Antibacterial Mechanisms and Skin-Protective Activities of Syzygium nervosum Leaf Extracts from Different Geographical Origins

Human skin is continuously exposed to microbial invasion and oxidative stress, highlighting the need for safe and effective natural skin-protective agents. This study aims to investigate the antibacterial, antioxidant, and anti-tyrosinase activities of methanolic leaf extracts of Syzygium nervosum collected from Chiang Mai (SNLM-CM) and Phayao (SNLM-PY), Thailand. LC–QTOF–MS analysis revealed distinct phytochemical profiles between the extracts, suggesting that geographical origin influenced their biological activities. Antibacterial activity against Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa, and Escherichia coli was evaluated, together with studies of bacterial membrane integrity. Antioxidant capacity was assessed using ABTS, DPPH, FRAP, and ORAC assays, while anti-tyrosinase activity and cytotoxicity toward normal human dermal fibroblasts (NHDF) were determined by MTT assay. Both extracts exhibited potent antibacterial activity, particularly against Gram-positive bacteria (S. aureus and S. epidermidis) likely through membrane disruption, as evidenced by propidium iodide uptake, intracellular DNA and protein leakage, and scanning electron microscopy observations. SNLM-CM showed stronger antioxidant activity, whereas SNLM-PY demonstrated greater anti-tyrosinase activity. The extracts maintained high NHDF viability at concentrations below 100 µg/mL and retained antibacterial efficacy when incorporated into a cleansing gel formulation. Overall, S. nervosum leaf extracts exhibit promising antibacterial, antioxidant, and anti-tyrosinase activities, warranting further investigation to optimize their safety and efficacy for dermatological and cosmetic use.