Growth of xylitol-resistant versus xylitol-sensitive Streptococcus mutans strains in saliva

Five Streptococcus mutans pairs (serotype c S. mutans 10449 and four clinical isolates of S. mutans: 123.1, LG1, OMFA, T10B) were used to find out if the xylitol-resistant (X^R) natural mutants of the corresponding xylitol-sensitive (X^S) S. mutans parental strains differ in their growth patterns in saliva. The isogenic X^R natural mutants of the parental S. mutans strains were selected after sequential cultivations in the presence of xylitol and glucose. The X^R/X^S strains pairs were grown in individual and pooled glucose-supplemented filter-sterilized salivas (one to five sequential cultivations). The two salivas used represented subjects with good or poor support of the growth of S. mutans in vivo. Protease and peptidase activities were determined from the saliva growth media and cell suspensions. Salivary protein profiles were analyzed using SDS-PAGE and native IEF before and after the cultivations. The growth properties of the X^R/X^SS. mutans pairs were similar in both individual and pooled salivas. Sequential cultivation of all strains did not show any differences in growth patterns. X^S strains were inhibited by the presence of xylitol (2% w/v) in pooled saliva, as shown for other glucose-supplemented media. Protease and peptidase activities of the X^R/X^SS. mutans pairs were low and of similar magnitude. Also, the general hydrolytic properties of most X^R/X^SS. mutans pairs appeared similar as judged by the small growth-induced changes in salivary protein profiles. In conclusion, saliva, the source of nutrients for salivary microorganisms in vivo, favored neither the X^R nor the X^S strains of S. mutans.