Impacts of NaHCO₃ on β-Lactam Binding to PBP2a Protein Variants Associated with the NaHCO₃-Responsive versus...

Methicillin-resistant Staphylococcus aureus (MRSA) regulates resistance to β-lactams via preferential production of an alternative penicillin-binding protein (PBP), PBP2a. PBP2a binds many β-lactam antibiotics with less affinity than PBPs which are predominant in methicillin-susceptible (MSSA) strains. A novel, rather frequent in vitro phenotype was recently identified among clinical MRSA bloodstream isolates, termed "NaHCO₃-responsiveness". This phenotype features β-lactam susceptibility of certain MRSA strains only in the presence of NaHCO₃. Two distinct PBP2a variants, 246G and 246E, have been linked to the NaHCO₃-responsive and NaHCO₃-non-responsive MRSA phenotypes, respectively. To determine the mechanistic impact of PBP2a variants on β-lactam susceptibility, binding profiles of a fluorescent penicillin probe (Bocillin-FL) to each purified PBP2a variant were assessed and compared to whole-cell binding profiles characterized by flow cytometry in the presence vs. absence of NaHCO₃. These investigations revealed that NaHCO₃ differentially influenced the binding of the fluorescent penicillin, Bocillin-FL, to the PBP2a variants, with binding intensity and rate of binding significantly enhanced in the 246G compared to the 246E variant. Of note, the NaHCO₃-β-lactam (oxacillin)-responsive JE2 strain, which natively harbors the 246G variant, had enhanced Bocillin-FL whole-cell binding following exposure to NaHCO₃. This NaHCO₃-mediated increase in whole-cell Bocillin-FL binding was not observed in the NaHCO₃-non-responsive parental strain, COL, which contains the 246E PBP2a variant. Surprisingly, genetic swaps of the mecA coding sites between JE2 and COL did not alter the NaHCO₃-enhanced binding seen in JE2 vs. COL. These data suggest that the non-coding regions of mecA may be involved in NaHCO₃-responsiveness. This investigation also provides strong evidence that the NaHCO₃-responsive phenotype in MRSA may involve NaHCO₃-mediated increases in both initial cell surface β-lactam binding, as well as ultimate PBP2a binding of β-lactams.