Scope and Predictive Genetic/Phenotypic Signatures of Bicarbonate (NaHCO 3 ) Responsiveness and β-Lactam Sensitization in Methicillin-Resistant Staphylococcus aureus

Addition of sodium bicarbonate (NaHCO ) to standard antimicrobial susceptibility testing medium reveals certain methicillin-resistant (MRSA) strains to be highly susceptible to β-lactams. We investigated the prevalence of this phenotype (NaHCO responsiveness) to two β-lactams among 58 clinical MRSA bloodstream isolates. Of note, ∼75% and ∼36% of isolates displayed the NaHCO responsiveness phenotype to cefazolin (CFZ) and oxacillin (OXA), respectively. Neither intrinsic β-lactam MICs in standard Mueller-Hinton broth (MHB) nor population analysis profiles were predictive of this phenotype. Several genotypic markers (clonal complex 8 [CC8]; I and t008) were associated with NaHCO responsiveness for OXA.