Abstract: In this study, SPF mice model has been developed to study the effects of enrofloxacin on fecal aerobic and anaerobic populations, selection of bacteria resistant to ciprofloxacin, maintenance of the intestinal barrier against exogenous E.coli. Enrofloxacin was administered via drinking water(1, 10 and 100 mg·L-1) to SPF femal mice for 48 d. Total aerobic flora counts, Enterococci counts and Bacteroides counts decreased significantly following exposure to 100 mg·L-1 enrofloxacin. Total aerobic flora counts were also affected by 10 mg·L-1 enrofloxacin. Enrofloxacin exposure at 1, 10 and 100 mg·L-1 increased the percentage of aerobic populations resistant to 1 ?g·ml-1 ciprofloxacin. 100 mg·L-1 enrofloxacin also increased the percentage of Bacteroides resistant to 16 ?g·ml-1 ciprofloxacin. The resistance colonization of some SPF mice was impaired at 1, 10 and 100 mg·L-1 enrofloxacin.The results indicated that enrofloxacin can partly decrease the aerobic populations, but pose little effects on anaerobic populations. In conclusion, the effects of low concentrations enrofloxacin on SPF mice fecal bacteria population and resistance colonization were not significant, the most important impact of low concentrations enrofloxacin on SPF mice intestinal microflora may be resistance selection and increase the percentage of resistant aerobic flora to ciproflxacin. The ADI of enrofloxacin and ciprofloxacin established by Ministry of Agriculture may affect human intestinal microflora by selecting resistant aerobic flora.

Key words: Residues, Intestinal microflora, Enrofloxacin, SPF mice