Inhibition of mycothione disulphide reductase and mycobacterial biofilm by selected South African plants

Abstract

The causative agent of tuberculosis is the bacterium Mycobacterium tuberculosis. It is the second leading cause of death worldwide. The development of resistance associated with this bacterium is of great concern, and the ability of the bacteria to form protective biofilms and persist for longer periods of time needs to be considered as a possible target. Plants have been considered as an additive to current treatments as an adjuvant, to aid the body in fighting the disease and as a source for new drug candidates to aid in the development of resistance of mycobacteria to available drugs. Twenty plants were selected based on their traditional usage against tuberculosis and related symptoms. The antimycobacterial activity of the ethanolic extracts from different genera and variable plant parts were evaluated using the microplate Alamar Blue assay. A biofilm formation inhibition assay was adapted and used to determine the inhibitory activity against Mycobacterium smegmatis biofilm formation. The cytotoxic effect of the plants was determined on U937 human macrophage cells using the XTT (2, 3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) cell proliferation kit. ADTNB-coupled Glutathione/Mycothiol disulfide reductase assay was used to determine the inhibitory activity against the human and bacterial reductases. Of the 20 plant extracts tested, it was found that Salvia africana-lutea, Ficus sur and Sphedamnocarpus pruriens showed moderate antimycobacterial activity with minimum inhibitory concentration values of 31.25, 62.5 and 62.5 μg/ml, respectively. Sphedamnocarpus pruriens (BF SI 1.00) and S.africana-lutea (BF SI 0.33) showed potential biofilm formation inhibition at concentrations, 62.2 and 95.8 μg/ml, respectively when compared to Ciprofloxacin (BF SI 0.32), which inhibited at a concentration of 1.9 μg/ml. Most of the plant extracts showed low to moderate cellular toxicity except Withania somnifera that showed a higher toxicity with an IC50 of 6.8 μg/ml (Actinomycin D0.0009 μg/ml). F.sur showed the highest selectivity index of 3.36. During mechanistic studies, S.africana-lutea showed inhibitory activity against Mycothiol disulfide reductase (Mtr) exhibiting an inhibitory concentration (IC50) of 102.3 μg/ml and showed lower inhibitory activity against Glutathione reductase (Gtr) with an IC50 of 224.1 μg/ml. This study indicates that South African plant extracts traditionally used to treat tuberculosis-related symptoms have reasonable antimycobacterial, antibiofilm and enzyme inhibitory activity with relatively low toxicity.