A novel peptidoglycan cross-linking enzyme for a β-lactam-resistant transpeptidation pathway

The beta-lactam antibiotics remain the most commonly used to treat severe infections. Because of structural similarity between the beta-lactam ring and the D-alanyl(4)-D-alanine(5) extremity of bacterial cell wall precursors, the drugs act as suicide substrates of the DD-transpeptidases that catalyze the last cross-linking step of cell wall assembly. Here, we show that this mechanism of action can be defeated by a novel type of transpeptidase identified for the first time by reverse genetics in a beta-lactam-resistant mutant of Enterococcus faecium. The enzyme, Ldt(fm), catalyzes in vitro the cross-linking of peptidoglycan subunits in a beta-lactam-insensitive LD-transpeptidation reaction. The specificity of Ldt(fm) for the L-lysyl(3)-D-alanine(4) peptide bond of tetrapeptide donors accounts for resistance because the substrate does not mimic beta-lactams in contrast to D-alanyl(4)-D-alanine(5) in the pentapeptide donors required for DD-transpeptidation. Ldt(fm) homologues are encountered sporadically among taxonomically distant bacteria, indicating that LD-transpeptidase-mediated resistance may emerge in various pathogens.