Rapid induction of resistance to the fluoroquinolone enrofloxacin in Pseudomonas putida NCTC 10936.

22 23 Fluoroquinolones are deemed indispensible antibiotics. However, a large number 24 of microbes rapidly becomes resistant against them upon exposure. This study focuses on 25 the relationship between exposure to low concentrations of enrofloxacin, such as often 26 encountered in an agricultural setting, and the resulting induction of resistance in 27 Pseudomonas putida NCTC10936. Low levels of enrofloxacin induced resistance to 28 much higher concentrations after 12-15 generation times. Upon exposure to non-lethal 29 concentrations, the MIC increased by a factor of 10, regardless of the dose of the 30 antibiotic. Both growth rate and the final density reached in batch cultures were affected 31 by enrofloxacin. All data can be explained by assuming that in a stochastic process 32 single, as yet undetermined, mutations occur during growth at low concentrations of 33 enrofloxacin. These results indicate that the exposure to low levels of fluoroquinolones 34 cause an unusually high risk of inducing resistance. 35