Inhibitory Efficacy of Medicinal Plant Extracts from the Family Acanthaceae Against Skin Pathogenic Bacteria, Antioxidant and Anti-Inflammatory Activities

Abstract

Medicinal plants in the Acanthaceae family have long been used in Thai traditional medicine. In this study, ethanolic extracts of four plant species, Andrographis paniculata, Thunbergia laurifolia, Acanthus ebracteatus, and Rhinacanthus nasutus, were prepared and their phytochemical and pharmacological properties were evaluated. Total phenolic and flavonoid content assays were used to investigate phytochemical content and phytochemical profiles were characterized with LC-MS, while antioxidant activities were assessed via colorimetric methods. The extracts were also tested for antibacterial activity against several skin pathogens, such as Pseudomonas aeruginosa, Cutibacterium acnes, Micrococcus luteus, Staphylococcus aureus, and methicillin-resistant S. aureus, using agar well diffusion and broth dilution methods. Cytotoxic effects on the RAW 264.7 macrophage cell were assessed using the MTT assay. The extracts were further evaluated for anti-inflammatory activity in an LPS-stimulated RAW 264.7 cell. Inflammatory gene expression was assessed by quantifying the mRNA levels of iNOS, COX-2, TNF-α, IL-1β, IL-6, and NF-κB genes using RT-qPCR. The results revealed that among the four species, T. laurifolia extract showed the highest phenolic and flavonoid levels, consistent with the greatest antioxidant activity. Moreover, LC-MS profiling annotated a range of secondary metabolites in the extracts. The inhibition zones were observed for all plant extracts when tested against the skin pathogenic bacteria except C. acnes, and A. paniculata did not inhibit P. aeruginosa, indicating the antibacterial activity of the extracts. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values differed across the extracts and bacterial strains. T. laurifolia exhibited the most potent activity against various bacterial pathogens, while A. paniculata was most effective against C. acnes at 0.98 mg/mL. In addition, all plant extracts reduced nitric oxide production and downregulated the expression of inflammatory genes. Among all plant species, A. paniculata showed the most potent activity with the highest NO inhibition of 65.68% at 60 µg/mL. Therefore, these findings support the antioxidant, antibacterial, and anti-inflammatory potential of A. paniculata, T. laurifolia, A. ebracteatus and R. nasutus, which are traditionally used in folk medicine.