Antibiotic-induced SOS response promotes horizontal dissemination of pathogenicity island-encoded virulence factors in staphylococci.

Although mobile genetic elements have a crucial role in spreading pathogenicity-determining genes among bacterial populations, environmental and genetic factors involved in the horizontal transfer of these genes are largely unknown. Here we show that SaPIbov1, a Staphylococcus aureus pathogenicity island that belongs to the growing family of these elements that are found in many strains, is induced to excise and replicate after SOS induction of at least three different temperate phages, 80alpha, phi11 and phi147, and is then packaged into phage-like particles and transferred at high frequency. SOS induction by commonly used fluoroquinolone antibiotics, such as ciprofloxacin, also results in replication and high-frequency transfer of this element, as well as of SaPI1, the prototypical island of S. aureus, suggesting that such antibiotics may have the unintended consequence of promoting the spread of bacterial virulence factors. Although the strains containing these prophages do not normally contain SaPIs, we have found that RF122-1, the original SaPIbov1-containing clinical isolate, contains a putative second pathogenicity island that is replicated after SOS induction, by antibiotic treatment, of the prophage(s) present in the strain. Although SaPIbov1 is not induced to replicate after SOS induction in this strain, it is transferred by the antibiotic-activated phages. We conclude that SOS induction by therapeutic agents can promote the spread of staphylococcal virulence genes.