Abstract
The emergence of drug-resistance pathogens of tuberculosis (TB) and other respiratory tract infections of bacterial origin unabatedly continues to be a global challenge. This is worsened by the increasing mortality rate associated with these infections annually. There is, therefore, need for unrelented search for new antibiotics or drugs to counter the menace of TB and allied diseases. Natural products with their extensive chemical diversity from medicinal plants provide matchless possibilities for discovering novel drug candidates. It is in line with this background that this study investigated the antimycobacterial activities of medicinal plants traditionally used for treating respiratory-cum-lung infections, including tuberculosis. In addition to this, the study also aimed at profiling and/or isolating the active compounds in these plants that might be responsible for the observed biological activities.
Hexane, chloroform and butanol:water (1:1, v/v) extracts of Vitellaria paradoxa Geartn., Drimia sanguinea (Schinz) Jessop, Lecaniodiscus cupanioides Planch. ex Benth, and Vitex doniana Sw. were screened in a preliminary investigation against Mycobacterium smegmatis (used as “surrogate” model for the virulent Mycobacterium tuberculosis), together with other panel of bacterial pathogens with history of respiratory tract infections in human, including Bacillus subtilis, Enterobacter cloacae, Enterococcus faecalis, Escherichia coli, Klebsiella aerogenes, Klebsiella oxytoca, Pseudomonas aeruginosa, Proteus mirabilis, Staphylococcus epidermidis and Staphylococcus aureus using microdilution techniques. The antimicrobial activities of the extracts were concentration dependent, with most extracts showed nearly the same pharmacological activity in terms of their minimum inhibitory concentration (MICs) from 0.10 to 3.33 mg/mL against most of the tested pathogens compared to the nalidixic acid and streptomycin used as positive controls.
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Five compounds, belonging to different classes of natural products, were isolated from chloroform extract of Vitex doniana stem-bark. The compounds, including 1-eicosene (1), n-hexadecanoic acid (2), β-sitosterol (3), 3-O-cinnamoyl-1β-11α-dihydroxy-α-amyrin (4), and 2α, 3β, 20α-trihydroxy-30-norolean-7-en-23-oic acid (5) showed a potential antimycobacterial activity. Compound 5 is isolated and reported, for the first time, from natural source. The compounds 1-4 are also isolated, for the first time, in pure forms from V. doniana. The isolated compounds inhibited the growth of Mycobacterium smegmatis (MIC; 0.5 – 1.0 mg/ml), though with less activity compared to the standard drug, streptomycin, but equal activity to that of nalidixic acid. These compounds also displayed inhibitory properties against Gram-negative as well as Gram-positive pathogens associated with respiratory tract infections, including Escherichia coli, Klebsiella oxytoca, Staphylococcus aureus and Enterococcus faecalis (MIC; 0.25 – 1.0 mg/ml). Compound 5 showed no cytotoxicity profile against human cervical cancer cell line at the tested concentration.
Together with 48 bioactive chemical constituents identified by Gas Chromatography-Mass Spectrometer (GC-MS), three cinnamoyl-based compounds, 11-hydroxy β-amyrin cinnamate (1), α-amyrin cinnamate (2), and sitosterol cinnamate (3) with pharmacological activities were isolated from Vitellaria paradoxa stem-bark. All the compounds had moderate inhibition against tested strains Mycobacterium smegmatis, Escherichia coli, Klebsiella aerogenes, Staphylococcus aureus, Enterobacter cloacae, Klebsiella oxytoca, and Enterococcus faecalis compared to the standard drugs (streptomycin and nalidixic acid). Compound 3 showed best inhibition (MIC, 0.125 mg/ml) among the isolated compounds against Mycobacterium smegmatis. It also showed better inhibition of other tested bacterial strains, with MICs varying from 0.0625 to 0.25 mg/ml. Compounds (1) and (2) showed no toxicity against the cervical cancer cell line (HeLa) (IC50; 82.53±4.07 and
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77.82±10.5 respectively). The activities of the isolates 1-3 against the tested bacterial strains are studied here for the first time.
The GC-MS analysis of Lecaniodiscus cupanioides leaves extract putatively identified phytochemicals known as antimicrobial agents, including phytol, 1-heptacosanol, β-citronellol, and hexadecanoic acid methyl ester. Phytol was found to inhibit the growth of Mycobacterium tuberculosis (the causative agent of tuberculosis), and mechanistically induce oxidative cell death in P. aeruginosa. 1-heptacosanol was growth-inhibitor of E. coli and S. aureus. The study revealed, for the first time, the bioactive constituents that can serve as potential antibacterial agents from the hexane leaves extract. Further phytochemical investigation of Lecaniodiscus cupanioides using column chromatography resulted in isolation of three known compounds, including eicosene (1), nonadecanol (2), and stigmasterol (3). To the best of my knowledge, compounds 1-3 were isolated and reported from L. cupanioides for the first time. The isolates 1-3 as well as the chromatographic fractions displayed moderate inhibition of Mycobacterium smegmatis, with MICs varying from 0.50 to 1.0 mg/ml. The study also revealed that the studied extract, column fractions and the isolated compounds exhibited no cytotoxicity against HeLa cell line. In another study, a total of forty-four chemical constituents were identified from Drimia sanguinea. The major constituents were phytol (6.70%), di-isooctyl phthalate (5.80%), octadecyl trifluoroacetate (5.79%), n-heptadecanol-1 (5.03%), 1-octadecanol (4.62%), undec-10-ynoic acid, undec-2-en-1-yl ester (4.54%), hexadecane (4.27%), 1-hexadecene (4.12%), (-)-caryophyllene oxide (3.88%), 9-eicosene, (E)- (2.92%), triacontane (2.53%) and α-sitosterol (2.30%). These compounds are expressed to have antimicrobial activities, and their presence in D. sanguinea extract provides scientific support for some of its uses in traditional medicine.
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All the isolates reported from the medicinal plants studied in this project exhibited antimycobacterial activity. In addition, the compounds from Vitex doniana and Vitellaria paradoxa showed growth-inhibition of other bacterial pathogens with the history of respiratory tract infections in human. Similarly, most of the isolates showed no or moderate cytotoxic effects on cancer cell line (HeLa). Besides, many of these phytochemical constituents are reported from these plants for the first time, with the exception of the isolates from Vitellaria paradoxa. In short, the findings from this project emphasis the significant roles medicinal plants could play, if fully exploit, in addressing the challenge of drug resistance found in current clinical drugs, including those for treating TB-related infections.
Keywords: Medicinal plants; respiratory infections; tuberculosis; antibacterial, cytotoxicity, phytochemicals.