Development of resistance to ciprofloxacin in nutrient-rich and nutrient-limited growth conditions in vitro by Pseudomonas aeruginosa isolates from patients with cystic fibrosis.

Five serial exposures of mucoid Pseudomonas aeruginosa from patients with cystic fibrosis to subinhibitory concentrations of ciprofloxacin resulted in stepwise increases in the MIC, with a mean proportional increase of 10. MICs were significantly lower in an iron-limited chemically defined medium than in Iso-Sensitest broth. The mucoid phenotype was maintained in chemically defined medium. Acquired resistance was retained either partially or completely in 85% of the isolates following 10 transfers in drug-free media. In cases in which susceptibility was regained, an increase in the MIC was observed on one further exposure to ciprofloxacin.