A New Method for the Detection and Enumeration of Xylitol-resistant (fructose-PTS-) Strains ofStreptococcus mutansfrom Pure Cultures and Human Saliva

Abstract

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 [ 14 C]xylitol by these cells whereas the [ 14 C]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-deoxy-[ 14 C]glucose. Keywords: S. mutans ; Xylitol; PTS; Detection; Enumeration; Mutant; Dental caries.