Evaluation of the antimycobacterial properties of plant extracts from Vernonia adoensis.

Abstract

Tuberculosis (TB), is a mycobacterial infection resulting from Mycobacterium uberculosis. Two million individuals die annually due to TB and currently, treating this infection is a massive challenge owing to the emergence of drug-resistant strains and co-infection with HIV. Medicinal plants, having been used since time immemorial for the treatment of TB and TB-related ailments, are a possible lead for the discovery of novel phytocompounds that can be exploited for the invention of more efficacious imycobacterial agents. Vernonia adoensis is a herbal medicine that is used as a natural therapy for TB. This study examined the antimycobacterial properties of V. adoensis against a model mycobacterial species Mycobacterium smegmatis and the plant’s mode of action. Crude extracts of V. adoensis were prepared from the flowers, leaves, root bark and roots of V. adoensis using solvents of varying polarity. The major biologically active class of phytochemicals were identified. The antimycobacterial activity of the prepared extracts was assessed using the broth microdilution assay with rifampicin as the reference drug. The plant extracts and phytochemicals were tested for their inhibitory and mycobactericidal activities against M. smegmatis. The consequence of the most powerful extract of V. adoensis on the transport of ciprofloxacin across the cell membrane was investigated. The extract's ability to damage membrane integrity resulting in protein and nucleic acid leakage in mycobacterial cells was determined and its ability to scavenge for free radicals was assessed using the DPPH method. In vitro determination of the effect of the plant extract on the survival of mycobacteria inside macrophages was conducted and the cytotoxic effects of the plant extract were investigated using sheep erythrocytes. The most potent extract from V. adoensis was the ethyl acetate leaf extract which had a minimum inhibitory concentration and minimum bactericidal concentration of 63 µg/ml and 125 µg/ml respectively against M. smegmatis. The most active phytochemical class was the terpenoid fraction with an MIC of 250 µg/ml. Nucleic acid and protein leakage in M. smegmatis cells were observed after they were subjected to the leaf extract. The ethyl acetate leaf extract didn’t show neither free radical scavenging activity nor effect on drug transport in mycobacterial cells. The extract showed minimal cytotoxic effects towards the sheep erythrocytes and post-treating M. smegmatis-infected murine macrophages with the leaf extract at a concentration of 126 µg/ml significantly reduced the viability of the mycobacteria. The result of the study supports the traditional use of V. adoensis leaves in the treatment of tuberculosis. It is suggested that cell membrane disruption resultant in protein and nucleic acid leakage could be the plant's mode of action. Since the plant extract was not cytotoxic, it is a potential candidate as a template for new antimycobacterials. Since a model non-pathogenic organism was used, further studies must be done using M. tuberculosis. Further studies may also include isolation of the active constituents of Vernonia adoensis, which may form a basis for discovering new compounds with better antimycobacterial activity than the drugs that are in use.