Characterization of Intracellular Activity of Antitubercular Constituents the Roots of Euclea natalensis

A Review of the Potential of Poly-(lactide-co-glycolide) Nanoparticles as a Delivery System for an Active Antimycobacterial Compound, 7-Methyljuglone

7-Methyljuglone (7-MJ) is a pure compound isolated from the roots of Euclea natalensis A. DC., a shrub indigenous to South Africa. It exhibits significant promise as a potential treatment for the highly communicable disease tuberculosis (TB), owing to its effective antimycobacterial activity against Mycobacterium tuberculosis. Despite its potential therapeutic benefits, 7-MJ has demonstrated in vitro cytotoxicity against various cancerous and non-cancerous cell lines, raising concerns about its safety for consumption by TB patients. Therefore, this review focuses on exploring the potential of poly-(lactide-co-glycolic) acid (PLGA) nanoparticles as a delivery system, which has been shown to decrease in vitro cytotoxicity, and 7-MJ as an effective antimycobacterial compound.

Antibody-Functionalized Polymer Nanoparticles for Targeted Antibiotic Delivery in Models of Pathogenic Bacteria Infecting Human Macrophages

The efficacy of antibody-functionalized poly(d,l-lactide-co-glycolide) (PLGA) nanoparticles (NPs), prepared by nanoprecipitation, carrying rifampicin (RIF) against planktonic, sessile, and intracellular Staphylococcus aureus and Escherichia coli is reported here. A biotinylated anti-S. aureus polyclonal antibody, which binds to structural antigens of the whole bacterium, was functionalized on the surface of RIF-loaded PLGA-based NPs by using the high-affinity avidin-biotin complex. This general strategy allows the binding of commercially available biotinylated antibodies. Coculture models of S. aureus ATCC 25923 and Escherichia coli S17 were used to demonstrate the preferential selectivity of the antibody-functionalized NPs against the Gram-positive bacterium only. At 0.2 μg/mL, complete S. aureus eradication was observed for the antibody-functionalized RIF-loaded NPs, whereas only a 5-log reduction was observed for the nontargeted RIF-loaded NPs. S. aureus is a commensal facultative pathogen having part of its live cycle intracellularly in both phagocytic and nonphagocytic cells. Those intracellular bacterial persisters, named small colony variants, have been postulated as reservoirs of relapsed episodes of infection and consequent treatment failure. At 0.5 μg/mL, the RIF-loaded NPs reduced in 2-log intracellular S. aureus-infecting human macrophages. The ability of those antibody-functionalized nanoparticles to prevent biofilm formation or to reduce the bacterial burden in already-formed mature biofilms is also reported here using S. aureus and E. coli single and cocultured biofilms. In the prevention of S. aureus biofilm formation, the antibody-functionalized NPs exerted a superior inhibition of bacterial growth (up to 2 logs) compared to the nonfunctionalized ones. This study demonstrates the selectivity of the synthesized immunonanoparticles and their antimicrobial efficacy in different scenarios, including planktonic cultures, sessile conditions, and even against intracellular infective pathogens.

Ethnobotanical uses, phytochemistry and biological activity of the genus Euclea: A review

Euclea (Ebenaceae) is a genus of flowering shrubs and trees widely distributed in Africa, the Comoro Islands, and Arabia. This review aimed to evaluate the ethnobotanical uses, phytochemistry, and biological activities of the genus Euclea on available research reports. This was achieved through PubMed, Medline, Google Scholar, Science Direct, Taylor and Francis Online, Wiley Online Library which provides access to scientific and medical research. The extensive literature survey revealed that plants that belong to this genus are used as folkloric medicine for the treatment of diabetes mellitus, toothache, diarrhea, cancer, malaria, leprosy, and genital and oral diseases in the case of HIV/AIDS-related diseases. To date, more than 40 secondary metabolites have been isolated and identified from these plants, especially from E natalensis and E. divinorum. Among these, naphthoquinones, terpenes, and flavonoids are potential secondary metabolites with profound biological activities. Euclea plant extracts and their bioactive compounds possess outstanding pharmacological properties, especially antimalarial, antidiabetic, anticancer, antimicrobial, and antioxidant properties.

