Expression of

Staphylococcus aureus is a leading cause of bacteremia, and infections caused by methicillin-resistant S. aureus (MRSA) strains are especially challenging to treat. MRSA strains are resistant to front-line beta-lactams due to PBP2a, a low-affinity penicillin-binding protein encoded by mecA. Daptomycin is used to treat MRSA infections but is not always effective. While daptomycin resistance has been well studied, the ability of S. aureus to tolerate daptomycin, a feature likely to slow clearance of the bacteria from the bloodstream, is less well understood. Here, using a panel of clinical bacteremia isolates, we show that MRSA strains are more tolerant of daptomycin than methicillin-susceptible S. aureus (MSSA) strains. This difference in tolerance is due to mecA and is independent of any changes in surface properties previously associated with altered daptomycin susceptibility. Instead, using a mecA transposon mutant, we found that a lack of this gene led to higher activity of the Agr quorum sensing system, resulting in an increased release of the phenol-soluble modulin toxins. Increased levels of these surfactant-like toxins prevented daptomycin from being inactivated by lipids released by the bacteria, leading to reduced antibiotic tolerance. Additionally, the clinical MRSA strains tested produced lower levels of toxins than the MSSA strains and inactivated daptomycin to a greater extent, explaining their enhanced tolerance. The expression of mecA in clinical MSSA strains reduced toxin production, increasing daptomycin inactivation and thereby enhancing tolerance. Together, these results demonstrate that mecA not only affects beta-lactam susceptibility but also compromises the efficacy of the last resort antibiotic daptomycin.IMPORTANCEThe incidence of Staphylococcus aureus bacteremia is on a steady incline in many parts of the world. Given the associated mortality rates have changed little in the last 10 years, this is a major health concern. One contributing problem is that antibiotics effective against the bacteria in vitro are failing to cure many patients, e.g., the use of daptomycin to treat methicillin-resistant Staphylococcus aureus (MRSA) infections. Here, we present a mechanistic study that may explain why this occurs. The expression of mecA, which confers the methicillin resistance of MRSA, reduces Agr activity, and this decreases the release of phenol-soluble modulins from the bacteria. Without these, the phospholipids that can block daptomycin activity are free to do so, rendering MRSA less sensitive to both antibiotics. This study, bridging clinical and molecular biology, provides an explanation for a significant clinical problem and may inform how antibiotic combinations should be managed in future trials.