Antibacterial and antioxidant activities, toxicity and physicochemical properties of Tetradenia discolor Phillipson and Tetradenia riparia (Hochst) Cood (Lamiaceae)

In Democratic Republic of Congo, traditional healers employ Tetradenia discolor, and T. riparia to manage bacterial gastroenteritis. We systematically evaluated methanolic leaf and stem-bark extracts of both taxa for antibacterial and antioxidant activities, acute and subacute oral toxicity, physicochemical parameters, and total phenolic, flavonoid, and tannin contents. Antibacterial activity was assessed by disc diffusion and tube macrodilution assays; radical-scavenging capacity by the DPPH assay. Physicochemical profiles were determined by gravimetric, spectrophotometric, and classical tests. Acute and subacute oral toxicity followed OECD protocols in Cavia porcellus models. All extracts inhibited Staphylococcus aureus and Salmonella typhi, yielding inhibition zones of 17–22.5 mm and MICs of 1.95–62.5 µg/mL. The T. discolor stem-bark extract exhibited the strongest bacteriostatic effect (zones 19.1–21.1 mm; MICs 1.95–15.63 µg/mL). DPPH IC₅₀ values ranged from 11.6 to 21.8 µg/mL, with the T. riparia stem-bark extract demonstrating the greatest radical-scavenging potency. Phytochemical screening identified coumarins, quinones, flavonoids, phenols, saponins, tannins, and terpenoids in all extracts. Notably, the T. riparia leaf extract contained the highest total phenolics (300 mg GAE/g), flavonoids (57 mg QE/g), and tannins (151 mg GAE/g). Both taxa displayed an LD₅₀ > 5000 mg/kg and produced no adverse effects at 200 mg/kg over 30 days. Total ash and acid-insoluble ash values ranged from 11.2–15.8% and 9.33–9.83%, respectively. These findings substantiate the safety and bioactivity of both extracts, highlight phenolics as the principal bioactive constituents, validate their ethnomedical use against gastroenteritis, and support the development of TD- and TR-based phytomedicines.