Phytochemical, Antiplasmodial, and Cytotoxic Investigation of Euclea natalensis A.DC. subsp. natalensis Leaves

Previous research shows that the root and bark extracts of Euclea natalensis have antiplasmodial activity, but the leaves have not been examined yet. This study investigated the phytochemical, antiplasmodial, and cytotoxic properties of the plant leaves. The activity against 3D7 Plasmodium falciparum was determined using the parasite lactate dehydrogenase assay, and the cytotoxicity against Vero and HeLa cells was evaluated using the MTT and resazurin assays, respectively. The bioactive compounds were isolated by chromatography, and their structures were established with spectroscopic and spectrometric techniques. The extract showed antiplasmodial activity (IC₅₀ =25.6 μg/mL) and was not cytotoxic against Vero cells (IC₅₀ =403.7 μg/mL). Purification of the extract afforded six flavonoid glycosides, four triterpenoids, and a coumarin. The glycosides showed antiplasmodial and cytotoxic activities, against HeLa cells, at 50 μg/mL, but the activity was reduced at 10 μg/mL. Naphthoquinones, which are among the predominant phytochemicals in the root and root bark of E. natalensis, were not detected in the leaves.

In vitro evaluation of the antibacterial and cytotoxic activities of the Euclea natalensis crude extract and fractions against oral infection agents

OBJECTIVE

This study aimed (i) to evaluate the antibacterial and cytotoxic activities of the crude extract and fractions obtained from Euclea natalensis A.D.C. roots against bacteria that cause periodontal disease and caries and (ii) to identify the isolated compounds.

DESIGN

The minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of the extract and fractions were determined by the microplate dilution assay. The cytotoxicity of the extract and fractions was evaluated by using the XTT colorimetric assay and normal human fibroblast cells (GM07492A, lung fibroblasts). The compounds present in the most promising fraction were determined by qualitative analysis through liquid chromatography coupled to mass spectrometry (HPLC-MS-ESI).

RESULTS

The MIC results ranged from 25 to > 400 μg/mL for the extract and from 1.56 to > 400 μg/mL for the fractions. To evaluate cytotoxicity, the tested concentrations of the extract and fractions ranged from 19.5 to 2500 μg/mL; IC₅₀ values between 625 and 1250 μg/mL were obtained. Analysis of the main bioactive fraction by HPLC-MS-ESI identified phenolic acids, coumarins, naphthoquinones, lignans, and fatty acids.

CONCLUSIONS

The E. natalensis root extract and fractions displayed good antibacterial activity against periodontal pathogenic and cariogenic bacteria. The antibacterial activity may be due to compounds present in the extract and fractions, which also showed low cytotoxicity to normal human cells. These data are relevant and encourage further research into this plant species, which may contribute to the discovery of new herbal medicines that will help to mitigate the problems caused by oral pathogenic bacteria.

New Investigations with Lupane Type A-Ring Azepane Triterpenoids for Antimycobacterial Drug Candidate Design

Twenty lupane type A-ring azepano-triterpenoids were synthesized from betulin and its related derivatives and their antitubercular activity against Mycobacterium tuberculosis, mono-resistant MTB strains, and nontuberculous strains Mycobacterium abscessus and Mycobacterium avium were investigated in the framework of AToMIc (Anti-mycobacterial Target or Mechanism Identification Contract) realized by the Division of Microbiology and Infectious Diseases, NIAID, National Institute of Health. Of all the tested triterpenoids, 17 compounds showed antitubercular activity and 6 compounds were highly active on the H37Rv wild strain (with MIC 0.5 µM for compound 7), out of which 4 derivatives also emerged as highly active compounds on the three mono-resistant MTB strains. Molecular docking corroborated with a machine learning drug-drug similarity algorithm revealed that azepano-triterpenoids have a rifampicin-like antitubercular activity, with compound 7 scoring the highest as a potential M. tuberculosis RNAP potential inhibitor. FIC testing demonstrated an additive effect of compound 7 when combined with rifampin, isoniazid and ethambutol. Most compounds were highly active against M. avium with compound 14 recording the same MIC value as the control rifampicin (0.0625 µM). The antitubercular ex vivo effectiveness of the tested compounds on THP-1 infected macrophages is correlated with their increased cell permeability. The tested triterpenoids also exhibit low cytotoxicity and do not induce antibacterial resistance in MTB strains.

