Antibacterial activity of Azadirachta indica, Pongamia pinnata, Psidium guajava, and Mangifera indica and their mechanism of action against Streptococcus mutans

Background: Curative plants have reportedly been used to make chewing sticks/toothbrushes intended for the treatment of oral diseases. Objective: The in vitro antibacterial activities of Azadirachta indica, Pongamia pinnata, Psidium guajava, and Mangifera indica were evaluated against Streptococcus mutans, along with the cytotoxicity and antioxidant and synergistic potentials. The effect of M. indica on the expression of crucial virulence genes spaP and gtfB of S. mutans was determined. Materials and Methods: The antibacterial activity was determined using a modified microdilution method. The antioxidant potential was evaluated using diphenyl picrylhydrazyl (DPPH), Griess reagent, and nitroblue tetrazolium calorimetric assays. The synergistic activity was investigated using a modified checkerboard method, while the cytotoxicity was determined according to a cell proliferation 2,3-Bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide salt assay. Reverse transcription was the chosen method for determining the difference in expression of the spaP and gtfB genes after treatment with the plant sample. Results: M. indica and A. indica had the highest antibacterial activity at concentrations of 0.3 mg/ml and 6.25 mg/ml, respectively. A. indica had the best free radical scavenging of DPPH, exhibiting 50% inhibition at 28.72 μg/ml; while M. indica showed better superoxide scavenging potential than the positive control quercetin. Both M. indica and A. indica had adequate activity against the nitric oxide-free radical (12.87 and 18.89 μg/ml, respectively). M. indica selectively reduced the expression of the gtfB gene, indicating a mechanism involving Glucotranferases, specifically targeting bacterial attachment. Conclusion: The overall good antibacterial activity of M. indica correlated with the general good antioxidant capacity, while showing a potentially unique mechanism of bacterial inhibition, targeting virulence gene expression.