficient or defective in von Willebrand disease and is involved in a large number of other diseases, including thrombotic thrombocytopenic purpura, Heyde's syndrome, and possibly hemolytic-uremic syndrome...

-binding protein. This virulence factor

Virulence factor

Virulence factors are molecules expressed and secreted by pathogens that enable them to achieve the following:* colonization of a niche in the host...

 may help S. aureus adapt to distinct animal hosts such as horses or ruminants.