Molecular mechanisms of underlying genetic factors and associated mutations for drug resistance in Mycobacterium tuberculosis

Nowadays, drug-resistant tuberculosis (DR-TB) and co-infected tuberculosis (CI-TB) strains are the leading cause for the enhancement of long-term morbidity and unpredicted mortality rates from this ghoulish acid fast-bacterium infection, globally. Unfortunately, the lack of/ample lethargic towards the development of compelling anti-TB regimens with a large-scale prevalence rate is a great challenge towards control of the pandemic situation. Indeed, the recent improvement in genomic studies for early diagnosis and understanding the mechanisms of drug resistance, as well as the identification of newer drug targets is quite remarkable and promising. Mainly, identification of such genetic factors, chromosomal mutations and associated pathways gives new ray of hope in current anti-TB drug discovery. This focused review provides molecular insights into the updated drug resistance mechanisms with encoded bacilli genetic factors as a novel target and potential source of development with screened-out newer anti-TB agents towards the control of MDR-TB soon.

Molecular Docking Suggests the Targets of Anti-Mycobacterial Natural Products

Tuberculosis (TB) is a major global threat, mostly due to the development of antibiotic-resistant forms of Mycobacterium tuberculosis, the causal agent of the disease. Driven by the pressing need for new anti-mycobacterial agents several natural products (NPs) have been shown to have in vitro activities against M. tuberculosis. The utility of any NP as a drug lead is augmented when the anti-mycobacterial target(s) is unknown. To suggest these, we used a molecular reverse docking approach to predict the interactions of 53 selected anti-mycobacterial NPs against known "druggable" mycobacterial targets ClpP1P2, DprE1, InhA, KasA, PanK, PknB and Pks13. The docking scores/binding free energies were predicted and calculated using AutoDock Vina along with physicochemical and structural properties of the NPs, using PaDEL descriptors. These were compared to the established inhibitor (control) drugs for each mycobacterial target. The specific interactions of the bisbenzylisoquinoline alkaloids 2-nortiliacorinine, tiliacorine and 13'-bromotiliacorinine against the targets PknB and DprE1 (-11.4, -10.9 and -9.8 kcal·mol-1; -12.7, -10.9 and -10.3 kcal·mol-1, respectively) and the lignan α-cubebin and Pks13 (-11.0 kcal·mol-1) had significantly superior docking scores compared to controls. Our approach can be used to suggest predicted targets for the NP to be validated experimentally, but these in silico steps are likely to facilitate drug optimization.

Diospyrin Modulates Inflammation in Poly I:C-Induced Macrophages via ER Stress-Induced Calcium-CHOP Pathway

Diospyrin, plant-derived bisnaphthoquinonoid, is known to have anticancer activity. However, pharmacological activity of diospyrin on viral infection is not well known. We investigated effects of diospyrin on macrophages induced by polyinosinic-polycytidylic acid (poly I:C), a mimic of double-stranded viral RNA. Various cytokines, intracellular calcium, nitric oxide (NO), phosphorylated p38 MAPK, and phosphorylated ERK1/2 as well as mRNA expressions of transcription factors were evaluated. Diospyrin significantly reduced NO production, granulocyte-macrophage colony-stimulating factor production, and intracellular calcium release in poly I:C-induced RAW 264.7. The phosphorylation of p38 MAPK and ERK1/2 was also significantly suppressed. Additionally, diospyrin inhibited mRNA levels of nitric oxide synthase 2, C/EBP homologous protein (CHOP), calcium/calmodulin dependent protein kinase II alpha, signal transducers and activators of transcription 1 (STAT1), STAT3, STAT4, Janus kinase 2, first apoptosis signal receptor, c-Jun, and c-Fos in poly I:C-induced RAW 264.7. Taken together, this study represents that diospyrin might have the inhibitory activity against viral inflammation such as excessive production of inflammatory mediators in poly I:C-induced RAW 264.7 via ER stress-induced calcium-CHOP pathway.

