Characterization of Peptidoglycan in Fem-deletion Mutants of Methicillin-resistant Staphylococcus aureus by Solid-State NMR

Compositional analysis of the peptidoglycan (PG) of a wild-type methicillin-resistant Staphylococcus aureus and its fem-deletion mutants has been performed on whole cells and cell walls using stable-isotope labeling and rotational-echo double-resonance NMR. The labels included [1-13C,15N]glycine and l-[e-15N]lysine (for a direct measure of the number of glycyl residues in the bridging segment), [1-13C]glycine and l-[e-15N]lysine (concentration of bridge links), and d-[1-13C]alanine and [15N]glycine (concentrations of cross-links and wall teichoic acids). The bridging segment length changed from 5.0 glycyl residues (wild-type strain) to 2.5 ± 0.1 (FemB) with modest changes in cross-link and bridge-link concentrations. This accurate in situ measurement for the FemB mutant indicates a heterogeneous PG structure with 25% monoglycyl and 75% triglycyl bridges. When the bridging segment was reduced to a single glycyl residue 1.0 ± 0.1 (FemA), the level of cross-linking decreased by more than 20%, resulting in a ...