Subinhibitory concentrations of β-lactam induce haemolytic activity in Staphylococcus aureus through the SaeRS two-component system

SaeRS is a two-component system that has been characterized as a positive regulatory system for the expression of several virulence factors, including coagulase, alpha-, beta- and gamma-haemolysins, nuclease, and fibronectin-binding proteins in Staphylococcus aureus. Previously, the SaeRS system was found to be induced at the transcriptional level by beta-lactam. Here, we found that subinhibitory concentrations of beta-lactam induce haemolytic activity in the S. aureus N315 strain but not in the saeRS null mutant KSA. Comparison of the transcriptional profile of the N315 and KSA strains by microarray analysis reveals that the SaeRS system modulates the regulation of coagulase (coa), alpha-, beta- and gamma-haemolysins (hla, hlb and hlg), nuclease (SA0746), fibrinogen-binding proteins (emp, efb, SA1000 and SA1004), fibronectin-binding protein B (fnbB), and 13 other genes. Further, the use of cefoxitin as a signal inducer reveals that the SaeRS system appears to modulate 22 additional genes as a secondary regulon, including the staphylococcal accessory regulators SarA and SarT and the Clp protease ATPase subunits ClpB and ClpL. These observations suggest that beta-lactam is able to induce the SaeRS system, which acts as a crucial signal transduction system for S. aureus pathogenicity rather than antimicrobial resistance.