Antibacterial and antioxidant activities of six plants consumed by the Tonkin snub-nosed monkey (Rhinopithecus avunculus) in Khau Ca area, Tuyen Quang province

Abstract

The Tonkin snub-nosed monkey (Rhinopithecus avunculus) is an endemic, critically endangered primate that inhabits karst forests in Vietnam. Its survival depends on a variety of food plants, making an understanding of their bioactive properties crucial for species conservation and nutritional ecology. This study aimed to evaluate the antibacterial and antioxidant activities of six plants consumed by the species in the Khau Ca area, Tuyen Quang province. Methanolic leaf extracts were analysed using standard assays, including DPPH (2,2-diphenyl-1-picrylhydrazyl) and ABTS (2,2′- azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) assays for antioxidant capacity and the agar dilution method for antibacterial activity. The results revealed that Polyalthiopsis floribunda extract is the most promising candidate for development into a broad-spectrum antimicrobial agent due to its consistent bactericidal activity against the tested Gram-negative and Gram-positive bacterial strains. The extracts were categorised into two main groups based on their mechanism of action: a bactericidal group (Polyalthiopsis floribunda) and a bacteriostatic group (Debregeasia squamata and Meliosma paupera). Both Celtis philippensis and Radermachera hainanensis extracts had the lowest IC₅₀ values among the six extracts, demonstrating strong antioxidant activity and comparable potential in both DPPH and ABTS assays. These findings highlight the ecological significance of these plants in supporting health maintenance and survival of the largest population of Tonkin snub-nosed monkeys in Vietnam.