부산대학교 도서관

ABSTRACTBypass of the d,d-transpeptidase activity of penicillin-binding proteins by an l,d-transpeptidase (Ldtfm) results in resistance to ampicillin and glycopeptides in Enterococcus faeciumM9, a mutant obtained by nine consecutive selection steps. Resistance requires activation of a cryptic locus for production of the essential tetrapeptide-containing substrate of Ldtfmand impaired activity of protein phosphatase StpA. Here, whole-genome sequencing revealed a high mutation rate for the entire selection procedure (79 mutations in 900 generations). Acquisition of a mutation in the mismatch repair gene mutLhad little impact on the frequency of rifampin-resistant mutants although the mutation spectrum of M9 was typical of impaired MutL with high transversion to transition (40/11) and substitution to deletion (51/28) ratios. M9 did not mainly accumulate neutral mutations since base substitutions occurred more frequently in coding sequences than expected (χ2= 5.0; P< 0.05) and silent mutations were underrepresented (χ2= 5.72; P< 0.02). None of the mutations directly affected recognition of the tetrapeptide substrate of Ldtfmby peptidoglycan synthesis enzymes. Instead, mutations appear to remodel regulatory circuits involving two-component regulatory systems and sugar metabolism. The high number of mutations required for activation of the l,d-transpeptidase pathway may strongly limit emergence of cross-resistance to ampicillin and glycopeptides by this mechanism.