Resistance emerging after pefloxacin therapy of experimental Enterobacter cloacae peritonitis.

Resistance emerging after pefloxacin therapy was investigated in an experimental Enterobacter cloacae infection. Mice were inoculated intraperitoneally (mean inoculum, 0.9 X 10(8) CFU) with one of four strains initially susceptible to quinolones and treated with a single 25-mg/kg dose of pefloxacin. This therapy produced a net decrease of bacterial counts in the peritoneal fluid, but with the of the isolates, posttherapy (PT1) strains emerged with decreased susceptibilities to quinolones (4- to 1,024-fold), to the structurally unrelated antibiotics (4- to 16-fold) chloramphenicol and trimethoprim, and sometimes to tetracycline and beta-lactam compounds. In a second set of experiments, new mice were similarly infected with PT1 strains and treated with up to five 25-mg/kg doses of pefloxacin. Compared with parent isolates, PT1 strains produced similar disease and peritoneal bacterial count in the control animals. In treated mice posttherapy (PT2) strains emerged that showed 8- to 64-fold increases in quinolone MICs compared with the PT1 strains inoculated. All PT1 and PT2 strains showed altered outer membrane protein patterns, principally marked by a decreased 37,000-molecular-weight band generally accompanied by an increased 42,000-molecular-weight band. Whole cells from all PT1 and PT2 strains, exposed to [14C]pefloxacin for 15 to 60 s, bound significantly less radioactivity than the corresponding parent strains. After partial purification, DNA gyrase extracted from the most resistant isolates (one PT1 and the PT2 strains) showed a 100- to 450-fold 50% inhibitory concentration increase for pefloxacin. Altogether, pefloxacin can select in vivo two types of resistant strain, one with only decreased permeability and another with decreased permeability combined with altered DNA gyrase.