RESUMO
The acquisition of resistance to amoxicillin, tetracycline, and enrofloxacin by Escherichia coli MG 1655 was examined by exposing growing cells to constant or stepwise increasing concentrations of these compounds. The minimal inhibitory concentration (MIC) of E. coli for amoxicillin increased from 4-8 to 32 µg/ml after growth in the presence of 1.25 or 2.5 µg/ml. By stepwise increasing the exposure, an MIC of 512 µg/ml was reached. This high MIC was maintained after removal of the antibiotics, whereas the lesser increase after exposure to low levels was reversed, indicating that the high MIC was due to a genetic change, but the lower one to phenotypic adaptation only. The MIC for tetracycline increased from 2 µg/ml to maximally 32 µg/ml. The MIC decreased to control levels in the absence of tetracycline, so no genetic changes seem to have occurred. The MIC for enrofloxacin increased from 0.25 µg/ml to maximally 512 µg/ml depending on the concentration during growth. These data mostly support the "radical-based" theory that bactericidal antibiotics induce a common mechanism that contributes to cell killing. Our findings indicate that exposure to low levels of antibiotics causes an increase in MIC above the concentration that the cells were exposed to. The implication is that exposure to low levels of antibiotics should be prevented as much as possible, because this causes resistance far more than high concentrations that inhibit growth or kill the cell and thus prevent acquisition of resistance.