Comparative study of five plasmid-mediated ceftazidimases isolated in Klebsiella pneumoniae

Five plasmid-mediated beta-lactamases conferring a high level of resistance to ceftazidime were isolated from Klebsiella pneumoniae strains. These ceftazidimases (CAZ) differed in their isoelectric point (from 5.3 to 8.2) and were encoded by large self-transferable plasmids of 85 kb (CAZ-2, CAZ-3) or greater than or equal to 150 kb (CAZ-1, CAZ-4, CAZ-5). The 85 kb plasmids seemed closely related to pCFF04 encoding CTX-1 enzyme and belonged to the same incompatibility group 7 or M. These beta-lactamases hydrolysed all beta-lactams with the exception of cephamycins and carbapenems. For CAZ-1, CAZ-2 and CAZ-3 producers, MICs of ceftazidime (32-256 mg/l) were higher than MICs of cefotaxime (0.12-2 mg/l) and aztreonam (1-16 mg/l). For the strains producing the beta-lactamases CAZ-4 and CAZ-5, MICs of aztreonam were the highest (greater than or equal to 256 mg/l). The impaired activities of cephalosporins and monobactams were restored equally well by 2 mg/l of clavulanate, sulbactam and CL-298741 for CAZ-2 producing strains (wild type and transconjugant). Sulbactam (2 mg/l) had a lower protective effect than other inhibitors on ceftazidime for CAZ-1 and CAZ-3 producing K. pneumoniae. The protective effect of sulbactam (2 mg/l) was lower than that of the other inhibitors on all beta-lactams for CAZ-4 and CAZ-5 producers. The enzymes CAZ-1, CAZ-2 and CAZ-3 derived from TEM beta-lactamase whereas CAZ-4 and CAZ-5 derived from SHV-1 enzyme.