Selection and characterization of cefepime-resistant gram-negative bacteria

The NCTC type strains and four clinical isolates of Enterobacter cloacae, Citrobacter freundii, Serratia marcescens, Morganella morganii, Providencia stuartii and Pseudomonas aeruginosa were exposed, in agar, to cefepime at 3, 5, and 10 x MIC and a breakpoint concentration of 16 mg/L. Mutants were selected at a frequency of approximately 10(-8) that had decreased susceptibility to cefepime and cefpirome, and species-dependent resistance to other beta-lactams. Any putative mutant with a greater than four-fold increase in the MIC was examined to determine its beta-lactamase expression and outer membrane protein (Omp) profile. Mutant strains of P. stuartii and M. morganii lacked an Omp of molecular mass similar to that of OmpF, and were cross-resistant to nalidixic acid. Mutant strains of E. cloacae had derepressed class I beta-lactamase production and lacked an Omp corresponding to OmpF, suggesting that in this species both parameters are necessary for decreased susceptibility. Derepressed beta-lactamases purified from mutant strains of E. cloacae, C. freundii and P. stuartii was able to hydrolyse cefepime, but not as quickly as TEM-10.