Luc Trahan

Transport and Phosphorylation of Xylitol by a Fructose Phosphotransferase System in Streptococcus mutans

The purpose of this work was to explain how the caries-preventive agent xylitol interferes with the growth of Streptococcus mutans. It was found that the xylitol-sensitive strain of S. mutans 27352 (serotype g) and LG1 (serotype c) took up 14C-xylitol when the labelled pentitol was added to cells growing at the expense of glucose. Uptake of xylitol by growing cells of S. mutans 27352 XR and LG1 XR, two xylitol-insensitive spontaneous mutants, and of S. mutans GS5-2, which was also insensitive to xylitol, was practically inexistent under the same conditions. Alkaline phosphatase treatment followed by enzymatic analysis and thin-layer chromatography revealed that the accumulated product was xylitol phosphate. Intracellular concentrations of 5–7 mM for resting cells and of up to 60 mM for growing cells were calculated. Xylitol was phosphorylated at the expense of phosphoenolpyruvate by toluenized cells of S. mutans LG1, but not by toluenized cells of GS5–2 and S. mutans LG1 XR. The phosphorylation of xylitol was dependent on phosphoenolpyruvate and required the presence of both soluble and membrane cellular fractions in the reaction mixture. This indicated that xylitol was transported and phosphorylated by a phosphoenolpyruvate: sugar phosphotransferase system. The phosphoenolpyruvate-dependent phosphorylation by isolated membranes of S. mutans LG1 in the presence of the soluble fraction was inhibited by fructose but not by glucose, mannose or galactose. Measurement of phosphoenolpyruvate: phosphotransferase activities in isolated membrane revealed that strain 27352 and LG1 had activities for fructose and xylitol; membrane from 27352 XR and LG1 XR had very little activity for fructose and xylitol. It was concluded that xylitol was transported and phosphorylated by a constitutive phosphoenolpyruvate:fructose phosphotransferase system in S. mutans. The data suggested that xylitol toxicity in S. mutans is caused by the intracellular accumulation of xylitol phosphate.

Effect of Xylitol Consumption on the Plaque-Saliva Distribution of Mutans Streptococci and the Occurrence and Long-term Survival of Xylitol-resistant Strains

Since the exposure of mutans streptococci to xylitol is known to select for xylitol-resistant (XR) natural mutants, the occurrence and long-term survival of such xylitol-resistant strains was evaluated in a cross-sectional sampling of participants of the Ylivieska xylitol study four years after the original two-year experimental period. Paraffin-stimulated whole saliva was first collected, and then plaque was collected and pooled. The salivary and dental plaque mutans streptococci were enumerated after growth on TSY20B agar. The proportion of XR strains was determined by autoradiography with 14C-xylitol. A strong and significant correlation (r = 0.645 and p = 0.005) between the number of mutans streptococci in saliva and in dental plaque was observed in non-consumers of xylitol. Such a correlation totally disappeared (r = 0.098 and p = 0.612) in xylitol-exposed consumers (habitual and former xylitol-consumers). The proportion of the salivary XR mutants (35%) in non-consumers (n = 16) was significantly lower than in the xylitol-exposed consumers (79%) (n = 27), (p = 0.0001) or in former consumers (75%) (n = 13), (p = 0.0008) or in the habitual consumers (83%) (n = 14), (p = 0.004). The proportion of XR mutants in dental plaque was, on the average, much lower than in the corresponding saliva. The proportion of XR in the plaque of xylitol non-consumers was half of that of the xylitol-exposed group, but the difference was not statistically significant. Taken together, the results suggest that: (1) xylitol consumption modifies the mutans streptococci distribution between the dental plaque and the saliva; (2) once selected by the habitual use of xylitol and established in the mouth, the XR natural mutant population maintains itself for up to four years after the withdrawal of the selective agent or at least under minimal and irregular consumption of the agent; and (3) an unknown characteristic of the XR mutants allows them to be more easily shed into the saliva from the plaque than the xylitol-sensitive parental strains.

Selection for Streptococcus mutans with an Altered Xylitol Transport Capacity in Chronic Xylitol Consumers

The effect of long-term consumption of refined xylitol on the natural populations of S. mutans in the human oral cavity has been investigated. Fifty four S. mutans strains were isolated from adults and children who had been consuming commercial food products containing xylitol for a period of from 1½ to 10 years. Twenty isolates were also obtained from control subjects who had never consumed xylitol-containing commercial food products. The inhibitory effect of xylitol on the isolated strains was determined by monitoring growth on glucose in the presence or absence of xylitol. This was used to define the sensitivity of each isolate to xylitol. Phosphoenolpyruvate:sugar phosphotransferase (PEP-PTS) activities were measured by means of the soluble and membrane fractions prepared from strains from both study populations. It was found that 87% of the fresh isolates from xylitol consumers were xylitol-resistant (XR), compared with only 10% of the strains isolated from the control subjects. The XR strains had low constitutive fructose PTS activity and very low xylitol-phosphorylating capacity. The xylitol-sensitive (Xs) strains, however, had much higher levels of constitutive fructose PTS activity and phosphorylated xylitol 16 times more rapidly than did the XR strains. Evidence for the phosphorylation of xylitol by a fructose PEP-PTS in the Xs strains was obtained. The growth inhibition by the intracellular accumulation of non-metabolizable toxic xylitol phosphate and its prevention by the presence of fructose are discussed.

