Genetic Context and Biochemical Characterization of the IMP-18 Metallo-β-Lactamase Identified in a Pseudomonas aeruginosa Isolate from the United States

The production of metallo-beta-lactamase (MBL) is an important mechanism of resistance to beta-lactam antibiotics, including carbapenems. Despite the discovery and emergence of many acquired metallo-beta-lactamases, IMP-type determinants (now counting at least 27 variants) remain the most prevalent in some geographical areas. In Asian countries, and notably Japan, IMP-1 and its closely related variants are most widespread. Some other variants have been detected in other countries and show either an endemic (e. g., IMP-13 in Italy) or sporadic (e. g., IMP-12 in Italy or IMP-18 in the United States) occurrence. The IMP-18-producing Pseudomonas aeruginosa strain PS 297 from the southwestern United States carried at least two class 1 integrons. One was identical to In51, while the other, named In133 and carrying the bla(IMP-18) gene cassette in the third position, showed an original array of five gene cassettes, including aacA7, qacF, aadA1, and an unknown open reading frame (ORF). Interestingly. In133 differed significantly from In96, the bla(IMP-18)-carrying integron identified in a P. aeruginosa isolate from Mexico. The meropenem and ertapenem MIC values were much lower for Escherichia coli strains producing IMP-18 (0.06 and 0.12 mu g/ml, respectively) than for strains producing IMP-1 (2 mu g/ml for each). Kinetic data obtained with the purified enzyme revealed lower turnover rates of IMP-18 than of other IMP-type enzymes with most substrates.