Factor Essential for Expression of Methicillin Resistance, Affects Glycine Content of Peptidoglycan in Methicillin-Resistant and Methicillin-Susceptible Staphylococcus aureus Strains

femA is a chromosomally encoded factor, occurring naturally in Staphylococcus aureus, which is essential for the expression of high-level methicillin resistance in this organism. The production of a low-affinity penicillin-binding protein, PBP2a or PBP2', which is intimately involved with methicillin resistance in S. aureus, is not influenced byfenmA. To elucidate a possible physiological function of the 48-kDa protein encoded byfemA, several related methicillin-resistant, methicillin-susceptible, and TnS51 insertionally inactivatedfemA mutants were analyzed for possible changes in cell wall structure and metabolism. Independent of the presence of mec, the methicillin resistance determinant, all femA mutants had a reduced peptidoglycan (PG) glycine content (up to 60% in the molar ratio of glycine/glutamic acid) compared to that of related fenmA parent strains. Additional effects offemA inactivation and the subsequent decrease in PG-associated glycine were (i) reduced digestion of PG by recombinant lysostaphin, (ii) unaltered digestion of PG by Chalaropsis B-muramidase, (iii) reduced cell wall turnover, (iv) reduced whole-cell autolysis, and (v) increased sensitivity towards ,-lactam antibiotics. Also, the PG-associated glycine content of a femA::Tn5Sl methicillin-susceptible strain