Background The purpose of this investigation was to evaluate the effect of probiotic on Streptococcus mutans and multi-species biofilms isolated from children with severe caries.Material 20 caries-active children (DMFS≥6) were selected as experimental group, unstimulated whole saliva (1 mL) was collected from the children before the meal.Subgingival plaque was removed from the tooth with a sterile curette.Streptococcus mutans were isolated from saliva.