Journal
ANTIBIOTICS-BASEL
Volume 11, Issue 4, Pages -Publisher
MDPI
DOI: 10.3390/antibiotics11040462
Keywords
penicillin-binding proteins; sodium bicarbonate; beta-lactams; methicillin-resistant Staphylococcus aureus
Categories
Funding
- National Institutes of Health [1R01-AI146078]
- UCLA CTSI [KL2-TR001882-05, 1U01-AI124319-01]
- German Center for Infection Research (DZIF)
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MRSA regulates resistance to beta-lactams through preferential production of the alternative penicillin-binding protein PBP2a. A novel phenotype, NaHCO3-responsiveness, has been linked to specific PBP2a variants in MRSA strains.
Methicillin-resistant Staphylococcus aureus (MRSA) regulates resistance to beta-lactams via preferential production of an alternative penicillin-binding protein (PBP), PBP2a. PBP2a binds many beta-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 beta-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 beta-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-beta-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 beta-lactam binding, as well as ultimate PBP2a binding of beta-lactams.
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