Medicinal Plants Used in Traditional Management of Cancer in Uganda: A Review of Ethnobotanical Surveys, Phytochemistry, and Anticancer Studies

The burden of neoplastic diseases is a significant global health challenge accounting for thousands of deaths. In Uganda, about 32,617 cancer cases were reported in 2018, accompanied by 21,829 deaths. In a view to identify some potential anticancer plant candidates for possible drug development, the current study was designed to compile the inventory of plants with reported anticancer activity used in rural Uganda and the evidences supporting their use in cancer therapy. An electronic survey in multidisciplinary databases revealed that 29 plant species belonging to 28 genera distributed among 24 families have been reported to be used in the management of cancer in Uganda. Anticancer plants were majorly from the families Bignoniaceae (7%), Caricaceae (7%), Fabaceae (7%), Moraceae (7%), and Rutaceae (7%). Most species occur in the wild (52%), though some are cultivated (48%). The growth habit of the plants is as trees (55%) or herbs (45%). Anticancer extracts are usually prepared from leaves (29%), bark (24%), roots (21%), and fruits (13%) through decoctions (53%), as food spices (23%) or pounded to produce ointments that are applied topically (10%). Prunus africana (Hook.f.) Kalkman, Opuntia species, Albizia coriaria (Welw. ex Oliver), Daucus carota L., Cyperus alatus (Nees) F. Muell., Markhamia lutea (Benth.) K. Schum., and Oxalis corniculata L. were the most frequently encountered species. As per global reports, Allium sativum L., Annona muricata L., Carica papaya L., Moringa oleifera Lam., Opuntia species, Prunus africana (Hook.f.) Kalkman, and Catharanthus roseus (L.) G. Don. are the most studied species, with the latter having vincristine and vinblastine anticancer drugs developed from it. Prostate, cervical, breast, and skin cancers are the top traditionally treated malignancies. There is a need to isolate and evaluate the anticancer potential of the bioactive compounds in the unstudied claimed plants, such as Cyperus alatus (Nees) F. Muell., Ficus dawei Hutch., Ficus natalensis Hochst., and Lovoa trichilioides Harms, and elucidate their mechanism of anticancer activity.

Synthesis and in vitro evaluation of antimycobacterial and cytotoxic activity of new α,β-unsaturated amide, oxazoline and oxazole derivatives from l-serine

The synthesis of 19 compounds derived from l-serine and analogs of p-substituted cinnamic acid is reported. Oxazolines 9 and oxazoles 10 have high antitubercular activity with Minimum Inhibitory Concentration (MIC) of 0.7812-25.0 µg/mL (3.21-100.3 µM), against two strains of Mycobacterium tuberculosis sensitive to first-line drugs Isoniazid (INH), Rifampicin (RIF), Ethambutol (EMB), Pyrazinamide (PZE) (H37Rv) and a clinical isolate resistant to INH, RIF and EMB (G122). The cytotoxic evaluation shows that oxazoles have low activity, finding viability>96% against the VERO cell line. The results show these compounds could be considered as future alternatives for antitubercular treatment.

The potential of plant systems to break the HIV-TB link

Tuberculosis (TB) and human immunodeficiency virus (HIV) can place a major burden on healthcare systems and constitute the main challenges of diagnostic and therapeutic programmes. Infection with HIV is the most common cause of Mycobacterium tuberculosis (Mtb), which can accelerate the risk of latent TB reactivation by 20-fold. Similarly, TB is considered the most relevant factor predisposing individuals to HIV infection. Thus, both pathogens can augment one another in a synergetic manner, accelerating the failure of immunological functions and resulting in subsequent death in the absence of treatment. Synergistic approaches involving the treatment of HIV as a tool to combat TB and vice versa are thus required in regions with a high burden of HIV and TB infection. In this context, plant systems are considered a promising approach for combatting HIV and TB in a resource-limited setting because plant-made drugs can be produced efficiently and inexpensively in developing countries and could be shared by the available agricultural infrastructure without the expensive requirement needed for cold chain storage and transportation. Moreover, the use of natural products from medicinal plants can eliminate the concerns associated with antiretroviral therapy (ART) and anti-TB therapy (ATT), including drug interactions, drug-related toxicity and multidrug resistance. In this review, we highlight the potential of plant system as a promising approach for the production of relevant pharmaceuticals for HIV and TB treatment. However, in the cases of HIV and TB, none of the plant-made pharmaceuticals have been approved for clinical use. Limitations in reaching these goals are discussed.

