Genetic analysis of Streptococcus mutans by nitrosoguanidine-induced mutation in synchronous replication.

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The frequency of nitrosoguanidine-induced mutation for streptomycin (Str)-, rifampicin (Rif)-and bacitracin (Bac)-resistance, and that of colonial mutation accompanied by elevated production of cell-associated insoluble glucan, were measured in Streptococcus mutans strains 102 and GS 5 during synchronous replication after release from chloramphenicol inhibition. A clear peak of mutagenesis for each drug-resistance was observed at certain times during synchronous replication. Peaks appeared in the order of Rif-r, Str-r and Bac-r during synchronous replication. At a definite time after the first peak, there was a second one. The distances between first and second peaks were identical for all markers and approximately 50 min which represents the doubling time of the organisms. The frequency of colonial mutation showed two peaks (P1sA and PlsB) after the peak for Bac-r, each of which was followed by the second peak 50 min after incubation. These results indicate that the method may be a valuable tool for gene analysis of S. mutans, and that the markers for Rif-r, Str-r, Bac-r, P1sA or PlsB in that order duplicate on the chromosomes of S. mutans.