Materials and Methods: This analytical case-control study was performed on 83 preschool children, 4-6 years old. Children were divided into two groups including 41 caries-active and 42 caries-free children. Non-stimulated saliva samples were collected and culture and polymerase chain reaction techniques were used. Statistical analysis was performed using t-test, Chi-square, ANOVA, and Kappa tests.

  Results: S. mutans and S. sobrinus were found in 65% and 21.6% of the samples respectively. S. mutans was isolated from 75.6% of caries-active and 54.8% of caries-free children. Figures for S. sobrinus were 29.2% and 14.3% respectively. Acid susceptibility of microorganisms isolated from saliva was 87.43 in caries-active children and 94.30 for caries-free children. Biofilm formation of microorganisms in caries-active and caries-free children was 0.77 and 0.73, respectively. 

  Conclusion: Frequency of S. mutans in caries-active children was significantly higher than cariesfree children, but the difference in frequency of S. sobrinus was not significant. Acid susceptibility of microorganisms in caries-active children was significantly lower, but the ability to form biofilm was not significantly different in two groups.