Investigating the Antimycobacterial, Antibiofilm, and Antioxidant Activities of Plant Extracts Against Mycobacterium smegmatis

Abstract

The persistence of Mycobacterium tuberculosis within biofilm-like structures underscores the need for alternative drug discovery strategies aimed at resistance mechanisms. Medicinal plants provide a rich source of chemically diverse compounds with broad biological activities, including potential antimycobacterial properties. This study investigated acetone stem extracts from Buddleja saligna, Combretum hereroense, and Olea europaea subsp. africana for their phytochemical composition, antioxidant capacity, and antimycobacterial activity against planktonic and biofilm forms of Mycobacterium smegmatis. Phytochemical profiles were analyzed using liquid chromatography–mass spectrometry and quantified through colorimetric assays. Antioxidant activity was assessed using DPPH radical scavenging and ferric reducing power assays, while antimycobacterial effects at MIC and sub-MIC levels were determined through microdilution and growth kinetic assays. Phytochemical composition and concentrations varied among extracts, with B. saligna exhibiting the highest levels of tannins (287.18 ± 0.19 mgGAE/g extract) and flavonoids (16.48 ± 0.05 mgQE/g extract) and showing the strongest antioxidant activity (17.66 ± 5.396 and 399.1 ± 3.717 µg/mL). C. hereroense had a notable antimycobacterial activity with an MIC of 0.16 mg/mL followed by B. saligna and O. europaea subsp. afriana with MIC values of 0.31 and 0.63 mg/mL, respectively. All extracts significantly inhibited early biofilm formation by over 80% at sub-MICs. However, the mature biofilms and sliding motility were less susceptible to the extracts. Overall, the results confirm the antioxidant and antimycobacterial potential of the selected plant extracts, while highlighting challenges in targeting mycobacterial biofilms.