Background. Dental caries is considered the most common infectious disease in humans worldwide. Cariogenesis is the outcome of a complex interaction between the host's oral flora and diet. The consumption of snacks such as cake, which have the potential to promote dental caries, has increased.Objectives. The aim of this study was to investigate the effect of including probiotic bacteria (Bacillus coagulans -B. coagulans) in consumed snack cake on the Streptococcus mutans (S. mutans) count and salivary pH.Material and methods. We conducted a randomized, double-blind, cross-sectional cohort study on 40 healthy volunteers. The subjects were divided into 2 groups. In the 1 st group, the subjects consumed probiotic cake as breakfast for 1 week and then, following a 4-week wash-out period, consumed regular cake as breakfast for 1 week. In the other group, the administration of probiotic and regular cake was reversed. For both groups, samples of at least 5 mL of non-stimulated saliva were collected using the spitting technique before and after the 1 st and the 6 th week. A colony counter was used to determine the number of S. mutans colonies. Salivary pH was measured before eating (8-9 a.m.). Results.We detected no statistically significant difference in the S. mutans count before and after the consumption of probiotic cake, but noted a statistically significant difference in the count before and after the consumption of regular cake. We did not detect a significant difference in salivary pH with respect to the consumption of probiotic and regular cake, although the consumption of both foods caused a drop in salivary pH.Conclusions. The addition of probiotic bacteria to sweet snack cake caused a minimal increase in the salivary count of S. mutans, a bacterial species with a definite role in cariogenesis, but did not impact salivary pH. Since probiotic cake has a slight impact on the S. mutans count, it is preferred over regular cake as a snack food. Cite asKoopaie M, Fatahzadeh M, Jahangir S, Bakhtiari R. Comparison of the effect of regular and probiotic cake (Bacillus coagulans) on salivary pH and Streptococcus mutans count.