EFFECT OF BETA-LACTAMASE EXPRESSION ON SUSCEPTIBILITY OF LOCAL ISOLATES OF ENTEROBACTER-CLOACAE, SERRATIA-MARCESCENS AND PSEUDOMONAS-AERUGINOSA TO BETA-LACTAM ANTIBIOTICS

During surveillance studies, a total of 66 strains of gram-negative bacilli (28 Enterobacter cloacae, 20 Serratia marcescens and 18 Pseudomonas aeruginosa) with a reduced susceptibility ty to third-generation cephalosporins, aztreonam and amikacin were isolated from documented infections. AU isolates were highly susceptible to imipenem and sparfloxacin. beta-Lactamase activity was demonstrated in ah isolates of E. cloacae and S. marcescens, and in 77% of P. aeruginosa isolates. Inducible beta-lactamase activity was detected in 80, 65 and 40% of E. cloacae, S. marcescens and P. aeruginosa isolates, respectively, when cefoxitin was used as an inducer. More inducible beta-lactamase producers were observed when imipenem was used as an inducer. Isolates off. cloacae, and to a lesser extent S. marcescens, expressed a wide spectrum of beta-lactamase activities. There was a good correlation between baseline beta-lactamase activity and the respective MIC of ceftazidime, cefotaxime and ceftriaxone, and to a lesser extent aztreonam in E. cloacae and S. marcescens isolates. Only one isolate of E. cloacae demonstrated an extended beta-lactamase activity. The data suggest that the resistance off. cloacae and S. marcescens isolates to beta-lactam antibiotics is largely dependent upon hyperproduction of beta-lactamase.