In-vitro Antibacterial Activity of Sri Lankan Traditional Rice (Oryza sativa L.) Extracts against Bacteria Causing Skin and Soft Tissue Infections

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The aim of this study was to evaluate the potential antibacterial activity of the extracts of selected parboiled and un-parboiled Sri Lankan traditional rice against bacteria causing skin and soft tissue infections. Methanolic extracts of five Sri Lankan traditional rice including Kalu Heenati, Pokkali, Rathdal, Kahawanu and Sudu Murunga were used for in vitro antibacterial analysis. Antibacterial activity was evaluated in both the parboiled and un-parboiled rice samples. Concentrations of rice extracts used for the assays were 1000 µg/mL and 2000 µg/mL from the each extract. The antibacterial activity was evaluated against common bacteria causing skin and soft tissue infections (Staphylococcus aureus (ATCC 25923), Pseudomonas aeroginosa (ATCC 27853), Escherichia coli (ATCC 25922) and three clinical isolates of Methicillin resistant staphylococcus aureus (MRSA)) by well diffusion method and viable colony count technique. According to the results, methanolic extracts of all the selected Sri Lankan traditional rice varieties exhibited a potent antibacterial activity against Staphylococcus aureus with minimum bactericidal concentrations (MBC) of 200 µg/mL (minimum incubation time (MIT); 30 min) for Rathdal, 200 µg/mL (MIT; 60 min) for Kalu Heenati, Pokkali and Kahawanu, and 2000 µg/mL (MIT; 60 min) forSudu Murunga. The largest inhibition zones were observed in the extracts of Kalu Heenati and Rathdal. Kalu Heenati, Pokkali and Rathdal showed an efficacious inhibitory effect against MRSA (MBC; 200 µg/mL, MIT; 60 min), whereas the highest inhibitory activity was observed for Rathdal. Only the extract of Kalu Heenati was slightly active against Pseudomonas aeroginosa. None of the rice extracts studied showed an antibacterial activity against Escherichia coli.Reduction and loss of antibacterial activity was detected in rice after subjected to parboiling. In conclusion, Sri Lankan traditional rice varieties with red pericarp are good sources of antibacterial compounds mainly against Gram positive bacteria. Methanolic extract of Rathdal and Kalu Heenati showed a high efficacious inhibitory effect against skin and wound pathogens of Staphylococcus aureus and MRSA.