Fractions and isolated compounds from Oxyanthus speciosus subsp. stenocarpus (Rubiaceae) have promising antimycobacterial and intracellular activity

BACKGROUND

Tuberculosis is a deadly disease caused by Mycobacterium species. The use of medicinal plants is an ancient global practice for the treatment and prevention of diverse ailments including tuberculosis. The aim of this study was to isolate and characterize antimycobacterial compounds by bioassay-guided fractionation of the acetone leaf extract of Oxyanthus speciosus.

METHODS

A two-fold serial microdilution method was used to determine the minimum inhibitory concentration (MIC) against mycobacteria. Cytotoxicity and nitric oxide inhibitory activity of the isolated compounds was determined to evaluate in vitro safety and potential anti-inflammatory activity. Intracellular efficacy of the crude extract against Mycobacterium-infected macrophages was also determined.

RESULTS

Two compounds were isolated and identified as lutein (1) and rotundic acid (2). These had good antimycobacterial activity against the four mycobacteria tested with MIC values ranging from 0.013 to 0.1 mg/mL. Rotundic acid had some cytotoxicity against C3A human liver cells. Lutein was not cytotoxic at the highest tested concentration (200 μg/mL) and inhibited nitric oxide production in RAW 264.7 macrophages by 94% at a concentration of 25 μg/mL. The acetone crude extract (120 μg/mL) of O. speciosus had intracellular antimycobacterial activity, reducing colony forming units by more than 90%, displaying bactericidal efficacy in a dose and time-dependent manner.

CONCLUSION

This study provides good proof of the presence of synergism between different compounds in extracts and fractions. It is also the first report of the antimycobacterial activity of lutein and rotundic acid isolated from Oxyanthus speciosus. The promising activity of the crude extract of O. speciosus both in vitro and intracellularly in an in vitro macrophage model suggests its potential for development as an anti- tuberculosis (TB) herbal medicine.

Review of Ethnomedicinal Uses, Phytochemistry and Pharmacological Properties of Euclea natalensis A.DC

Euclea natalensis is traditionally used as herbal medicine for several human diseases and ailments in tropical Africa. This study reviews information on ethnomedicinal uses, botany, phytochemical constituents, pharmacology and toxicity of E. natalensis. Results of this study are based on literature search from several sources including electronic databases, books, book chapters, websites, theses and conference proceedings. This study showed that E. natalensis is used as traditional medicine in 57.1% of the countries where it is indigenous. Euclea natalensis has a high degree of consensus on abdominal pains, antidote for snake bites, diabetes, diarrhoea, malaria, roundworms, stomach problems, toothache, venereal diseases and wounds. Several ethnopharmacological studies have shown that crude extracts and chemical compounds from E. natalensis demonstrated many biological activities both in vitro and in vivo, which included antibacterial, antidiabetic, antifungal, antimycobacterial, antiviral, antioxidant, antiplasmodial, larvicidal, antischistosomal, molluscicidal, dentin permeability and hepatoprotective activities. Future studies should focus on the mechanism of biological activities of both crude extracts and chemical compounds from the species, as well as structure-function relationships of bioactive constituents of the species.

Plants in our combating strategies against Mycobacterium tuberculosis: progress made and obstacles met

CONTEXT

Traditionally used plants for treating chest-related problems/tuberculosis (TB) have not been evaluated in detail and hence a thorough study is needed in this regard. This knowledge may find application in developing new anti-TB drugs.

OBJECTIVE

This article elaborates on studying the activity of medicinal plants against different forms of Mycobacterium tuberculosis (Mtb) using different model strains, in vitro and ex vivo assays for studying the tuberculocidal activity and discusses the results from different studies on the activity against different forms of Mtb and human immunodeficiency virus-tuberculosis (HIV-TB) co-infection.

METHODS

Scientific databases such as PubMed, Elsevier, Scopus, Google scholar, were used to retrieve the information from 86 research articles (published from 1994 to 2016) related to the topic of this review.

RESULTS

Twenty-three plant species have been reported to possess active molecules against multi-drug resistant (MDR) isolates of Mtb. Seven plants were found to be active against intracellular Mtb and six against dormant bacilli. Seven plants were synergistically effective when combined with anti-TB drugs. Six studies suggest that the beneficial effects of plant extracts are due to their wide array of immuno-modulatory effects manifested by the higher expression of cytokines. Some studies have also shown the dual activity (anti-HIV and anti-TB) of plants.

