Combination antibiotic exposure selectively alters the development of vancomycin intermediate resistance in staphylococcus aureus

Invasive methicillin-resistant Staphylococcus aureus (MRSA) treated with vancomycin (VAN) is associated with reduced VAN susceptibility and treatment failure. VAN combination therapy is one strategy to improve response, but comprehensive assessments of combinations to prevent resistance are limited. This study identifies optimal combinations to prevent the emergence of VAN-intermediate Staphylococcus aureus (VISA). Two standard MRSA and two heterogeneous VISA (hVISA) strains were exposed for 28 days in vitro to VAN alone, VAN with cefazolin (CFZ), fosfomycin (FOF), gentamicin (GEN), meropenem (MEM), rifampin (RIF), piperacillin-tazobactam (TZP) or trimethoprim-sulfamethoxazole (SXT). In addition to VAN susceptibility testing, cell wall thickness (CWT), carotenoid content, and membrane fluidity were determined for Mu3. VAN plus any β-lactam limited the VAN MIC increase to 1-4 mg/L throughout the 28-day exposure, with CFZ and TZP as the most effective agents (VAN MIC=1-2 mg/L). Similar MIC trends occurred with the lipo-/glyco-peptide agents daptomycin and telavancin, where β-lactam combinations with VAN prevented MIC increases to these agents as well. Combinations with non β-lactams were ineffective in preventing VAN MIC increases with VAN MICs of 4-16 mg/L emerging during weeks 2-4 of treatment. VAN plus β-lactam decreased CWT significantly whereas VAN plus other antibiotics significantly increased the CWT. No correlation was observed between carotenoid content or membrane fluidity and antibiotic exposure. Only the combination exposures of VAN plus β-lactam suppress the development of VISA. Rational selection of VAN plus β-lactam should be further explored as a long-term combination treatment of MRSA infections due to their ability to suppress VAN resistance.