Mutation of lipoprotein processing pathway gene lspA or inhibition of LspA activity by globomycin increases MRSA resistance to -lactam antibiotics

The Staphylococcus aureus cell envelope comprises numerous components, including peptidoglycan (PG), wall teichoic acids (WTA), lipoteichoic acids (LTA), targeted by antimicrobial drugs. MRSA resistance to methicillin is mediated by the mecA-encoded {beta}-lactam-resistant transpeptidase, penicillin binding protein 2a (PBP2a). However, PBP2a-dependent {beta}-lactam resistance is also modulated by the activity of pathways involved in the regulation or biosynthesis of PG, WTA or LTA. Here, we report that mutation of the lipoprotein signal peptidase II gene, lspA, from the lipoprotein processing pathway, significantly increased {beta}-lactam resistance in MRSA. Mutation of lgt, which encodes diacylglycerol transferase (Lgt) responsible for synthesis of the LspA substrate did not impact {beta}-lactam susceptibility. Consistent with previous reports, lgt and lspA mutations impaired growth in chemically defined media, but not in complex broth. MRSA exposure to the LspA inhibitor globomycin also increased {beta}-lactam resistance. Mutation of lgt in an lspA background restored {beta}-lactam resistance to wild type. The lspA mutation had no effect on PBP2a expression, PG composition or autolytic activity indicating a potential role for WTA or LTA. The lspA and lgt mutants exhibited marginally increased resistance to the D-alanine pathway inhibitor D-cycloserine. In addition, mutation of lgt and multicopy lspA expression, but not mutation of lspA, significantly increased susceptibility to the lipoteichoic acid synthase inhibitor Congo red revealing complex interplay between lipoprotein processing mutations and the expression/stability of cell surface glycopolymers. These findings indicate that accumulation of the LspA substrate, diacylglyceryl lipoprotein, increases MRSA resistance to {beta}-lactam antibiotics through impacts on cell envelope components other than PG.