Title: Alternative Sweeteners

Xylitol 347
grow and metabolize optimally in a relatively acidic environment, and to live
under microaerobic or strictly anaerobic conditions, as met in the depth of pits
and fissures (134–137).
Several studies have suggested that the predominance of S. mutans in cariogenic
plaque depends on the ability of this bacterium to remain metabolically
active even in a relatively acidic environment. In fact, S. mutans is more active
at pH 5 than at pH 7, whereas many other members of the plaque flora become
metabolically inactive under such conditions. This observation suggests that the
frequent ingestion of surcrose gives a competitive edge to S. mutans over the
other plaque microbes. In this way, a circulus vitiosus for the formation of a
more cariogenic plaque is formed (138–140).
In this regard, the use of xylitol as a sucrose substitute becomes an extremely
attractive means to control and prevent dental caries for two different
reasons. First, no acid is formed from xylitol by the dental plaque. In fact, during
and after chewing of a xylitol-sweetened gum, an elevation rather than a decrease
of the plaque pH is observed (141,142). Under such conditions, however, the
metabolism of S. mutans is, as mentioned previously, not optimally active, and
other bacteria may successfully compete with S. mutans. Second, in addition
to this indirect, pH-mediated effect, xylitol appears to inhibit the growth and
metabolism of S. mutans in a more direct way. Several experiments have shown
that the addition of xylitol to a glucose-containing medium reduced the growth
of S. mutans (143–150). This inhibition by xylitol appears to be related to the
accumulation of xylitol-5-phosphate and xylulose-5-phosphate within the cells
(151–156). Probably as a result of the intracellular accumulation of these metabolites,
the ability of S. mutans to adhere to surfaces is decreased (157,158), and
disintegration of the ultrastructure of the cells may occur (159).
In line with these results, lower S. mutans counts were found in the plaque
and/or saliva of xylitol-treated human volunteers (160–166). These observations
suggest that the formation of xylitol-tolerant strains of S. mutans after chronic
xylitol exposure does not annihilate its inhibitory effect (167,168).
E. General Effects on Dental Plaque
Because dental plaque plays a crucial role in the formation of tooth decay, many
investigators have examined the effects of xylitol on dental plaque. It is well
established that frequent consumption of sucrose promotes the growth of a voluminous,
sticky dental plaque. Xylitol is not a substrate for oral microorganisms,
and it therefore does not favor plaque formation. Consequently, studies comparing
the effects of sucrose and xylitol on dental plaque have demonstrated consistently
that the plaque weights are lower in the xylitol-treated subjects than in the
positive controls consuming sucrose (6). Studies suggest that xylitol inhibits the