Antimycobacterial and antibiofilm studies on selected South African plants

Tuberculosis (TB) remains the leading cause of death from a single infectious agent globally, compounded by the rise of drug-resistant Mycobacterium tuberculosis and the limitations of prolonged anti-TB chemotherapy. This study evaluated selected South African medicinal plants with ethnomedicinal relevance, for antimycobacterial activity and antibiofilm activity, with a view to identifying promising anti-TB plants. Eleven plants were extracted with dichloromethane (DCM), ethanol (EtOH) and distilled water (dH 2 O), and screened in vitro against M. tuberculosis H37Rv and M. smegmatis mc 2 155 strains using microtitre plate colorimetric assays. Extracts with promising activity were further investigated for possible herb-drug interactions with bedaquiline, ciprofloxacin and rifampicin standards. In vitro antiproliferative and hepatoprotective effects of the extracts were determined against human non-tumorigenic liver cells (HepG2). Mechanistic enzymatic effects of the promising extracts on mycothiol disulfide reductase (MTR) and glutathione reductase (GTR) were determined. Finally, an in silico molecular docking against key TB enzymes. Pelargonium citronellum and Hypoestes forskaolii were the promising anti-TB plants. The DCM extract of P. citronellum showed antimycobacterial activity against M. smegmatis (IC 50 of 353.35 µg/mL), while the EtOH extract of H. forskaolii showed notable antibiofilm activity (IC 50 of 74.46 µg/mL). Ciprofloxacin exhibited a fractional inhibitory concentration of 5 when co-administered with t he EtOH extract of P. citronellum , indicating an antagonistic herb-drug interaction in vitro . Furthermore, the extracts showed higher cytotoxicity effects on human hepatocytes compared to their antibiofilm activity, with no substantial level of hepatoprotection. The EtOH extract of P. citronellum inhibited the activity of mycobacterial proteins, mycothiol reductase (MTR) at 0.01 µg/mL (Km = 0.73 µM; Vmax = 1.18 µM), and glutathione reductase (GTR) at 0.55 µg/mL (Km = 3.97 µM; Vmax = 96.3 µM) in vitro . Finally, in silico studies showed strong binding affinity between the TB proteins, MTR and GTR, and the major monoterpenoids of P. citronellum –nerol, geraniol and trans-geranic acid. While this study concludes that P. citronellum and H. forskaolii may have some anti-TB properties, an extensive phytochemical, in vivo pharmacological and toxicological studies on the plants may be required to fully unravel their anti-TB potential.