Role of the NaHCO₃ Transporter MpsABC in the NaHCO₃-β-Lactam-Responsive Phenotype in Methicillin-Resistant...

Methicillin-resistant Staphylococcus aureus (MRSA) infections are an increasing concern due to their intrinsic resistance to most standard-of-care β-lactam antibiotics. Recent studies of clinical isolates have documented a novel phenotype, termed NaHCO₃ responsiveness, in which a substantial proportion of MRSA strains exhibit enhanced susceptibility to β-lactams such as cefazolin and oxacillin in the presence of NaHCO₃. A bicarbonate transporter, MpsAB (membrane potential-generating system), was recently found in S. aureus, where it plays a role in concentrating NaHCO₃ for anaplerotic pathways. Here, we investigated the role of MpsAB in mediating the NaHCO₃ responsiveness phenotype. Radiolabeled NaH¹⁴CO₃ uptake profiling revealed significantly higher accumulation in NaHCO₃-responsive vs nonresponsive MRSA strains when grown in ambient air. In contrast, under 5% CO₂ conditions, NaHCO₃-responsive (but not nonresponsive) strains exhibited repressed uptake. Oxacillin MICs were measured in four prototype strains and their mpsABC deletion mutants in the presence of NaHCO₃ supplementation under 5% CO₂ conditions. NaHCO₃-mediated reductions in oxacillin MICs were observed in the responsive parental strains but not in mpsABC deletion mutants. No significant impact on oxacillin MICs was observed in the nonresponsive strains under the same conditions. Transcriptional and translational studies were carried out using both quantitative reverse transcription-PCR (qRT-PCR) and mpsA-green fluorescent protein (GFP) fusion constructs; these investigations showed that mpsA expression and translation were significantly upregulated during mid-exponential-phase growth in oxacillin-NaHCO₃-supplemented medium in responsive versus nonresponsive strains. Taken together, these data show that the NaHCO₃ transporter MpsABC is a key contributor to the NaHCO₃-β-lactam responsiveness phenotype in MRSA. IMPORTANCE MRSA infections are increasingly difficult to treat, due in part to their resistance to most β-lactam antibiotics. A novel and relatively common phenotype, termed NaHCO₃ responsiveness, has been identified in which MRSA strains show increased susceptibility in vitro and in vivo to β-lactams in the presence of NaHCO₃. A recently described S. aureus NaHCO₃ transporter, MpsAB, is involved in intracellular NaHCO₃ concentration for anaplerotic pathways. We investigated the role of MpsAB in mediating the NaHCO₃ responsiveness phenotype in four prototype MRSA strains (two responsive and two nonresponsive). We demonstrated that MpsABC is an important contributor to the NaHCO₃-β-lactam responsiveness phenotype. Our study adds to the growing body of well-defined characteristics of this novel phenotype, which could potentially translate to alternative targets for MRSA treatment using β-lactams.