Characterization of IMI-1 beta-lactamase, a class A carbapenem-hydrolyzing enzyme from Enterobacter cloacae

In 1984, a year prior to the U.S. approval of imipenem for clinical use, a wound isolate and a bile isolate of Enterobacter cloacae were obtained from two patients in a California hospital, These isolates were resistant to imipenem, penicillins, and inhibitor combinations; early cephalosporins such as cephalothin, cefamandole, and cefoxitin; and cefoperazone, However, they were susceptible (MICs, <4 mu g/ml) to cefotaxime, ceftriaxone, ceftazidime, and moxalactam. Both strains produced an apparent TEM-I beta-lactamase; an inducible NmcA-type imipenem-hydrolyzing beta-lactamase, IMI-1, with a pI of 7.05; and an inducible beta-lactamase with a pi of 8.1, typical of an E. cloacae AmpC beta-lactamase, Purified IMI-1 hydrolyzed imipenem and benzylpenicillin at modest rates, but more slowly than cephaloridine, The enzyme was inhibited by clavulanic acid and tazobactam. EDTA did not inhibit the cephaloridine-hydrolyzing activity. The beta-lactamase gene encoding IMI-1, imiA1, was cloned from E, cloacae 1413B. Sequence analysis identified the imiA1 gene as encoding a class A serine beta-lactamase. Both the imiA1 DNA and encoded amino acid sequences shared greater than 95% identity with the NmcA gene and its encoded protein, DNA sequence analysis also identified a gene upstream of imiA1 that shares >95% identity with nmcR and that may encode a regulatory protein, In conclusion, IMI-1, a carbapenem-hydrolyzing beta-lactamase inhibited by clavulanic acid, was identified as a group 2f, class A, carbapenem-hydrolyzing cephalosporinase.