Background and purpose: Dental caries is a pathological infectious disease. It begins with the formation of dental plaques which is a structurally and functionally organized biofilm. Streptococcus mutans is the most important bacterium in the formation of dental plaque and dental caries. This study aimed at evaluating the antibacterial and antibiofilm activity of Quercus infectoria galls against Streptococcus mutans. Materials and methods: The bacterial strain used in this study was Streptococcus mutans (ATCC: 25923). Extracts were prepared by Soxehlet apparatus and maceration. They were then dissolved in sterile distilled water to a final concentration of 0.16 to 10.00 mg/ml. The antimicrobial activities of the extracts were determined using well diffusion method. The antibiofilm activities of the extracts were examined in a microdilution assay using TTC. Statistical analysis was performed in SPSS V.18. Results: The methanol, ethanol, and acetone extracts of Quercus infectoria galls showed strong inhibitory effects against Streptococcus mutans. The MIC values of extracts were similar and ranged from 160µg/ml to 320µg/ml, whereas the MBC values ranged from 320µg/ml to 640µg/ml. Aqueous extracts of oak galls did not show antimicrobial activity. The extracts of Quercus infectoria galls strongly inhibited the formation of Streptococcus mutans biofilms at concentrations higher than 19.5µg/ml. Conclusion: The extracts of Quercus infectoria galls displayed similarities in their antibacterial activity against Streptococcus mutans. Also, they were found effective in preventing biofilm formation of Streptococcus mutans. The galls of Quercus infectoria are considered potentially good sources of antimicrobial agent

Keywords: Antibacterial activity, dental plaque, oak gall, plant extract, Streptococcus mutans

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