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.