FIG. 5.

Effect of novobiocin on the amplification process. The effect of sublethal concentrations of the DNA gyrase inhibitor novobiocin (Novo) on the frequency of appearance of hyperresistant mutants was assayed by determination of the number of CFU per milliliter on plates containing different concentrations of ampicillin. A dose-dependent decrease in the frequency of appearance of hyperresistant mutants was seen in the presence of novobiocin. Data are represented as the logarithm of the mean plus the standard deviation from three independent experiments. The amplification status of the blaA locus was monitored by hybridization with a blaA-specific probe in the hyperresistant mutants obtained in 300 μg of ampicillin per ml and different amounts of novobiocin. Both the frequency and the degree of amplification were affected by increasing amounts of novobiocin. C, control (strain Y56).