Fig. 6: Components of the S. aureus cell wall machinery associated with resistance to EDTA and DTPMP.
a Relevant peptidoglycan synthesis and teichoic acid modification proteins are illustrated. Peptidoglycan monomers, synthesised as lipid-linked precursors (lipid II), are transported across the cytoplasmic membrane prior to insertion into the sacculus network by PBPs101. PBP2 has both glycosyltransferase (GT) and transpeptidase (TP) activities required for elongation of glycan chains and formation of peptide bonds, respectively102. VraF, as part of a complex with VraG and alongside GraXRS, is involved in regulation of various genes that affect cell surface charge, most notably the dltXABCD operon whose products populate teichoic acids with D-Alanine (D-Ala), adding a greater positive charge (+). FmtA is an esterase that removes D-Ala from teichoic acid embedded in peptidoglycan and is implicated in removing positive charge. Lipoteichoic acids (LTA) and wall teichoic acids (WTA) are also indicated. b–d Effect of different mutations isolated in this study that promote EDTA and DTPMP resistance. All of the strains show a thicker cell wall, likely due to mutations in PBP2 that produce longer glycan chains with fewer cross-links. The two EDTA-selected strains (JN170/JN206) also lack fmtA. The three DTPMP strains have mutations affecting the GT domain of PBP2, while JN208 also has a null allele in vraF. Several of these strains display an increased negative (–) charge. Resistance (r) and sensitivity (s) to cell wall-targeting agents, moenomycin (MmA), osmotic stress (NaCl), Triton X-100 (Trx) and lysostaphin (Lys).