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 mechanisms of action of xylitol, a caries-preventive natural carbohydrate sweetener. Xylitol is taken up and accumulated as non-metabolizable, toxic xylitol phosphate via 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-resistant mutans populations in humans in an oral environment containing sugars of dietary origin, we wanted to test the hypothesis that xylitol-resistant cells could be selected from mutans streptococci strains during 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 glucose, fructose, sucrose, or lactose in the presence and absence of xylitol. Depending on the growth sugar, the presence of xylitol resulted in the selection of xylitol-resistant populations for several of the six strains tested, but not necessarily in the presence of all four sugars. All six strains rapidly became xylitol-resistant when grown on glucose in the presence of xylitol. All three fresh isolates became xylitol-resistant after 9 to 16 transfers in the presence of fructose or sucrose plus xylitol, while none of the laboratory strains became xylitol-resistant after 16 transfers in the presence of these sugars. The growth rates of 12 xylitol-resistant mutants in the presence of eight sugars suggested the existence of various types of xylitol-resistant mutants. The data partially explain the occurrence of xylitol-resistant mutans populations in long-term xylitol consumers and suggest a mechanism consistent with a selection process. Since various preliminary results suggest that xylitol-resistant natural mutants may be less virulent and less cariogenic than their parent strains, this selection process may alter, for the better, the mutans streptococci population of the plaque and play a role in the caries-preventive action of xylitol.

Positive selection for resistance to 2-deoxyglucose gives rise, in Streptococcus salivarius, to seven classes of pleiotropic mutants, including ptsH and ptsI missense mutants.

We have used the toxic non‐metabolizabie glucose/ mannose analogue 2‐deoxygiucose to isolate a comprehensive collection of mutants of the phosphoenoipyruvate:sugar phosphotransferase system from Streptococcus salivarius. To increase the range of possible mutations, we isolated spontaneous mutants on different media containing 2‐deoxyglucose and various metabolizable sugars, either lactose, meli‐biose, galactose or fructose. We found that the frequency at which 2‐deoxygiucose‐resistant mutants Were isolated varied according to the growth substrate. The highest frequency was obtained with the combination galactose and 2‐deoxygiucose and was 15‐fold higher than the rate observed with the mixture melibiose and 2‐deoxygiucose, the combination that gave the lowest frequency. By combining results from: (i) Western biol analysis of IIIMan, a specific component of the phosphoenolpyruvate:mannose phosphotransferase system in S. salivarius; (ii) rocket immunoelectrophoresis of HPr and EI, the two general energy‐coupling proteins of the phosphotransferase system; and (iii) from gene sequencing, mutants could be assigned to seven classes. Class 1 was composed of strains devoid of IIIManL, a low‐molecular‐weight form of IIIManL (35200), class 2 was composed of strains exhibiting a reduced level of IIIManL, class 3 was composed of strains devoid of both forms of IIIMan (IIIManL as well as IIIManH, the high‐molecular‐weight form of IIIMan (38900)), class 4 was composed of mutants bearing a mutation in ptsH, the gene encoding HPr, class 5 was composed of mutants bearing a mutation in ptsl, the gene encoding EI, class 6 was composed of 2‐deoxygiucose‐resistant strains without any apparent defect in PTS components, and class 7 was composed of strains possessing both forms of IIIMan but abnormal levels of HPr and/or EI without any mutation in the ptsH and/or the ptsI genes. Preliminary characterization of representative strains of each class is reported.

Control of sugar utilization in the oral bacteria Streptococcus salivarius and Streptococcus sanguis by the phosphoenolpyruvate: Glucose phosphotransferase system

Three different Strep. salivarius (G2, G5 and G29) and two Strep. sanguis (GS3 and GS12) mutants affected in the phosphoenolpyruvate: glucose phosphotransferase system were selected on agar plates containing lactose and 2-deoxyglucose. All 5 were defective in a membrane-bound component of the transport system and grew less rapidly than the parent strain in 5 mM glucose-containing medium. Mutants G2 and G29 grew poorly in the presence of 5 mM mannose. Growth on mixed substrates revealed that the mutants and wild-type parents behaved differently. Wild-type strains in medium containing glucose plus another sugar (lactose, galactose, melibiose, raffinose or trehalose for Strep. salivarius and lactose, galactose or trehalose for Strep. sanguis) always exhausted most of the glucose before utilizing the other sugar. The mutants used the second sugar concurrently or preferentially to glucose. In medium containing glucose plus fructose or mannose, the wild types consumed both sugars concurrently whereas the mutants utilized the second sugar before glucose. Mutants G2 and G5 were insensitive to repression by fructose and released glucose into the medium when grown in the presence of 0.4 per cent lactose. Mutant G5 also released galactose. Sugar release was not detected with the wild types. The Strep. salivarius mutants contained normal levels of glucokinase and beta-galactosidase but G5 was almost totally devoid of galactokinase activity after growth on lactose. On galactose, the activity was restored. It seems that the phosphoenolpyruvate: glucose phosphotransferase system is involved in the regulation of sugar utilization in these two streptococci.

