Antibiotic susceptibility and beta-lactamase production in clinical isolates of Enterobacter spp.

The in vitro susceptibility of 237 clinical isolates of Enterobacter spp. (E. aerogenes, E. agglomerans and E. cloacae; 41, 64 and 132 respectively) to 16 different antibiotics is described. Four quinolones (ciprofloxacin, lomefloxacin, norfloxacin and ofloxacin), two new cephalosporins (cefpirome and cefepime) and imipenem, all showed high activity against the three Enterobacter species tested (MIC50 less than or equal to 0.125 mg/l, MIC90 less than or equal to 0.5 mg/l). Also the aminoglycosides gentamicin and tobramycin were highly active antibiotics (MIC50 less than or equal to 0.5 mg/l, MIC90 less than or equal to 1.0 mg/l). The susceptibility of beta-lactam-antibiotics to beta-lactamase produced by Enterobacter spp. was evaluated, and imipenem and cefepime were found to be most stable. Different methods for detection of inducible beta-lactamases were used, the agar dilution method being more sensitive than the double-disc diffusion test. Elevated beta-lactamase production was detected, via induction, in 83% of E. aerogenes strains and 70% of E. cloacae strains, with cefamandole used as the substrate and cefoxitin as the inducer. Constitutive, high level enzyme production was detected in 7 and 13% respectively of the E. aerogenes and the E. cloacae strains. In all the strains of E. agglomerans, 10% of E. aerogenes and 13% of E. cloacae, no beta-lactamases could be detected with the methods studied.