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.(ABSTRACT TRUNCATED AT 250 WORDS)

Among 139 Streptococcus mutans fresh isolates tested, using the deferred-antagonism test and Streptococcus sanguis Ny 101 as the indicator strain, we observed that the frequency of detection of inhibition zones was reduced by 19% (from 53 to 34%) when arginine (1%) was used in the overlay agar. Among pigmented strains, the frequency of mutacin-like production was 70%. The frequency with which inhibition zones were detected varied from 7 to 91%, depending on the indicator strain used. These results indicate that the ability to detect the presence of mutacin-like substances varies widely and is dependent to a great extent on the methodology used.

A simple procedure for the identification and enumeration, in large bacterial populations, of Streptococcus mutans of both xylitol-resistant and xylitol-sensitive phenotypes was developed. Since xylitol-resistant cells have lost the capacity to take up xylitol from the extracellular medium, due to a defect in their fructose-PTS, there is no uptake of [14C]xylitol by these cells whereas the [14C]xylitol taken up by the sensitive cells is used to visualise them by autoradiography. The method was shown to be suitable for laboratory strains as well as for natural populations of S. mutans ofthe human oral cavity. The in vitro selection of the xylitol-resistant phenotype during consecutive periods of growth at the expense of glucose in the presence of xylitol was followed. Likewise, it was found that the average percentage of xylitol-resistant fresh isolates from two human subjects increased from 0 per cent to 78 per cent over a xylitol-consuming period of 17 d, while it increased from 0 per cent to 8 per cent only in the control subjects who were consuming sorbitol and mannitol instead of xylitol. The superiority of this method resides in the possibility of processing many samples simultaneously. Moreover, it can be applied successfully to the detection and enumeration of other types of PTS mutants if the radioactive xylitol is replaced with a sugar analogue of the defective PTS. A strain of S. salivarius, defective in mannose-PTS, was identified with 2-deo~y-['~C]glucose.

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