The NaHCO 3 -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 β-lactams on standard laboratory testing. A novel phenotype frequently identified in MRSA strains, termed 'NaHCO 3 -responsiveness', is a property whereby strains are susceptible in vitro to many β-lactams in the presence of NaHCO 3 . Specific mecA genotypes, repression of mecA /PBP2a expression and perturbed maturation of PBP2a by NaHCO 3 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 NaHCO 3 -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 (NaHCO 3 -responsive) and C36 (NaHCO 3 - 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 NaHCO 3 -nonresponsive (resistant to the β-lactam, oxacillin [OXA]), in the presence of NaHCO 3 . Moreover, these swap variants had lost NaHCO 3 -mediated repression of mecA /PBP2a expression. In contrast, C36 swap variants harboring the NaHCO 3 -responsive mecA genotypes remained NaHCO 3 -nonresponsive phenotypically, and still exhibited nonrepressible mecA /PBP2a expression. These data demonstrate that in addition to the mecA genotype, NaHCO 3 -responsiveness may also depend on strain-specific genetic backgrounds.