Identification of mutations responsible for acquisition of ampicillin resistance by L,D-transpeptidation in Enterococcus faecium D344S. Enterococcus faecium strain:D344S

Nine consecutive selection steps on increasing concentrations of beta-lactam and glycopeptide antibiotics resulted in by-pass of the D,D-transpeptidase activity of penicillin-binding proteins by an L,D-transpeptidase (Ldtfm) in Enterococcus faecium D344S. The resulting mutant, designated M9, was highly resistant to ampicillin and glycopeptides. Whole genome sequencing revealed a high mutation rate for the entire selection procedure (79 mutations in 900 generations) involving impaired activity of mismatch repair protein MutL which led to high transversion to transition and deletion to substitution ratios. Most mutations are expected not to be neutral and to predominantly affect regulatory circuits.