A study investigated the effects of xylitol on mutans streptococci in an in vitro 6-species oral biofilm model. Each biofilm contained one of the following: a laboratory reference strain, a fresh isolate, a xylitol-sensitive, or a xylitol-resistant strain of mutans streptococci or Streptococcus sobrinus. The biofilms were grown on hydroxyapatite discs which were pellicle-coated, and were fed with a medium supplemented with glucose/sucrose medium 3 times per day for 45 minutes; between feedings, the biofilms were incubated in saliva. Before or after feeding, biofilms were exposed to either 7.5% xylitol, 7.5% sorbitol or to the control (saliva) for 20 minutes. After 64.5 hours, biofilms were harvested and the microbial composition was analyzed by nonselective and selective culturing. Variability among the strains in terms of their ability to colonize biofilms was observed. Yet, the response patterns in the biofilms to the 4-polyol treatments were similar. None of the mutans streptococci were inhibited by xylitol provided either before or after feeding. Sorbitol given before feeding did not affect microbial growth whereas sorbitol provided after feeding showed a slight, although statistically significant, increase in mutans streptococci counts for some of the tested strains. This occurred at the expense of Streptococcus oralis, which decreased in numbers. The study concluded that the present findings do not support the claims that xylitol reduces mutans streptococci in plaque by futile metabolic cycles.
(Source: Giertsen E, Arthur RA, Guggenheim B. Effects of xylitol on survival of mutans streptococci in mixed-six-species in vitro biofilms modelling supragingival plaque. Caries Research, 2011, Volume 45, Number 1)