E. coli nalidixic acid survival curves. As observed for ampicillin, the survival rates at low nalidixic acid concentrations are too high to be accounted for by spontaneous mutation. Cells surviving prior antibiotic exposure show dramatically improved resistance rates, yet reversion to antibiotic sensitivity is very common, with over 95% of cells from 40 μg/ml nalidixic acid not surviving re-exposure to the same antibiotic concentration. No previous nalidixic acid exposure, triangles. From 40 μg/ml nalidixic acid, circles.