Novel mechanism of beta-lactam resistance due to bypass of DD-transpeptidation in Enterococcus faecium

The peptidoglycan structure of in vitro selected ampicillin-resistant mutan t Enterococcus faecium D344M512 and of the susceptible parental strain D344 S was determined by reverse phase high performance liquid chromatography an d mass spectrometry. The muropeptide monomers were almost identical in the two strains. The substantial majority (99.3%) of the oligomers from the sus ceptible strain D344S contained the usual D-alanyl --> D-asparasginyl (or D -aspartyl)-L-lysyl cross-link (D-Ala --> D-Asx-L-Lys) generated by beta-lac tam-sensitive DD-transpeptidation. The remaining oligomers (0.7%) were prod uced by beta-lactam-insensitive LD-transpeptidation, because they contained L-Lys --> D-ASX-L-Lys cross-links. The muropeptide oligomers of the ampici llin-resistant mutant D344M512 contained only these L-Lys --> D-ASX-L-Lys c ross-links indicating that resistance was due to the bypass of the beta-lac tam-sensitive DD-transpeptidation reaction. The discovery of this novel res istance mechanism indicates that DD-transpeptidases cannot be considered an ymore as the sole essential transpeptidase enzymes.