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

Abstract

The β-lactam antibiotics remain the most commonly used to treat severe infections. Because of structural similarity between the β-lactam ring and the D-alanyl4-D-alanine5 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 β-lactam-resistant mutant of Enterococcus faecium. The enzyme, Ldtfm, catalyzes in vitro the cross-linking of peptidoglycan subunits in a β-lactam-insensitive LD-transpeptidation reaction. The specificity of Ldtfm for the L-lysyl3-D-alanine4peptide bond of tetrapeptide donors accounts for resistance because the sub-strate does not mimic β-lactams in contrast to D-alanyl4-D-alanine5 in the pentapeptide donors required for DD-transpeptidation. Ldtfm homologues are encountered sporadically among taxonomically distant bacteria, indicating that LD-transpeptidase-mediated resistance may emerge in various pathogens.