Characterization of plasmid pAX22, encoding VIM-1 metallo-β-lactamase, reveals a new putative mechanism of In70 integron mobilization.

OBJECTIVES VIM-type enzymes are among the most widespread acquired metallo-β-lactamases among Gram-negative pathogens. Integron In70 is a class 1 integron that has emerged as a successful genetic support for blaVIM-1 (one of the most prevalent blaVIM allelic variants) in Gram-negative non-fermenters, and is usually chromosome borne. The objective of this study was to characterize plasmid pAX22 from Achromobacter xylosoxidans AX22, which represents the only In70-harbouring plasmid known so far, to gather insights into the mechanisms of evolution and dissemination of In70-like elements. METHODS The complete sequence of pAX22 was obtained by pyrosequencing and assembled with Roche Newbler software. The draft sequence, completed using a PCR-based strategy, was annotated via the BASys tool and compared with known sequences using BLAST algorithms. RESULTS The backbone of pAX22 showed significant similarity with that of pNOR-2000, a blaVIM-2-harbouring plasmid from Pseudomonas aeruginosa, and with the TnCP23 transposon. The three elements differed from each other mainly by the class 1 integron cassette arrays and by some integron-associated structures. In pAX22, In70 was associated with a novel putative transposon, Tn7017, composed of a defective Tn402-like transposon carrying In70 and the ISPa17 insertion sequence. CONCLUSION Plasmid pAX22 belongs to a lineage of plasmids circulating among Gram-negative non-fermenters. In70 was probably acquired by pAX22 by transposition of Tn7017, revealing a novel putative mechanism of In70 mobilization. Our results highlight the potential role that ISPa17 could have in mobilizing defective Tn402-like transposons carrying class 1 integrons.