The phage-related chromosomal islands of Gram-positive bacteria

Abstract

Staphylococcus aureus pathogenicity islands (SaPIs) are highly mobile phage-related mobile genetic elements in staphylococci. Elements similar to SaPIs have been detected in related Gram-positive bacteria, prompting a name change to phage-related chromosomal islands (PRCIs). PRCIs are normally quiescent until they are induced by a helper phage that inactivates the repressor Stl. The mechanism of induction seems to vary between Stl molecules and requires helper phages specific for the SaPI. SaPIs are packaged by capsid proteins encoded by the helper phage, but helper phage DNA is excluded. Therefore, SaPIs can hijack the helper phage to mediate their own spread. In some cases autonomous replication of the SaPIs takes place. There are several proposed models for replication mechanisms in these SaPIs. Recently, high-frequency transfer of SaPIs to Listeria monocytogenes was demonstrated, suggesting that silent phage-mediated intergeneric transfer may be an important but, to date, unrecognized mode of horizontal gene transfer among bacteria.