Introduction: Although various synthetic antimicrobials have been used to control Listeria monocytogenes in foods, therapeutic herbal plants could be considered as antimicrobial alternatives because of consumers' demand.
Purpose: This study evaluated the antimicrobial activities of therapeutic herbal plants on L. monocytogenes and their cytotoxicity on Caco-2 cell.
Methods: The paper disks impregnated with 69 herbal plants were placed on brain heart infusion agar, where L. monocytogenes strains were overlaid. After incubation (37°C, 24 h), clear zone diameters were measured to select the herbal extracts showing antilisterial activity. L. monocytogenes strains were inoculated in 96-well plates containing 100 ¥ìl of two-fold serial diluents of the selected herbal extracts, followed by incubation (37 °C, 24 h) to determine MICs (minimal inhibition concentration). Moreover, aqueous portions of the wells that displayed no growth were spread-plated on BHI agar plates, and the plates were incubated at 37°C for 24 h to determine MBCs (minimal bactericidal concentration). The stability of the selected extracts were also examined to food-related conditions such as heat (60-100°C), NaCl (0-6%), and pH (4-7). In addition, two-serial dilutions of the selected herbal extracts were added to the wells containing Caco-2 cells and incubated at 37°C for 2 days in 5% CO2 atmosphere to evaluate cytotoxicity of the herbal extracts.
Results: Of 69 herbal extracts, Psoraleae semen L. (Bogolji) and Sophorae radix L. (Gosam) extracts showed more than 10 mm of clear zone diameter, and their MICs were 8-16 µg/ml and 128-256 µg/ml depending on strains for Bogolji and Gosam, respectively. The MBCs were also 16 (Bogolji) and 256 µg/ml (Gosam). Heating, NaCl and acidic condition did not affect (P > 0.05) the antilisterial activity of Bogolji and Gosam. Cytotoxic activities were observed only at high concentration (128 µg/ml) of Bogolji extract.
Significance: The results indicate that Bogolji and Gosam could be used as potential phytochemicals to control L. monocytogenes.