Plasmid-mediated transfer of the bla(NDM-1) gene in Gram-negative rods.

The latest threat of multidrug-resistant Gram-negative bacteria corresponds to the emergence of carbapenemase NDM-1 (New Delhi metallo-β-lactamase) producers, mostly in Enterobacteriacae. Five bla(NDM) (-1) -positive plasmids of different incompatibility groups (IncL/M, FII, A/C and two untypeable plasmids) from clinical Enterobacteriaceae were evaluated for conjugation properties and host specificity. Successful conjugative transfers were obtained using all tested enterobacterial species as recipients (Escherichia coli, Klebsiella pneumoniae, Salmonella typhimurium and Proteus mirabilis) and all plasmid types. Conjugation frequencies varied from 1 × 10(-4) to 6 × 10(-8) transconjugants per donor. Higher conjugation rates were obtained for two plasmids at 30 °C compared with that observed at 25 and 37 °C. Carbapenems used as selector did not lead to higher conjugation frequencies. None of the five plasmids was transferable to Acinetobacter baumannii or Pseudomonas aeruginosa by conjugation. This work underlines how efficient the spread of the carbapenemase bla(NDM) (-1) gene could be among Enterobacteriaceae.