The effect of MurM and a branched cell wall structure on penicillin resistance in Streptococcus pneumoniae

The aminoacyltransferase MurM is an important penicillin resistance determinant in Streptococcus pneumoniae . This enzyme attaches a serine or alanine to the dipeptide side chain of lipid II, resulting in branched muropeptides that can be crosslinked to stem peptides in the peptidoglycan layer by penicillin binding proteins (PBPs). Deletion of murM results in only linear muropeptides, and more importantly a significant reduction in resistance. Highly penicillin resistant pneumococci are known to express low-affinity PBPs, an altered MurM protein, and possess a highly branched cell wall structure. It has therefore been hypothesized that MurM, and thus branched muropeptides, are essential for resistance because they are better substrates for low-affinity PBPs. In this study, we found that neither the version of murM nor elevated levels of cell wall branching affected the resistance level. To further support this, we investigated whether branched muropeptide substrates compete better than linear versions with penicillin at the active site of low-affinity PBPs and quantified changes to the stem peptide composition of the resistant Pen6 strain in response to subinhibitory concentrations of penicillin. We found that the level of cell wall branching decreased during penicillin exposure. Together our results do not support the idea that elevated levels of branched muropeptides (more active MurM) are important for either the function of low-affinity PBPs or the cell’s response to penicillin. Nevertheless, since a functional MurM enzyme is important for resistance, we speculate that it might indirectly influence other functions related to cell wall synthesis and remodelling needed for a resistant phenotype.