Acquisition of Resistance to Beta-Lactams by Enterobacter cloacae and Serratia marcescens in the Clinical Field.

We compared the drug resistance patterns of clinical isolates of Enterobacter cloacae and Serratia marcescens in 1983 and 1990 at Nagasaki University Hospital. Furthermore, beta-lactamase production by drug-resistant strains of several organisms was investigated. In 1990, the number of E.cloacae or S. marcescens strains isolated was about half of that in 1983. Most of the E.cloacae strains were isolated from the respiratory tract in both years. A large proportion of S. marcescens strains were isolated from the urinary tract in 1983, but in 1990 these strains were isolated from the respiratory tract. An increase in E. cloacae strains and S. marcescens strains resistant to ceftizoxime or to latamoxef was not noted when comparing 1990 with 1983. The beta-lactamase prepared from resistant strains isolated in 1990 showed a higher potency than that from susceptible strains isolated in the same year, and a larger amount of betalactamase was produced by the resistant strains. Beta-lactamase production by resistant strains of E. cloacae, Citrobacter freundii, and S. marcescens was found to be due to derepression. These strains resistant to beta-lactams were susceptible to imipenem.