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 NaHCO(3) (termed ‘NaHCO(3)-responsiveness’). This increased β-lactam susceptibility phenotype has been verified in both ex vivo and in vivo models. Mechanistic studies to-date have implicated NaHCO(3)-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 NaHCO(3)-responsive (MRSA 11/11) vs. non-responsive (COL) strains, in the presence vs. absence of NaHCO(3)-β-lactam co-exposures. These investigations revealed that NaHCO(3) 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 NaHCO(3)-OXA exposure in the NaHCO(3)-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 NaHCO(3)-responsiveness phenotype in MRSA.