Effect of ciprofloxacin on fecal flora of patients with cystic fibrosis and other patients treated with oral ciprofloxacin.

Ciprofloxacin is a fluorinated carboxyquinolone that inhibits Enterobacteriaceae, staphylococci, and Pseudomonas at low concentrations. It has poor activity against Bacteroides fragilis. In this study, the effect of administration of ciprofloxacin on bowel flora was determined in patients treated for different infections. Patients, aged 22 to 70 years, were treated with 500 mg of ciprofloxacin every 12 hours or 750 mg every eight hours for seven to 42 days. Some patients had advanced cystic fibrosis; other patients had infections with resistant bacteria. Infecting organisms were Pseudomonas aeruginosa, Staphylococcus aureus, Serratia, and Acinetobacter. Sites of infections were lung, soft tissue, and urinary tract. Stool samples were evaluated initially, during therapy, and after therapy. No resistant gram-negative aerobic species emerged; five patients had yeast colonization, staphylococci were found in three patients, and streptococci were found in one patient. Ciprofloxacin did not select resistant gram-negative bacteria in the stool, although sputum isolates showed increases in minimal inhibitory concentrations. Resistant bacteria were not selected in the fecal flora of patients who had received beta-lactam and aminoglycoside antibiotics before therapy with ciprofloxacin.