EMERGENCE OF MULTIPLE XYLITOL-RESISTANT (FRUCTOSE PTS-) MUTANTS FROM HUMAN ISOLATES OF MUTANS STREPTOCOCCI DURING GROWTH ON DIETARY SUGARS IN THE PRESENCE OF XYLITOL

The growth inhibition of mutans streptococci is one of the proposed me chanisms of action of xylitol, a caries-preventive natural carbohydrat e sweetener. Xylitol is taken up and accumulated as non-metabolizable, toxic xylitol phosphate aia a constitutive fructose PTS, and selects, during in vitro growth at the expense of glucose, for natural xylitol -resistant mutants that lack constitutive fructose PTS activity. Since long-term xylitol consumption leads to the emergence of xylitol-resis tant mutans populations in humans in an oral environment containing su gars of dietary origin, we wanted to test the hypothesis that xylitol- resistant cells could be selected from mutans streptococci strains dur ing in vitro growth on fructose, sucrose, or lactose. Three laboratory strains and three fresh mutans streptococcal isolates were repeatedly transferred in trypticase-yeast extract medium supplemented with gluc ose, fructose, sucrose, or lactose in the presence and absence of xyli tol. Depending on the growth sugar, the presence of xylitol resulted i n the selection of xylitol-resistant populations for several of the si x strains tested, but not necessarily in the presence of all four suga rs. All six strains rapidly became xylitol-resistant when grown on glu cose in the presence of xylitol. All three fresh isolates became xylit ol-resistant after 9 to 16 transfers in the presence of fructose or su crose plus xylitol, while none of the laboratory strains became xylito l-resistant after 16 transfers in the presence of these sugars. The gr owth rates of 12 xylitol-resistant mutants in the presence of eight su gars suggested the existence of various types of xylitol-resistant mut ants. The data partially explain the occurrence of xylitol-resistant m utans populations in long-term xylitol consumers and suggest a mechani sm consistent with a selection process. Since various preliminary resu lts suggest that xylitol-resistant natural mutants may be less virulen t and less cariogenic than their parent strains, this selection proces s may alter, for the better, the mutans streptococci population of the plaque and play a role in the caries-preventive action of xylitol.