Isolation and characterization of Streptococcus mutans mutants defective in adherence and aggregation

A method was developed which enriched for mutants of Streptococcus mutants that exhibit defects in adherence to glass, aggregation, or both. Mutants were isolated from derivatives of strains PS14 (serotype c) and 6715 (serotype g) after mutagenesis with either ethyl methane sulfonate or nitrous acid. Cell survival after mutagenesis was kept above 1 to 2% to enhance the probability that mutants resulted from single mutational events. A total of 117 mutants were isolated; they also displayed non-wild type colony morphology on mitis salivarius agar. These mutants were examined for adherence and aggregation after overnight growth in sucrose-containing medium, aggregation of nongrowing cells in the presence of 200 μg of surcrose per ml or 20 μg of dextran per ml, and dextranase production on blue dextran agar plates. Although we isolated mutants which exhibited a variation from the parent strain in only one of the traits tested, the majority of mutants exhibited defects in two or more characteristics. Thirty-eight stable mutants of independent origin were categorized into 13 separate phenotypic groups.