Impact of ciprofloxacin in the human-flora-associated (HFA) rat model: comparison with the HFA mouse model

The ecological impact of different doses of ciprofloxacin was investigated in an experimental germ-free rat model into which human fecal flora was inoculated. Animals received oral doses (gavage) of 0, 0.25, 2.5, and 25 mg/kg body weight (bw) of ciprofloxacin once daily for 5 weeks. All doses of ciprofloxacin significantly reduced aerobic populations. Elimination of Enterobacteriaceae and reduction of bifodibacteria were noticed in the group treated with 25 mg/kg of the antibiotic. The rest of the intestinal flora was not affected. These effects were reversible after the treatment ended. The percentage of resistant enterococci increased in rats treated with 2.5 and 25 mg/kg; however, this increase was not statistically significant. There was a significant (P < 0.05) emergence of ciprofloxacin-resistant Bacteroides fragilis group with 25 mg/kg bw, which is equivalent to a human therapeutic dosage of the antibiotic. The MIC values and the percentage of resistance remained elevated 2 weeks after the end of treatment in this anaerobic population. Although sub-populations of enterococci and Enterobacteriaceae showed decreased susceptibility after ciprofloxacin administration, resistance was not evident. The ability of an exogenous strain of Salmonella to colonize the intestine of animals treated with 25 mg/kg of ciprofloxacin confirmed that the drug disrupted the colonization barrier effect of the indigenous flora at the high dose level tested. No changes in the metabolic parameters occurred during the antibiotic treatment. The results obtained in the HFA rat model were similar to those obtained in our previous study using the HFA mice model where ciprofloxacin at 0.125, 1.25, and 12.5 mg/kg bw induced a decrease of enterococci and Enterobacteriaceae populations. The high dose of ciprofloxacin also induced a decrease in bifidobacteria counts, an increase in levels of resistant B. fragilis group and a significant (P < 0.05) disruption of the colonization resistance of the barrier flora in HFA mice.