The cariogenic characters of xylitol-resistant and xylitol-sensitive Streptococcus mutans in biofilm formation with salivary bacteria

Streptococcus mutans metabolize carbohydrates, such as glucose and sucrose, to produce acid and enhance biofilm formation with the early colonizing bacteria to induce dental caries. Xylitol has been used as a reliable substitute for carbohydrate to inhibit the acid production of S. mutans. However, long-term xylitol consumption leads to the emergence of xylitol-resistance in S. mutans. The aim of this study was to investigate the cariogenic trait of Xylitol-resistant (XR) S. mutans using biofilm formation and coaggregation of xylitol-sensitive (XS) and XR S. mutans with salivary bacteria and their glucosyltransferases expression. When XS or XR S. mutans were incubated in brain heart infusion broth with bacteria from human saliva, XR S. mutans exhibited reduction in biofilm formation in comparison to XS S. mutans. The coaggregation between XR S. mutans and S, gordonii, S. mitis, S. oralis or S. sanguinis was less pronounced than that of XS S. mutans in the presence of sucrose. However, there was no difference in the coaggregation between XR and XS S. mutans in the sucrose-free condition. The level of gtfB and gtfC mRNA expression of XR S. mutans was lower than that of XS S. mutans, whilst the level of gtfD mRNA expression did not differ between the two strains. The reduction of biofilm formation in XS S. mutans due to decrease in glucosyltransferases expression suggests that XR S. mutans may be less cariogenic than XS S. mutans.