The study shows that adding sodium bicarbonate (baking‑soda) to lab tests can make some MRSA bacteria become sensitive again to common antibiotics like oxacillin and cefazolin, especially when the natural immune peptide LL‑37 is present. This suggests a possible way to treat certain MRSA infections with cheaper, less toxic drugs, but it’s still early‑stage research and not a ready‑to‑use protocol for people.

Endovascular infections caused by methicillin-resistant <i>Staphylococcus aureus</i> (MRSA) are a major health care concern, especially infective endocarditis (IE). Standard antimicrobial susceptibility testing (AST) defines most MRSA strains as "resistant" to &#x3b2;-lactams, often leading to the use of costly and/or toxic treatment regimens. In this investigation, five prototype MRSA strains, representing the range of genotypes in current clinical circulation, were studied. We identified two distinct MRSA phenotypes upon AST using standard media, with or without sodium bicarbonate (NaHCO₃) supplementation: one highly susceptible to the antistaphylococcal &#x3b2;-lactams oxacillin and cefazolin (NaHCO₃ responsive) and one resistant to such agents (NaHCO₃ nonresponsive). These phenotypes accurately predicted clearance profiles of MRSA from target tissues in experimental MRSA IE treated with each &#x3b2;-lactam. Mechanistically, NaHCO₃ reduced the expression of two key genes involved in the MRSA phenotype, <i>mecA</i> and <i>sarA</i>, leading to decreased production of penicillin-binding protein 2a (that mediates methicillin resistance), in NaHCO₃-responsive (but not in NaHCO₃-nonresponsive) strains. Moreover, both cefazolin and oxacillin synergistically killed NaHCO₃-responsive strains in the presence of the host defense antimicrobial peptide (LL-37) in NaHCO₃-supplemented media. These findings suggest that AST of MRSA strains in NaHCO₃-containing media may potentially identify infections caused by NaHCO₃-responsive strains that are appropriate for &#x3b2;-lactam therapy.