FIG. 8.

Analysis of mutation frequencies. (A) YB955 and isogenic strains of YB955 carrying the comA and/or comK allele were tested for their ability to produce His⁺, Met⁺, and Leu⁺ revertants during exponential growth as described in Materials and Methods. The revertants were scored and recorded 48 h after plating. The median (r) is the mean of the 19th and 20th values of r (observed number of mutants per culture in a 38-test-tube fluctuation test) when the r's are ranked. The number of mutations per culture (m) was calculated with the Lea-Coulson formula, r/m − ln(m) = 1.24. Three parallel cultures were used to determine the N_t by titration. The mutation rates were calculated with the formula m/2N_t (45). Results presented are the average mutation rates from three individual fluctuation tests. Error bars represent 1 standard error. (B) YB955 and His⁺, Met⁺, and Leu⁺ revertants (that arose on the sixth day following plating) were tested for their abilities to generate growth-dependent spontaneous rifampin resistance mutations (see Materials and Methods for details). The mutation rates for rifampin resistance were also determined by the method described above for panel A but with a 40-test-tube fluctuation test. The rifampin-resistant bacteria were selected on TBAB containing 5 μg of rifampin/ml (Fluka Biochemicals). After 24 h of incubation at 37°C, the number of rifampin-resistant colonies was determined. Results presented are the average mutation rates from three individual fluctuation tests. Error bars represent 1 standard error.