Impact of bicarbonate-β-lactam exposures on methicillin-resistant staphylococcus aureus (Mrsa) gene expression in bicarbonate-β-lactam-responsive vs. non-responsive strains

Methicillin-resistant Staphylococcus aureus (MRSA) infections represent a difficult clinical treatment issue. Recently, a novel phenotype was discovered amongst selected MRSA which exhibited enhanced β-lactam susceptibility in vitro in the presence of NaHCO₃ (termed ‘NaHCO₃-responsiveness’). This increased β-lactam susceptibility phenotype has been verified in both ex vivo and in vivo models. Mechanistic studies to-date have implicated NaHCO₃-mediated repression of genes involved in the production, as well as maturation, of the alternative penicillin-binding protein (PBP) 2a, a necessary component of MRSA β-lactam resistance. Herein, we utilized RNA-sequencing (RNA-seq) to identify genes that were differentially expressed in NaHCO₃-responsive (MRSA 11/11) vs. non-responsive (COL) strains, in the presence vs. absence of NaHCO₃-β-lactam co-exposures. These investigations revealed that NaHCO₃ selectively repressed the expression of a cadre of genes in strain 11/11 known to be a part of the sigB-sarA-agr regulon, as well as a number of genes involved in the anchoring of cell wall proteins in MRSA. Moreover, several genes related to autolysis, cell division, and cell wall biosynthesis/remodeling, were also selectively impacted by NaHCO₃-OXA exposure in the NaHCO₃-responsive strain MRSA 11/11. These outcomes provide an important framework for further studies to mechanistically verify the functional relevance of these genetic perturbations to the NaHCO₃-responsiveness phenotype in MRSA.