Bioassay-guided isolation of antibacterial compounds from the leaves of Tetradenia riparia with potential bactericidal effects on food-borne pathogens

Abstract

Ethnopharmacological relevance: Tetradenia riparia (commonly known as ginger bush) is frequently used in traditional African medicine to treat foodborne infections including diarrhoea, gastroenteritis, and stomach ache.

Aim of the study: The present study aims to identify in Tetradenia riparia the compounds active against foodborne pathogens.

Materials and methods: Dried Tetradenia riparia leaf powder was consecutively extracted with hexane, ethyl acetate, methanol and water. The hexane extract was counter-extracted with methanol:water (9:1), and after evaporation of the methanol, this phase was extracted with dichloromethane. The water extract was counter-extracted with butanol. All these fractions were tested against a panel of foodborne bacterial pathogens. A bioassay-guided purification was performed to isolate antimicrobial compounds using Staphylococcus aureus as a target organism. Further, antibiofilm activity was evaluated on S. aureus USA 300.

Results: The dichloromethane fraction and ethyl acetate extract were the most potent, and therefore subjected to silica gel chromatography. From the dichloromethane fraction, one active compound was crystalized and identified using NMR as 8(14),15-sandaracopimaradiene-7alpha, 18-diol (compound 1). Two active compounds were isolated from the ethyl acetate extract: deacetylumuravumbolide (compound 2) and umuravumbolide (compound 3). Using a microdilution method, their antimicrobial activity was tested against eight foodborne bacterial pathogens: Shigella sonnei, S. flexneri, Salmonella enterica subsp. enterica, Escherichia coli, Micrococcus luteus, S. aureus, Enterococcus faecalis, and Listeria innocua. Compound 1 had the strongest activity (IC₅₀ ranging from 11.2 to 212.5 μg/mL), and compounds 2 and 3 showed moderate activity (IC₅₀ from 212.9 to 637.7 μg/mL and from 176.1 to 521.4 μg/mL, respectively). Interestingly, 8(14),15-sandaracopimaradiene-7alpha, 18-diol is bactericidal, and also showed good antibiofilm activity with BIC₅₀ (8.8 ± 1.5 μg/mL) slightly lower than for planktonic cells (11.4 ± 2.8 μg/mL).

Conclusions: These results support the traditional use of this plant to conserve foodstuffs and to treat gastrointestinal ailments, and open perspectives for its use in the prevention and treatment of foodborne diseases.

Keywords: 8(14),15-sandaracopimaradiene-7alpha 18-diol; Antimicrobial activity; Bacterial foodborne pathogens; Biofilm; Deacetylumuravumbolide; Tetradenia riparia; Umuravumbolide.