Increased prevalence and clonal dissemination of multidrug-resistant Pseudomonas aeruginosa with the blaIMP-1 gene cassette in Hiroshima

Objectives: The aim of this study was to evaluate the dissemination of metallo-β-Iactamase (MBL)-encoding genes among multidrug-resistant (MDR) Pseudomonas aeruginosa isolates recovered from major hospitals in the Hiroshima region. Methods: 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≥16mg/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). Results: 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. Conclusions: 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 In 113-derived bla IMP-1 gene cassette array appears to be clonally expanding.