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Relevance of resistance levels to carbapenems and integron-borne bla_IMP-1, bla_IMP-7, bla_IMP-10 and bla_VIM-2 in clinical isolates of Pseudomonas aeruginosa

¹ Department of Microbiology and Immunology, Showa University School of Medicine, Tokyo, Japan

² Department of Clinical Pathology, Showa University School of Medicine, Tokyo, Japan

³ Clinical Laboratory, Showa University Hospital, Showa University, Tokyo, Japan

Correspondence
Wei-Hua Zhao
whzhao{at}med.showa-u.ac.jp

Received February 4, 2009
Accepted April 15, 2009

Molecular detection and surveillance of the resistance genes harboured by Pseudomonas aeruginosa are becoming increasingly important in assessing and controlling spread and colonization in hospitals, and in guiding the treatment of infections. This study analysed the resistance mechanisms of carbapenem-resistant clinical isolates of P. aeruginosa and identified the associated integron-borne metallo-β-lactamase (MBL)-encoding genes. Twenty-seven imipenem (IPM)-resistant clinical isolates of P. aeruginosa were divided into three groups according to their resistance levels to carbapenems. Strains bearing bla_IMP-10 showed extremely high-level resistance to IPM, with MICs of 512–2048 µg ml^–1. By comparison, strains bearing bla_IMP-1, bla_IMP-7 and bla_VIM-2 showed an intermediate level of resistance, with MICs of 32–256 µg ml^–1. The non-MBL-producing strains showed a low level of resistance, with MICs of 8–32 µg ml^–1. The same trend in resistance levels was also observed when resistance to other carbapenems, such as meropenem and panipenem, was determined. DNA sequencing showed that the MBL-encoding gene cassettes were carried by class 1 integrons. The bla_IMP-1, bla_IMP-7 and bla_IMP-10 gene cassettes were preceded by a hybrid P_ant promoter, TGGACA-N₁₇-TAAACT, and the bla_VIM-2 gene cassette was preceded by a weak promoter, TGGACA-N₁₇-TAAGCT. Most of the MBL-encoding genes were linked to one or two resistance genes encoding aminoglycoside-modifying enzymes, such as aac(6')Iae, aac(6')II, aacA7, aacC4, aadA1, aadA2 and aadA6, highlighting the multidrug-resistant properties of these clinical isolates.