In Vitro Flouride Resistance in a Cariogenic Streptococcus

Author Institution: Department of Botany and Microbiology, University of ArkansasStable fluoride-resistant mutants of Streptococcus mutans GS-5 were isolated with a stepwise selection procedure. First-step mutants were isolated at a frequency of 6.4 X 10~10 and demonstrated six maximal levels of resistance ranging from 400-1000 ug/ml sodium fluoride. Second-step mutants with higher levels of resistance were isolated at a frequency of 1.4 X 10~8. Second-step mutants demonstrated two maximal levels of resistance, 1600 and 3000 ug/ml sodium fluoride. Other than fluoride resistance, the characteristics of both first- and second-step mutants were similar to those of the parental strain. Growth rates did differ, however. First-step mutants exhibited slightly longer mass doubling times than the parental strain (average of 50 vs 45 min, respectively). Second-step mutants exhibited substantially longer mass doubling times (average of 71.5 min). The results suggest that fluoride resistance may be regulated by more than one gene, and that high levels of resistance may be due to a cumulative effect of at least two genes