Ability of Bicarbonate Supplementation to Sensitize Selected Methicillin-Resistant Staphylococcus aureus (MRSA) Strains to β-Lactam Antibiotics in an Ex Vivo Simulated Endocardial Vegetation Model

Abstract

Supplementation of standard growth media (cation-adjusted Mueller-Hinton Broth [CAMHB]) with bicarbonate (NaHCO3) significantly increases {beta}-lactam susceptibility of selected MRSA strains (\"NaHCO3-responsive\"). This \"sensitization\" phenomenon translated to enhanced {beta}-lactam efficacy in a rabbit model of endocarditis. The present study evaluated NaHCO3-mediated {beta}-lactam MRSA sensitization using an ex vivo pharmacodynamic model, featuring simulated endocardial vegetations (SEVs), to more closely mimic the host microenvironment. Four previously described MRSA strains were used: two each exhibiting in vitro \"NaHCO3-responsive\" or \"NaHCO3-nonresponsive\" phenotypes. Cefazolin (CFZ) and oxacillin (OXA) were evaluated in CAMHB{+/-}NaHCO3. Intra-SEV MRSA killing was determined over 72 hr exposure. In both NaHCO3-responsive strains, supplementation with 25 mM or 44 mM NaHCO3 significantly reduced {beta}-lactam MICs to below the OXA susceptibility breakpoint ([≤] 4 mg/L) resulting in bactericidal activity ([≥] 3 log kill) in the model for both OXA and CFZ. In contrast, neither in vitro-defined NaHCO3-nonresponsive MRSA strains showed significant sensitization in the SEV model to either {beta}-lactam. At both NaHCO3 concentrations, the fractional time-above-MIC was >50% for both CFZ and OXA in the NaHCO3-responsive MRSA. Also, in RPMI+10% LB media (proposed as a more host-mimicking microenvironment and containing 25 mM NaHCO3), both CFZ and OXA exhibited enhanced bactericidal activity against each NaHCO3-responsive strain in the SEV model. Neither CFZ nor OXA exposures selected for high-level {beta}-lactam-resistant mutants within SEVs. Thus, in this ex vivo model of endocarditis, in the presence of NaHCO3 supplementation, both CFZ and OXA are highly active against MRSA strains that demonstrate similar enhanced susceptibility in NaHCO3-supplemented media in vitro.