Karthikeyan K Kumarasamy MPhil a, Mark A Toleman PhD b, Prof Timothy R Walsh PhD b , Jay Bagaria MD c, Fafhana Butt MD d, Ravikumar Balakrishnan MD c, Uma Chaudhary MD e, Michel Doumith PhD c, Christian G Giske MD f, Seema Irfan MD g, Padma Krishnan PhD a, Anil V Kumar MD h, Sunil Maharjan MD c, Shazad Mushtaq MD c, Tabassum Noorie MD c, David L Paterson MD i, Andrew Pearson PhD c, Claire Perry PhD c, Rachel Pike PhD c, Bhargavi Rao MD c, Ujjwayini Ray MD j, Jayanta B Sarma MD k, Madhu Sharma MD e, Elizabeth Sheridan PhD c, Mandayam A Thirunarayan MD l, Jane Turton PhD c, Supriya Upadhyay PhD m, Marina Warner PhD c, William Welfare PhD c, David M Livermore PhD c, Neil Woodford PhD c. The Lancet Infectious Diseases, Early Online Publication, 11 August 2010
Background
Gram-negative Enterobacteriaceae with resistance to carbapenem conferred by New Delhi metallo-β-lactamase 1 (NDM-1) are potentially a major global health problem. We investigated the prevalence of NDM-1, in multidrug-resistant Enterobacteriaceae in India, Pakistan, and the UK.

Methods
Enterobacteriaceae isolates were studied from two major centres in India—Chennai (south India), Haryana (north India)—and those referred to the UK's national reference laboratory. Antibiotic susceptibilities were assessed, and the presence of the carbapenem resistance gene blaNDM-1 was established by PCR. Isolates were typed by pulsed-field gel electrophoresis of XbaI-restricted genomic DNA. Plasmids were analysed by S1 nuclease digestion and PCR typing. Case data for UK patients were reviewed for evidence of travel and recent admission to hospitals in India or Pakistan.

Findings
We identified 44 isolates with NDM-1 in Chennai, 26 in Haryana, 37 in the UK, and 73 in other sites in India and Pakistan. NDM-1 was mostly found among Escherichia coli (36) and Klebsiella pneumoniae (111), which were highly resistant to all antibiotics except to tigecycline and colistin. K pneumoniae isolates from Haryana were clonal but NDM-1 producers from the UK and Chennai were clonally diverse. Most isolates carried the NDM-1 gene on plasmids: those from UK and Chennai were readily transferable whereas those from Haryana were not conjugative. Many of the UK NDM-1 positive patients had travelled to India or Pakistan within the past year, or had links with these countries.

Interpretation
The potential of NDM-1 to be a worldwide public health problem is great, and co-ordinated international surveillance is needed.

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