Background: Metallo-β-lactamase-mediated carbapenem resistance has been increasingly reported in Pseudomonas, Acinetobacter and other Gram-negative bacilli (GNB) in many countries. A few studies showed highly variable structure of MBL-gene cassette-carrying integrons. The aim of this study was to determine the structure of bla_VIM-2-carrying integrons in Pseudomonas and Acinetobacter.

Methods: bla_VIM-2-carrying GNB were isolated at a Korean hospitals during the years 1995-1999 and 2005. The size of bla_VIM-2-carrying integrons was estimated by the PCR products. Representative integrons were sequenced by the dideoxy-chain termination method. The MICs of antimicrobial agents were tested by the CLSI agar dilution methods.

Results: During the years 1995-1999 and 2005, the approximate size of the bla_VIM-2-carrying class 1 integrons was 3-7 kb in 35 Pseudomonas isolates and 3-5 kb in 24 Acinetobacter isolates. The integrons carried one-five resistance gene cassettes in addition to the bla_VIM-2 cassette. Other resistance gene cassettes found were bla_OXA-1, aacA1, aac(6’)-I, and aac(6’)-II. Interestingly, sequences homologous to part of a putative class II intron were inserted into the recombination site of the last cassette in four of nine integrons. The class 1 integron from P. aeruginosa isolates had fused orf/IntI1 in a downstream leftward inverted repeat (IRi).

Conclusion: According to period, the size and structure of bla_VIM-2-carrying integrons are quite variable, but an identical one is also present in a different genus, indicating high mobility of the bla_VIM-2 cassette and horizontal transfer of the whole integron. We suggest that the class 1 integron containing the bla_VIM-2 gene is spreading horizontally among Gram- negative bacilli and is undergoing continuous development in Korea. (Korean J Clin Microbiol 2012;15:131-138)