CONCLUSION

We emphasize on identifying plants based on traditional uses and testing their extracts/phytomolecules against MDR strains, intracellular Mtb as well as against dormant Mtb. This will help in future to shorten the current therapeutic regimens for TB and also for treating HIV-TB co-infection.

The chemistry and bioactivity of Southern African flora II: flavonoids, quinones and minor compound classes

This review is intended to highlight the relevance of natural products in drug discovery paying particular attention to those derived from Southern African medicinal plants with diverse biological activities.

Review of the chemistry and pharmacology of 7-Methyljugulone

BACKGROUND

Naphthoquinone is a class of phenolic compounds derived from naphthalene. 7-Methyljuglone (7-MJ) is a naphthoquinone also known as ramentaceone or 6-Methyl-8-hydroxy-1,4-naphthoquinone or 5-Hydroxy-7-methyl-1,4-naphthoquinone or 7-Methyl-5-hydroxy-1,4-naphthoquinone or 5-Hydroxy-7-methyl-,1,4-naphtoquinone or 7-Methyl-5-hydroxynaphthalene-1,4-dione. This compound is a biologically active naphtoquinone, with a molecular weight of 188 g/mol mostly isolated in the genus Diospyros and Euclea.

OBJECTIVES

This review was aimed at providing available chemically and pharmacological data on 7-MJ.

METHODS

The chemical and pharmacological data were retrieved from the well-known scientific websites such as Pubmed, Google Scholar, Reaxys, Scirus, Scopus, Sciencedirect, Web-of-knowledge and Scifinder.

RESULTS

7-MJ was reported to have a variety of pharmacological activities such as antibacterial, antifungal, anticancer, antitubercular, anti-inflammatory and antiviral activities. The hemi-synthesis of the compound have been described.

CONCLUSIONS

The present review pooled out together the knowledge on 7-MJ, and can serve as the start point for future research and valorization accomplishments.

Synergistic Antimycobacterial Actions of Knowltonia vesicatoria (L.f) Sims

Euclea natalensis A.DC., Knowltonia vesicatoria (L.f) Sims, and Pelargonium sidoides DC. are South African plants traditionally used to treat tuberculosis. Extracts from these plants were used in combination with isoniazid (INH) to investigate the possibility of synergy with respect to antimycobacterial activity. The ethanol extract of K. vesicatoria was subjected to fractionation to identify the active compounds. The activity of the Knowltonia extract remained superior to the fractions with a minimum inhibitory concentration (MIC) of 625.0 μg/mL against Mycobacterium smegmatis and an MIC of 50.00 μg/mL against M. tuberculosis. The K. vesicatoria extract was tested against two different drug-resistant strains of M. tuberculosis, which resulted in an MIC of 50.00 μg/mL on both strains. The combination of K. vesicatoria with INH exhibited the best synergistic antimycobacterial activity with a fractional inhibitory concentration index of 0.25 (a combined concentration of 6.28 μg/mL). A fifty percent inhibitory concentration of this combination against U937 cells was 121.0 μg/mL. Two compounds, stigmasta-5,23-dien-3-ol (1) and 5-(hydroxymethyl)furan-2(5H)-one (2), were isolated from K. vesicatoria as the first report of isolation for both compounds from this plant and the first report of antimycobacterial activity. Compound (1) was active against drug-sensitive M. tuberculosis with an MIC of 50.00 μg/mL.

Antimicrobial activity of medicinal plants against oral microorganisms

Ethanol extracts of eight plant species used traditionally in South Africa for the treatment of oral diseases were investigated for in vitro antimicrobial activity against oral pathogens namely Actinobacillus actinomycetemcomitans, Actinomyces naeslundii, Actinomyces israelii, Candida albicans, Porphyromonus gingivalis, Privotella intermedia and Streptococcus mutans using the disk diffusion method. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of ethanol extracts were determined against these microorganisms using micro dilution. The cytotoxicity and therapeutic index (TI) of selected active extracts were also determined. Out of eight plants, six (Annona senegalensis, Englerophytum magalismontanum, Dicerocarym senecioides, Euclea divinorum, Euclea natalensis, Solanum panduriforme and Parinari curatellifolia) exhibited MIC values ranging from 25.0 mg/ml to 0.8 mg/ml. Gram negative bacteria were found to be more resistant to the plant extracts than Gram positive bacteria, except for Euclea natalensis which inhibited all three Gram negative bacteria tested in this study. All plant extracts showed moderate cytotoxicity on the Vero cell line. The fifty percent inhibitory concentration (IC(50)) of all plants tested range from 92.3 to 285.1 microg/ml.

