Fluoride‐sensitivity of growth and acid production of oral <i>Actinomyces</i>: comparison with oral <i>Streptococcus</i>

<jats:title>ABSTRACT</jats:title><jats:sec><jats:label /><jats:p><jats:italic>Actinomyces</jats:italic> are predominant oral bacteria; however, their cariogenic potential in terms of acid production and fluoride sensitivity has not been elucidated in detail and compared with that of other caries‐associated oral bacteria, such as <jats:italic>Streptococcus</jats:italic>. Therefore, this study aimed to elucidate and compare the acid production and growth of <jats:italic>Actinomyces</jats:italic> and <jats:italic>Streptococcus</jats:italic> in the presence of bicarbonate and fluoride to mimic conditions in the oral cavity. Acid production from glucose was measured by pH‐stat at pH 5.5 and 7.0 under anaerobic conditions. Growth rate was assessed by optical density in anaerobic culture. Although <jats:italic>Actinomyces</jats:italic> produced acid at a lower rate than did <jats:italic>Streptococcus</jats:italic>, their acid production was more tolerant of fluoride (ID<jats:sub>acid production</jats:sub> 50 = 110–170 ppm at pH 7.0 and 10–13 ppm at pH 5.5) than that of <jats:italic>Streptococcus</jats:italic> (ID<jats:sub>acid production</jats:sub> 50 = 36–53 ppm at pH 7.0 and 6.3–6.5 ppm at pH 5.5). Bicarbonate increased acid production by <jats:italic>Actinomyces</jats:italic> with prominent succinate production and enhanced their fluoride tolerance (ID<jats:sub>acid production</jats:sub> 50 = 220–320 ppm at pH 7.0 and 33–52 ppm at pH 5.5). Bicarbonate had no effect on these variables in <jats:italic>Streptococcus</jats:italic>. In addition, although the growth rate of <jats:italic>Actinomyces</jats:italic> was lower than that of <jats:italic>Streptococcus</jats:italic>, <jats:italic>Actinomyces</jats:italic> growth was more tolerant of fluoride (ID<jats:sub>growth</jats:sub> 50 = 130–160 ppm) than was that of <jats:italic>Streptococcus</jats:italic> (ID<jats:sub>growth</jats:sub> 50 = 27–36 ppm). These results indicate that oral <jats:italic>Actinomyces</jats:italic> are more tolerant of fluoride than oral <jats:italic>Streptococcus</jats:italic>, and bicarbonate enhances the fluoride tolerance of oral <jats:italic>Actinomyces</jats:italic>. Because of the limited number of species tested here, further study is needed to generalize these findings to the genus level.</jats:p></jats:sec>