Synergistic drug strategy against mastitis-associated MRSA strains.

AIMS: To investigate the resistance profiles of mastitis-associated methicillin-resistant Staphylococcus aureus (MRSA) and to evaluate the synergistic antibacterial effects of antibiotic combinations. METHODS: Twenty clinical isolates obtained from patients with lactational mastitis underwent antimicrobial susceptibility testing. Six multidrug-resistant (MDR) isolates were selected to assess pairwise combinations of 10 antibiotics by checkerboard assays and to confirm bactericidal synergy via time-kill testing. In vivo efficacy of the combinations was assessed in a Galleria mellonella infection model. RESULTS: Over 90% of the isolates were MRSA, with high resistance to oxacillin, clindamycin, and gentamicin; several also displayed elevated minimum inhibitory concentrations (MICs) of vancomycin. Vancomycin plus oxacillin (VAN+OXA) or rifampicin (VAN+RIF) showed consistent in vitro synergy. VAN+OXA produced the greatest killing, reducing bacterial counts by > 3 log₁₀ CFU/mL at 24 h. In vivo, both combinations significantly improved larval survival versus monotherapy across multiple strains. CONCLUSIONS: VAN combined with OXA or RIF exhibited robust synergy against mastitis-linked MDR-MRSA in vitro and in vivo. These findings highlight that the combination of VAN with OXA or RIF could be a promising strategy for treating mastitis caused by drug-resistant MRSA.