Tn551-mediated insertional inactivation of the fmtB gene encoding a cell wall-associated protein abolishes methicillin resistance in Staphylococcus aureus

A Tn551 insert in a gene termed fmtB was shown to reduce oxacillin as well as Triton X-100 resistance in highly methicillin-resistant Staphylococcus a ureus (MRSA) COL. Backcrosses of fmfS::Tn551 into S. aureus COL and into tw o genetically distinct MRSA strains, KSA8 and NCTC10443, confirmed the link age of fmtB::Tn551 with loss of oxacillin resistance. The fmtB gene codes f or a protein of a deduced molecular mass of 263 kDa that contains 17 tandem repeats of 75 amino acids and a C-terminal LPXTG cell wall-sorting motif. Immunoblots with anti-FmtB antibodies confirmed its localization in the cel l wall fraction. The fmtB gene was mapped downstream of the phosphoglucosam ine mutase operon glmM which catalyses formation of glucosamine-1-phosphate . Oxacillin resistance was not restored in fmtB mutants by trans-complement ation with fmtB. However, although GlmM production was not affected by fmtB inactivation, oxacillin resistance was increased in fmtB mutants by introd ucing a plasmid-borne glmM gene, presumably by GlmM overexpression. Interes tingly, a similar phenotypic complementation was obtained in fmtB mutants b y including substrate level concentrations of N-acetylglucosamine or glucos amine in the growth medium. Inactivation of the fmtB gene seems therefore t o have an indirect effect on methicillin resistance which can be relieved b y increasing the production of the cell wall precursor glucosamine-1-phosph ate.