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

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 carbapen em-resistant clinical isolates of P. aeruginosa and identified the associated integron-borne metallo-beta-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 mu g 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 32256 mu g ml(-1). The non-MBL-producing strains showed a low level of resistance, with MICs of 832 mu g 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-N-17-TAAACT, and the bla(VIM-2) gene cassette was preceded by a weak promoter, TGGACA-N-17-TAAGCT. Most of the MBL-encoding genes were linked to one or two resistance genes encoding aminoglycoside-modifying enzymes, such as aac(6')lae, aac(6')ll, aacA7, aacC4, aadA1, aadA2 and aadA6, highlighting the multidrug-resistant properties of these clinical isolates.