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During July to December from 2004 to 2006, a surveillance of eight major hospitals in the Hiroshima region identified 387 non-duplicate isolates resistant to imipenem (MIC ≥ 16 mg/L). They were screened for resistance to amikacin (MIC ≥ 64 mg/L) and ciprofloxacin (MIC ≥ 4 mg/L) and MBL-encoding genes. The structure of the variable regions of the integrons was determined using PCR mapping. Clonality was assessed using PFGE and multilocus sequence typing (MLST).

The frequency of MBL-positive isolates in MDR P. aeruginosa isolates significantly increased from 42.3% in 2004 to 81.4% in 2006. Most of the MBL-positive isolates produced IMP-1 followed by VIM-2. The bla_IMP-1 and bla_VIM-2 genes were present in class 1 integrons. Characterization of the variable regions of the integron showed the presence of six different gene cassette arrays in bla_IMP-1 cassettes and a single array in bla_VIM-2 cassettes. The IMP-1 producers belonged to two clonal lineages using PFGE and MLST analyses and the integron variations correlated well with the clonal complexes. Among them, strains positive for a newly identified In113-derived bla_IMP-1 gene cassette array were most widely distributed in Hiroshima.

This study shows a dramatic increase in MBL genes, primarily bla_IMP-1, in MDR P. aeruginosa isolates in Hiroshima during these 3 years. In addition, MDR P. aeruginosa with the newly discovered In113-derived bla_IMP-1 gene cassette array appears to be clonally expanding.