Our work focuses on the genetic identification of the bacterial factors necessary for the formation and the maintenance of mature Staphylococcus aureus and Staphylococcus epidermidis biofilms. These approaches may lead to a better understanding of the biofilm lifestyle as well as to strategies to control pathogenic biofilms. The group works with SOS response and horizontal gene transfer. 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. In S. aureus strains, several related pathogenicity islands (PIs) have been described, that belong to the growing family of these elements that are found in many strains. These PIs encode virulence factors, including toxins and genes involved in biofilm formation (persistence and resistence). Our studies have demonstrated that these islands are induced to excise and replicate following SOS induction of different temperate phages, and are then packaged into phage-like particles and transferred at high frequency. SOS induction by commonly used fluoroquinolone antibiotics, as well as beat-lactams antibiotics, also results in replication and high-frequency transfer of this element, suggesting that such antibiotics may have the unintended consequence of promoting the spread of bacterial virulence factors. (José Penadés)