The study found that giving certain beta‑lactam antibiotics (especially cloxacillin) to MRSA bacteria that were resistant to the drug daptomycin can make those bacteria sensitive to daptomycin again. This happens because the bacteria pick up extra changes in a gene called mprF, which also makes them easier for the natural immune peptide LL‑37 to kill.

Daptomycin-nonsusceptible (DAP-NS) <i>Staphylococcus aureus</i> often exhibits gain-in-function mutations in the <i>mprF</i> gene (involved in positive surface charge maintenance). Standard &#x3b2;-lactams, although relatively inactive against methicillin-resistant <i>S. aureus</i> (MRSA), may prevent the emergence of <i>mprF</i> mutations and DAP-NS. We determined if &#x3b2;-lactams might also impact DAP-NS isolates already possessing an <i>mprF</i> mutation to revert them to DAP-susceptible (DAP-S) phenotypes and, if so, whether this is associated with specific penicillin-binding protein (PBP) targeting. This study included 25 DAP-S/DAP-NS isogenic, clinically derived MRSA bloodstream isolates. MICs were performed for DAP, nafcillin (NAF; PBP-promiscuous), cloxacillin (LOX; PBP-1), ceftriaxone (CRO; PBP-2), and cefoxitin (FOX; PBP-4). Three DAP-NS isolates were selected for a 28-day serial passage in subinhibitory &#x3b2;-lactams. DAP MICs and time-kill assays, host defense peptide (LL-37) susceptibilities, and whole-genome sequencing were performed to associate genetic changes with key phenotypic profiles. Pronounced decreases in baseline MICs were observed for NAF and LOX (but not for CRO or FOX) among DAP-NS versus DAP-S isolates ("seesaw" effect). Prolonged (28-d) &#x3b2;-lactam passage of three DAP-NS isolates significantly reduced DAP MICs. LOX was most impactful (&#x223c;16-fold decrease in DAP MIC; 2 to 0.125&#x2009;mg/liter). In these DAP-NS isolates with preexisting <i>mprF</i> polymorphisms, accumulation of additional <i>mprF</i> mutations occurred with prolonged LOX exposures. This was associated with enhanced LL-37 killing activity and reduced surface charge (both <i>mprF</i>-dependent phenotypes). &#x3b2;-lactams that either promiscuously or specifically target PBP-1 have significant DAP "resensitizing" effects against DAP-NS <i>S. aureus</i> strains. This may relate to the acquisition of multiple <i>mprF</i> single nucleotide polymorphism (SNPs), which, in turn, affect cell envelope function and metabolism.