A 9-year surveillance for multidrug-resistant (MDR) Pseudomonas aeruginosa in the Hiroshima region showed that the number of isolates harboring the metallo-β-lactamase gene blaIMP-1 abruptly increased after 2004, recorded the highest peak in 2006, and showed a tendency to decline afterwards, indicating a history of an epidemic. PCR mapping of the variable regions of the integrons showed that this epidemic was caused by the clonal persistence and propagation of an MDR P. aeruginosa strain harboring the blaIMP-1 gene and an aminoglycoside 6′-N-acetyltransferase gene, aac(6′)-Iae in a class I integron (In113), whose integrase gene intl1 was disrupted by an IS26 insertion. Sequence analysis of the representative strain PA058447 resistance element containing the In113-derived gene cassette array showed that the element forms an IS26 transposon embedded in the chromosome. It has a Tn21 backbone and is composed of two segments sandwiched by three IS26s. In Japan, clonal nationwide expansion of an MDR P. aeruginosa NCGM2.S1 harboring chromosomally encoded In113 with intact intl1 is reported. Multilocus sequence typing and genomic comparison strongly suggest that PA058447 and NCGM2.S1 belong to the same clonal lineage. Moreover, the structures of the resistance element in the two strains are very similar, but the sites of insertion into the chromosome are different. Based on tagging information of the IS26 present in both resistance elements, we suggest that the MDR P. aeruginosa clone causing the epidemic in Hiroshima for the past 9 years originated from a common ancestor genome of PA058447 and NCGM2.S1 through an IS26 insertion into intl1 of In113 and through IS26-mediated genomic rearrangements.
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