Sequential mutagenesis of drug resistance in Streptococcus mutans during synchronous replication

The frequency of nitrosoguanidine-induced mutation for streptomycin-, bacitracin- and rifampicin-resistance was measured in Streptococcus mutans during synchronous replication after release from chloramphenicol inhibition. A clear peak of mutagenesis for each marker was observed at certain times during synchronous replication. These times were different for individual markers; the times of peaks for streptomycin-, bacitracin- and rifampicin-resistance were 13, 22 and 12 min, respectively. At a definite time after the first peak, there was a second one. The distances between the first and second peaks during the synchronous replication were identical for all markers and approximately 50 min which represents the doubling time of the organisms. These results indicate that nitrosoguanidine causes sequential mutagenesis for these three markers in Streptococcus mutans when the growth is resumed after chromosome alignment, so that the methods may be useful in determining the sequence of gene replication for various markers in Streptococcus mutans.