Fluoride resistance and adherence of selected strains of Streptococcus mutans to smooth surfaces after exposure to fluoride.

The fluoride resistance and smooth surface adherence characteristics of Streptococcus mutans were examined using tooth model and radioactive cell assays. Resistance to 600 ppmF by S. mutans isolated from the plaque of radiation-induced xerostomia patients receiving daily topical applications of a caries preventive 1% NaF gel was transient. Resistance induced in vitro in two strains of S. mutans by exposure to gradually increasing levels of NaF was apparently permanent. Smooth surface adherence by both fluoride-sensitive and -resistant strains of S. mutans 6715 in a tooth model system was slightly diminished by 1% NaF gel. Fluoride-resistant strains retained 89 to 93% of their adherence capability in 600 ppmF, as determined by the cell radiolabeling assay.