Peptidoglycan cross-linking in Staphylococcus aureus. An apparent random polymerisation process.

The peptidoglycan of Staphylococcus aureus contains relatively short glycan chains and is highly cross-linked via its peptide chains. The material from wild-type (strain H) and mutants H28, H4B and MR-1 was freed from the teichoic-acid-linked component and then hydrolysed by Chalaropsis muramidase to yield disaccharide-repeating units of the glycan with attached peptides either non-cross-linked (monomer) or joined to similar units by one (dimer), two (trimer) or more (oligomer) peptide cross links. The resulting fragments were separated by high-resolution HPLC so that distinguishable components as large as nonamer could be identified. Extrapolation showed that, in S. aureus H, H28 and MR-1, oligomers at least as large as eicosamer formed part of the smooth distribution of oligomer fragments, whereas in strain H4B (PBP4-) the maximum size was around dodecamer. The oligomer distribution profile was related to the polymerization theories of Flory, which allow a distinction to be made between a monomer addition model, whereby each oligomer can only be synthesized by the addition of a single monomer unit to its next lower homologue, and a random addition model, in which an oligomer can be formed by linkage of any combination of its constituent smaller units. In S. aureus close approximation to the random addition model for oligomer synthesis and hence for peptidoglycan cross-linking was observed, both in PBP4+ and PBP4- mutants. The implications for secondary cross-linking in S. aureus cell wall formation are inescapable, although the possibility of an endopeptidase/transpeptidase providing later modification of the peptidoglycan is not completely ruled out.