Novel Mechanism of β-Lactam Resistance Due to Bypass of DD-Transpeptidation in Enterococcus faecium

Abstract The peptidoglycan structure of in vitro selected ampicillin-resistant mutant Enterococcus faecium D344M512 and of the susceptible parental strain D344S was determined by reverse phase high performance liquid chromatography and mass spectrometry. The muropeptide monomers were almost identical in the two strains. The substantial majority (99.3%) of the oligomers from the susceptible strain D344S contained the usuald-alanyl → d-asparaginyl (ord-aspartyl)-l-lysyl cross-link (d-Ala → d-Asx-l-Lys) generated by β-lactam-sensitive DD-transpeptidation. The remaining oligomers (0.7%) were produced by β-lactam-insensitive LD-transpeptidation, because they contained l-Lys →d-Asx-l-Lys cross-links. The muropeptide oligomers of the ampicillin-resistant mutant D344M512 contained only these l-Lys → d-Asx-l-Lys cross-links indicating that resistance was due to the bypass of the β-lactam-sensitive DD-transpeptidation reaction. The discovery of this novel resistance mechanism indicates that DD-transpeptidases cannot be considered anymore as the sole essential transpeptidase enzymes.