Prevalence and molecular characterisation of New Delhi metallo-β- lactamases NDM-1, NDM-5, NDM-6 and NDM-7 in multidrug-resistant Enterobacteriaceae from India

The growing prevalence of carbapenem resistance in Enterobacteriaceae worldwide is a major concern. New Delhi metallo-β-lactamase (NDM)-mediated carbapenem resistance has been identified in Enterobacteriaceae from numerous countries including those of the Indian subcontinent. Currently, seven NDM β-lactamase variants (NDM-1 to -7) have been identified. This study evaluated the detection and molecular characterisation of NDM variants in Enterobacteriaceae at a tertiary care hospital in India. A total of 464 isolates were tested; 57 (12.3%) were resistant or showed reduced susceptibility to imipenem and meropenem. All carbapenem-resistant isolates were bla _NDM-positive by PCR, but 13 isolates bore variants that differed in sequence from bla_NDM-1. NDM-5, NDM-6 and NDM-7 were identified in two, eight and three isolates, respectively. bla_NDM variants were located on plasmids of >100 kb with IncF, IncA/C and untypeable replicon types. Genes encoding the 16S rRNA methyltransferases RmtB, RmtC and ArmA as well as those for AmpC β-lactamases were also located on the same plasmids as bla_NDM in different combinations. The prevalence of NDM-5 to -7 variants was significantly higher in Escherichia coli (P = 0.015) and they were more frequently isolated from the urology ward (P = 0.037) than NDM-1. The mortality rate was comparable between patients infected with isolates positive for bla_NDM-1 and bla_NDM variants [25% (11/44) vs. 23% (3/13)]. Expression of bla_NDM variants in E. coli using the same promoter showed that NDM-7 conferred higher resistance to imipenem. The diverse genotypic features of bla_NDM indicate rapid evolution of NDM resulting from their wide spread in the Indian subcontinent.