FIG 5.

Ciprofloxacin promotes bidirectional phenotype-switching. (A) Wild-type SH1000 and a umuC mutant were grown in the absence (□) or presence (■) of a subinhibitory concentration of ciprofloxacin for 16 h, and the frequency of gentamicin-resistant SCVs was determined. The data represent the mean average of 10 independent cultures, and error bars represent the standard deviations. Values which differ from those seen in media lacking ciprofloxacin are highlighted (*). (B) Stability of SCV isolates that arose in wild-type SH1000 populations in the presence of ciprofloxacin (n = 6). The data are presented and analyzed as described in the legend to Fig. 4. (C) The frequency of SCV reversion to the wild type was determined in the absence (CTL) or presence of ciprofloxacin (Cipro) or paraquat (Para). Reversion rates that differ from the those found on media without supplements are highlighted (*). (D) Stability of SCVs that arose in the umuC background in the absence of oxidants or ciprofloxacin (n = 8).