Xylitol's effect on cariogenic bacteria.
A) Xylitol's starvation effect.
Cariogenic (decay-causing) bacteria have trouble metabolizing (feeding on) xylitol.
When xylitol (a five-carbon sugar-alcohol) is available to cariogenic bacteria, they transport it through their cellular wall via an uptake system that's intended for fructose (a five-carbon sugar).
During this process, it's transformed into xylitol phosphate, a compound that these bacteria cannot metabolize (break down).
The bacteria starve.
Over time, as the level of unmetabolized xylitol phosphate accumulates within the bacteria, it has the effect of becoming toxic to them. That's because it interferes with metabolic pathways that they rely upon for their nourishment.
This means, a regular exposure of xylitol in a person's mouth creates a situation where, at minimum, cavity-forming bacteria tend to become starved or, at the extreme, die. This creates a situation where the growth and well being of colonies of cariogenic bacteria is impaired.
Why this is beneficial.
A decrease in the number of cariogenic bacteria in dental plaque will correlate with a decreased level of acidic byproducts formed by their digestion of sugars. As a result, the overall acidity of the plaque will be less, thus helping to maintain an environment where tooth decay is less likely to occur.
B) Xylitol-induced population changes.
The presence of xylitol in a person's mouth favors the growth of less virulent strains of cariogenic bacteria.
A person's long-term exposure to xylitol will tend to affect the population size of various types of cariogenic bacteria that live in their mouth.
Within the population of cavity-causing bacteria living in dental plaque, some will have a (genetically) altered fructose uptake system (one that can't transport xylitol). Because of this, these bacteria will not accumulate toxic levels of xylitol phosphate.
The population shift.
This means that these xylitol-resistant bacteria will continue to thrive whereas their xylitol-sensitive counterparts will not. Over time, the numbers of sensitive bacteria will decrease and the numbers of resistant ones will grow and ultimately predominate.
How this is beneficial.
Research suggests that xylitol-resistant strains of cariogenic bacteria are less virulent (less capable of cauing tooth decay) than their counterparts.
One theory suggests that xylitol-resistant bacteria have impaired adhesive properties.
The ability of bacteria to cause decay depends, in part, on their ability to adhere to a tooth's surface. Since xylitol-resistant strains have difficulty in this regard, their ability to create the conditions needed for tooth demineralization is reduced.