For resistance induction, an initial bacterial suspension of E.coli ATCC 25922 was prepared at a concentration of 0.5McFarland. This was sown onto 50brain infusion broth agar (Becton Dickinson, StLouis, Missouri) dishes using a calibrated 10μl loop and aerobically incubated at 37°C for 18h to 24h. The resulting colonies were then diluted in brain infusion broth (Becton Dickinson, StLouis,Missouri)andsownonbraininfusion agar (Becton Dickinson, StLouis, Missouri) with an initial enrofloxacin concentration of 0.008μg/ml, which was doubled up at each step to 655.36μg/ml (11thstep, T11) in which resistantstrainswereobtained