Novel Variant of New Delhi Metallo-β-lactamase, NDM-20, in Escherichia coli

The spread of carbapenem-resistant Enterobacteriaceae (CRE) mediated by New Delhi metallo-β-lactamase (NDM) poses a serious challenge to clinicians and has become a major public health concern. NDM has been evolving into variants that possess different hydrolysis activity toward antibiotics, so as to affect treatment strategy. In addition, very few studies on NDM variants have focused on animal-derived bacterial isolates. Our study reports a novel NDM variant, NDM-20, in an isolate of Escherichia coli CCD1 recovered from the food animal swine in China. The isolate that was assigned to ST1114, exhibited high level resistance to all β-lactams tested, including aztreonam and carbapenems. The gene of bla(NDM-20) was located on an IncX3-type plasmid, surrounded by multiple insertion sequences. Sequencing analysis demonstrated that bla(NDM-20) contained three point mutations at positions 262 (G→T), 460 (A→C), and 809 (G→A), compared with bla(NDM-1), and just one point mutation at position 809 (G→A), relative to bla(NDM-5). Functional analysis revealed that the bla(NDM-20) transformant, DH5α+pHSG398/NDM-20, exhibited a higher resistance to ertapenem than that of bla(NDM-1) transformant DH5α+pHSG398/NDM-1. Kinetic parameter analysis showed that NDM-20 had increased enzymatic activity against some penicillins and cephalosporins but decreased carbapenemase activity relative to NDM-5. The identification of NDM-20 further confirms the evolution and prevalence of NDM variants in bacteria of food-animal origin.