Effective transfer of a 47 kb NDM-1-positive plasmid among Acinetobacter species.

OBJECTIVES To investigate the link between two NDM-1-positive Acinetobacter isolates from the same hospital, the plasmid profiles of the isolates were examined. These two isolates were found from a surveillance programme within 3 months from two patients without obvious physical contact or hospitalization time overlap. METHODS Antimicrobial susceptibility tests, genome sequencing of both isolates and plasmid transfer experiments were performed. A comparative study of similar plasmids was performed using BLAST analysis. RESULTS The antimicrobial susceptibility of the isolates (Acinetobacter soli M131 and Acinetobacter pittii MS32) and their Escherichia coli transconjugants revealed a conjugative plasmid that carried the carbapenem resistance determinant. Eleven plasmids were observed in M131 and three in MS32. Each isolate shared an identical plasmid that carried the blaNDM-1 gene. This 47 271 bp plasmid harbours a conserved blaNDM-1-containing region that is flanked by ISAba125 and ISAba11 elements, and also contains a Ti-type conjugative operon. The plasmid is nearly identical in sequence to those of Acinetobacter isolates from China. In contrast to the mobilization of the blaNDM-1 sequence in Enterobacteriaceae, which is mainly by transposition, this plasmid moves as a whole among Acinetobacter species. Consistently, this plasmid was found to transfer effectively by in vitro conjugation to several Acinetobacter species. CONCLUSIONS The clinical and laboratory findings suggest that Acinetobacter species may serve as a reservoir of this blaNDM-1 plasmid. Our study demonstrates the potential of applying genome sequencing to the surveillance of antimicrobial-resistant bacteria.