| Antimicrobial residues in tissue of food-producing animals could pose a risk on human intestinal flora. Ciprofloxacin was applied widely, which has resulted in lots of drug-resistance bacteria appearance. Present study was designed to assess effects of low levels of ciprofloxacin on representative human gut bacteria and its resistance a in vitro gut model in which both the antibacterial and test strains were exposed to conditions by vibrating simulating the human gastrointestinal tract, and two resistance induction models with no digestion in gut. The in vitro gut model used ATCC25922, ATCC29212 and isolates of Escherichia coli sensitive strains & Salmonella spp sensitive strains from the feces of healthy volunteer as indicator strains. The amount of ATCC25922 & ATCC29212 inoculated were 10~5CFU/mL, 10~6CFU/mL & 10~8CFU/mL and the amount of isolates were 10~6CFU/mL. In the no gut model, the amount of ATCC25922 & ATCC29212 were 10~6CFU/mL. The concentrations of ciprofloxacin were respectively 0.1, 0.5, 2 μg/mL and control groups with no drug in all test models. All bacteria were cultivated anaerobically at 37℃ for one day or seven successive days. The sensitivity of test strains to ten antimicrobial (ciprofloxacin, cefalexin. enrofloxacin, penicillinum G, gentamycin sulfate, tetracycline, erythromycin, ampicillin, hloramphenicol, sulfamethazine) were tested on solid media by the agar doubling dilution procedure recommended by NCCLS, and their viable number after one day culture were counted on plate by fold serial dilution technique. After the treatment, the gyrA of the ciprofloxacin-resistant bacteria were amplified by polymerase chain reaction. Their DNA sequence alignments were analyzed to study the mechanism of antibiotic resistance.Escherichia coli & Salmonella spp were restrained in the two models when the concentration of ciprofloxacin was above 0.1 u g/mL, however Enterococcus spp were not evidently restrained. Escherichia coli & Salmonella spp induced for seven days were killed when the concentration of ciprofloxacin was above 0.5 u g/mL in the no gut models. Escherichia coli or Salmonella spp whose amount inculated were 105CFU/mL and 106CFU/mL were killed too, when the concentration of ciprofloxacin was 2 u g/mL. Those Escherichia coli and Salmonella spp which were not killed could produce antibiotic resistance to ciprofloxacin & enrofloxacin, but could not produce antibiotic resistance to other antibiotics. Ciprofloxacin whose concentration was 0.1 u g/mL did not make the sensitivity of Enterococcus spp change much. The gyrA of the antibiotic-resistant bacteria had mutation points. The mutation points led the 83rd and 87th amino acids of GyrA to change. The results showed that the sensitivity of bacteria to ciprofloxacin was lower before the treatment, and its antibiotic resistance could not be induced. The concentrations of ciprofloxacin had possive relativity with antibiotic resistance to Escherichia coli & Salmonella spp; The length of cultivation had possive relativity, too. But the amount of the bacteria initially inoculated had negative relativity. The complex gut environment could reduce the effect of ciprofloxacin. The antibiotic-resistant bacteria had mutation on the level of molecule.In a word, the low levels of ciprofloxacin have antibiotic resistance selective pressure. But the effect of low levels of ciprofloxacin on the selection of resistance in intestinal bacteria become very feeble in the complex environment for the complex gut environment can reduce the effect of ciprofloxacin. So the acceptable daily intake of ciprofloxacin drafted by China may be appropriate at the present time. However, more studies should be developed on low levels of ciprofloxacin resistance to human gut bacteria.
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