Population analyses of the susceptibility to ciprofloxacin of eight clinical strains of Pseudomonas aeruginosa.

Population analyses of the susceptibility to ciprofloxacin of eight strains of Pseudomonas aeruginosa, including mucoid strains and strains resistant to aminoglycosides or anti-Pseudomonas beta-lactams, were carried out. All strains were sensitive as judged by the broth-dilution technique, but four strains were found to yield resistant mutants with a frequency of less than 10(-6). Two strains yielded mutants homogeneously sensitive to ciprofloxacin at a level 8-16 times the MIC of the parent strains. Two other strains yielded mutants resistant to different higher concentrations of ciprofloxacin. One of these mutants was examined for production of ciprofloxacin-inactivating enzyme; no enzyme production could be detected. Cross-resistance was found with another quinolone antibiotic, ofloxacin, but not with aminoglycosides or beta-lactam antibiotics.