Alterations in the cellular envelope of spontaneous IIIManL-defective mutants of Streptococcus salivarius

In Streptococcus salivarius, the phosphoenolpyruvate: mannose phosphotransferase system (PTSMan) transports and concomitantly phosphorylates mannose, glucose, fructose and 2-deoxyglucose. PTSMan consists of a membrane Enzyme II and two forms of Enzyme III (IIIMan) having molecular masses of 38.9 kDa (IIIManH) and 35.2 kDa (IIIManL) respectively. We have previously reported the isolation of spontaneous mutants lacking IIIManL, and showed that they exhibited abnormal growth when cultured in mixtures of sugars containing glucose. The mutants also synthesize several cytoplasmic glucose-repressible proteins during growth on glucose and some of them constitutively express a fructose PTS which is induced by fructose in the parental strain. We have now investigated the properties and composition of the cellular envelope of three S. salivarius IIIManL-defective mutants (strains A37, B31 and G29) after growth on glucose. The mutants have altered sensitivity to various toxic compounds that interfere with cell-envelope functions. The mutants also exhibited altered membrane-protein profiles when analysed by two-dimensional PAGE and modified total lipid and phosphorus contents and lipid/protein ratio. In one mutant (strain G29), the proportion of the phospholipids separated by TLC was different from the parental strain. Electron microscopy indicated that one mutant (strain A37) possessed more fimbriae than the parental strain. The results suggested that these IIIManL-defective mutants were affected in a global regulatory gene controlling several cellular or physiological functions, many of these being related to the cellular envelope.

Inactivation of the Streptococcus mutans fxpC Gene Confers Resistance to Xylitol, a Caries-preventive Natural Carbohydrate Sweetener

Xylitol is transported by Streptococcus mutans via a constitutive phosphoenolpyruvate:fructose phosphotransferase system (PTS) composed of a IIABC protein. Spontaneous xylitol-resistant strains are depleted in constitutive fructose-PTS activity, exhibit additional phenotypes, and are associated with the caries-preventive properties of xylitol. Polymerase chain-reactions and chromosome walking were used to clone the fxp operon that codes for the constitutive fructose/xylitol-PTS. The operon contained three open reading frames: fxpA, which coded for a putative regulatory protein of the deoxyribose repressor (DeoR) family, fxpB, which coded for a 1-phosphofructokinase, and fxpC, which coded for a IIABC protein of the fructose-PTS family. Northern blot analysis revealed that these genes were co-transcribed into a 4.4-kb mRNA even in the absence of fructose. Inactivation of the fxpC gene conferred resistance to xylitol, confirming its function. The fxp operon is also present in the genomes of other xylitol-sensitive streptococci, which could explain their sensitivity to xylitol.

Xylitol-resistant mutans streptococci strains and the frequency of xylitol consumption in young children

In clinical studies, the beneficial effects of xylitol have been demonstrated in association with its daily, habitual use [1]. Furthermore, regular maternal xylitol consumption has been shown to decrease transmission of mutans streptococci (ms) to the child [2]. Regular xylitol consumption has been connected with selection of ms which have lost their sensitivity to the inhibitory action of xylitol [3]. In rat experiments, these so-called “xylitol-resistant” (Xr) ms strains have been shown to be less cariogenic than the “xylitolsensitive” (Xs) ms strains [4]. In fact, several in vitro studies suggest that the Xr strains may be less virulent than the Xs strains of ms [5], while other in vitro studies have shown no difference in the virulence properties of these strains [6,7]. Very little is known about the frequency, quantity, and duration of xylitol consumption needed to select the Xr strains of ms in the oral cavity. Dental plaque is the major biofilm in the oral cavity. The present study aimed to investigate associations between prevalence of Xr strains in dental plaque and the frequency of xylitol use in young children.

A Comparative Study of Enzymes Involved in Glucose Phosphorylation in Oral Streptococci

The properties of two enzymes involved in the phosphorylation of glucose were studied in three oral streptococci species. The glucokinase of Streptococcus mutans had a lower affinity for glucose and ATP than did those from S. salivarius and S. sanguis. The enzyme had an identical pH optimum (pH 8.0) in all three bacteria. However, the results from the phosphoenolpyruvate phosphotransferase system showed a different pattern when its activity was measured using 2-deoxyglucose with toluenized cells. Uptake studies of 2-deoxyglucose also revealed that the three microorganisms had different affinities for this compound. This glucose analogue strongly inhibited the acid production of S. salivarius, but did not affect the glycolysis of the other two bacteria.