Acidogenicity and acidurance of fluoride-resistant Streptococcus sobrinus in vitro.

OBJECTIVE

To induce fluoride-resistant mutants of Streptococcus sobrinus in vitro and to study the acidogenic and acid tolerance of fluoride-resistant strains.

METHODS

Fluoride-resistant mutants 6715-FR and S1-FR were obtained by subculturing S. sobrinus 6715 and freshly isolated S. sobrinus SSMU-S1 to media TSA or TSB, which contained different concentrations of fluoride. The lactic production and growth of fluoride-resistant strains and their respective parent strains were measured by gas chromatography and ultra-violet visible spectrophotometer.

RESULTS

The stable fluoride-resistant mutants of S. sobrinus had been induced successfully in vitro. The acid production of 6715-FR or S1-FR was significantly greater than their parent strains in the presence of 0.5 or 5.0 mM fluoride. In the absence of fluoride, there were 2 situations: The lactic production of F-resistant S. sobrinus was less than or equal to its parent strains when initial pH value was above 5.0 while the result was the opposite when pH value was below 5.0. The F-resistant strains and the parent strains of S. sobrinus could reduce the suspension pH to identical values in the absence of fluoride, and the F-resistant strains were more aciduric in the presence of fluoride.

CONCLUSIONS

The F-resistant strains of S. sobrinus had higher acidogenic and aciduric ability than the wild-type strains, and were expected to have stronger cariogenic potential at plaque fluoride levels.