Cefpirome Treatment Results in Limited Selection of Stable Derepressed Enterobacter cloacae Mutants in the Intestinal Flora of Rats Treated for an Experimental Klebsiella pneumoniae Pulmonary Infection

Background: Fourth-generation cephalosporins have been developed to improve their potency, that is, low minimal inhibitory concentrations (MICs) and to prevent Resistance Selection of derepressed AmpC-producing mutants in comparison to third-generation cephalosporins as ceftazidime. Objectives: We investigated the role of the administered cefpirome dose on the efficacy of treatment of a Klebsiella pneumoniae lung Infection as well as in the selection of resistant Enterobacter cloacae isolates in the intestines of rats treated for a K. pneumoniae lung Infection. Materials and Methods: Rats with K. pneumoniae lung Infection received therapy with cefpirome doses of 0.4 to 50 mg/kg/day b.i.d. for 18 days. Resistance Selection in intestinal E. cloacae was monitored during 43 days. Mutants were checked for β-lactamase activity, mutations in their structural ampC gene, ampD gene, and omp39-40 gene. Results: A 45% and 100% rat survival rate was obtained by administration of 3.1 and 12.5 mg/kg b.i.d. of cefpirome. A significant correlation was demonstrated in the reduction of the susceptible E. cloacae isolates with %fT_>MIC at days 7, 14, 22, and 29. Cefpirome E. cloacae mutants, with increased cefpirome MICs, were obtained in only four rats. Conclusions: The treatment with cefpirome resulted in less selection of derepressed mutants in comparison to ceftazidime as shown by their low number per gram of feces and in a limited number of Animals.