Mutans streptococci, in particular Streptococcus mutans and Streptococcus sobrinus, are generally considered to be the principal microbial pathogen of dental caries. The objective of the study was to isolate S. mutans and S. sobrinus, identify them by PCR, and to compare their presence with the caries status and caries risk in Mongolian preschool and school children. Forty one preschool children aged 3-5 years and 40 school children aged 12-15 years were enrolled in this study. As assessed using Cariostat test, 75.6% of preschool children had high caries risk and 37.5% of school children had high caries risk. In preschool children, the prevalence of S. mutans and S. sobrinus were 100% and 36.6%, respectively; 63.4% were positive for S. mutans alone and 36.6% were positive for both S. mutans and S. sobrinus. In school children, the prevalence of S. mutans and S. sobrinus were 100% and 25.0%, respectively; 75.0% carried S. mutans alone and 25.0% had both S. mutans and S. sobrinus. The percentage of children positive for both S. mutans and S. sobrinus in the high caries risk group were significantly higher than those in the low risk group of either preschool (42.0% vs. 10.0%, P,0.001) or school children (46.6% vs. 12.0%, P,0.001). Moreover, the caries status of children positive for both S. mutans and S. sobrinus were significantly higher than those positive for S. mutans alone (P,0.01 for preschool children, and P,0.05 for school children). traits such as the synthesis of extracellular polysaccharides from sucrose, lactic acid production by metabolism of dietary carbohydrates and aciduricity, which are fundamental to their virulence 2-8). Various caries activity tests, cultivation, enzyme assays, enzyme-linked immunosorbent assays and species-specific DNA probes have been used for caries prediction and detection of putative pathogens 9-13). One of these caries activity tests, the Cariostat ® test, developed by Shimono, is colorimetric test based on the evaluation of acidogenicity of plaque microorganisms 13) , and has been proven to be a good predictor of dental caries, especially in young children 13-16) .
