Resistance studies with ofloxacin.

The selection of ofloxacin-resistant mutants from susceptible wild-type bacterial populations was investigated by three methods. Resistant mutants selected from populations of Escherichia coli (NCTC 10418) and Staphylococcus aureus (NCTC 6571) by single-step passage at either four or eight times the MIC occurred at a frequency of less than 1 x 10(-10). Ofloxacin-resistant mutants of Pseudomonas aeruginosa (NCTC 10662), selected at four times the MIC, occurred with a 100-fold greater frequency at 3.3 X 10(-8), however, mutants of this species selected at eight times the MIC, occurred at a frequency of less than 1 X 10(-10). Sequential selection of ofloxacin-resistant mutants during multiple passages at 1/2 MIC, resulted in a 16- to 32-fold increase in MICs for isolates of P. aeruginosa and Klebsiella aerogenes, a four- to eight-fold increase in MICs for some strains of E. coli and a two- to four-fold increase in MICs for Staph. aureus and Streptococcus spp. During prolonged exposure of populations of E. coli (NCTC 10418) and Staph. aureus (NCTC 6571) to concentrations of ofloxacin at 1, 10 and 100 times the MIC no resistant-mutants were selected after seven days incubation. In similar experiments with P. aeruginosa (NCTC 10662) ofloxacin-resistant mutants were selected from populations exposed to the MIC which required 64 mg/l ofloxacin for inhibition. No ofloxacin-resistant mutants were selected from populations of P. aeruginosa (NCTC 10662) exposed to either ten or 100 times the MIC.(ABSTRACT TRUNCATED AT 250 WORDS)