FIG. 1.

Time course of induced tetracycline resistance of E. coli AG100 (solid circles) and AG100A (open triangles) strains and the reversal of induced resistance by transfer to drug-free medium. Strains were induced to tetracycline resistance (+TET) by serial transfer of inoculae from cultures that contained the highest concentration of the antibiotic under which the strains grew to media containing increasing concentrations of the drug and incubated until they yielded prominent evidence of growth. Reversal of resistance was induced by serial transfer to drug-free medium (-TET). MICs were periodically determined and confirmed in solid media by the tetracycline E-test (AB Biodisk, VIVA Diagnostica, Huerth, Germany) according to the manufacturer's instructions.