The NaHCO3-Responsive Phenotype in Methicillin-Resistant Staphylococcus aureus (MRSA) Is Influenced by mecA Genotype

Methicillin-resistant Staphylococcus aureus (MRSA) strains are a leading cause of many invasive clinical syndromes, and pose treatment difficulties due to their in vitro resistance to most beta-lactams on standard laboratory testing. A novel phenotype frequently identified in MRSA strains, termed 'NaHCO3-responsiveness', is a property whereby strains are susceptible in vitro to many beta-lactams in the presence of NaHCO3. Specific mecA genotypes, repression of mecA/PBP2a expression and perturbed maturation of PBP2a by NaHCO3 have all been associated with this phenotype. The aim of this study was to define the relationship between specific mecA genotypes and PBP2a substitutions, on the one hand, with NaHCO3-responsiveness in vitro. Mutations were made in the mecA ribosomal binding site (RBS -7) and at amino acid position 246 of its coding region in parental strains MW2 (NaHCO3-responsive) and C36 (NaHCO3- nonresponsive) to generate 'swap' variants, each harboring the other's mecA-RBS/coding region genotypes. Successful swaps were confirmed by both sequencing, as well as predicted swap of in vitro penicillin-clavulanate susceptibility phenotypes. MW2 swap variants harboring the nonresponsive mecA genotypes became NaHCO3 -nonresponsive (resistant to the ,6-lactam, oxacillin [OXA)), in the presence of NaHCO3. Moreover, these swap variants had lost NaHCO3-mediated repression of mecA/PBP2a expression. In contrast, C36 swap variants harboring the NaHCO3-responsive mecA genotypes remained NaHCO3-nonresponsive phenotypically, and still exhibited nonrepressible mecA/PBP2a expression. These data demonstrate that in addition to the mecA genotype, NaHCO3-responsiveness may also depend on strain-specific genetic backgrounds.