The potential of South African plants against Mycobacterium infections

ETHNOPHARMACOLOGICAL RELEVANCE

In South Africa, tuberculosis (TB) caused by Mycobacterium tuberculosis is the most commonly notified disease and the fifth largest cause of mortality, with one in ten cases of TB resistant to treatment in some areas. Many plants are used locally in traditional medicine to treat TB-related symptoms.

AIM OF THE STUDY

The aim was to summarize currently available knowledge on South African plants used to treat TB symptoms, and antimycobacterial efficacy of plant-derived extracts and compounds.

MATERIALS AND METHODS

The traditional uses of plants for respiratory ailments and TB were collated and tabulated. The antimycobacterial activity tests of extracts and chemical constituents of several of these plants and others using different methods and target organisms were summarized.

RESULTS

Almost 180 plants used for TB-related symptoms in South African traditional medicine were documented. About 30% of these have been tested for antimycobacterial efficacy, mostly against fast-growing, non-pathogenic Mycobacterium species.

CONCLUSIONS

Many plant species are used in traditional South African medicine to alleviate symptoms of TB, and several interesting leads have originated for further inquiry following in vitro antimycobacterial activity evaluation. However, much work remains to be done on the systematic assessment of anti-TB efficacy of local plants against pathogenic Mycobacterium species, both in vitro and in vivo.

Purified compounds and extracts from Euclea species with antimycobacterial activity against Mycobacterium bovis and fast-growing mycobacteria

Naphthoquinones and other compounds with antimycobacterial activity against Mycobacterium tuberculosis have previously been isolated from Euclea species. In this study, several constituents of Euclea natalensis and E. undulata, as well as organic extracts of the leaves, were assessed for efficacy against the zoonotic pathogen, Mycobacterium bovis. Also included in the battery of test organisms were M. bovis BCG and the fast-growing species M. smegmatis and M. fortuitum. The acetone extract of E. natalensis had potent activity against M. bovis (MIC=26 microg/ml). The naphthoquinone 7-methyljuglone was the most active compound, with an MIC as low as 1.55 microg/ml against pathogenic M. bovis. M. bovis BCG was not as susceptible to the test compounds as the pathogenic strain, but similar patterns of activity were observed between all the strains tested. M. smegmatis appeared to be a better predictor of antimycobacterial activity against pathogenic M. bovis (and M. tuberculosis), while MIC values obtained using M. fortuitum correlated well with those of M. bovis BCG.

Activity of 7-methyljuglone derivatives against Mycobacterium tuberculosis and as subversive substrates for mycothiol disulfide reductase

The naphthoquinone 7-methyljuglone (5-hydroxy-7-methyl-1,4-naphthoquinone) has previously been isolated and identified as an active component of root extracts of Euclea natalensis which displays antitubercular activity. Herein, a series of synthetic and plant-derived naphthoquinone derivates of the 7-methyljuglone scaffold have been prepared and evaluated for antibacterial activity against Mycobacterium tuberculosis. Several of these compounds have been shown to operate as subversive substrates with mycothiol disulfide reductase. The absence of a direct correlation between antitubercular activity and subversive substrate efficiency with mycothiol disulfide reductase, might be a consequence of their non-specific reactivity with multiple biological targets (e.g. other disulfide reductases).

Activity of 7-methyljuglone in combination with antituberculous drugs against Mycobacterium tuberculosis

The recent increase in the incidence of tuberculosis with the emergence of multidrug-resistant (MDR) cases has lead to the search for new drugs that are effective against MDR strains of Mycobacterium tuberculosis and can augment the potential of existing drugs against tuberculosis. In the present study, we investigated the activities of a naphthoquinone, 7-methyljuglone, isolated from the roots of Euclea natalensis alone and in combination with other antituberculous drugs against extracellular and intracellular M. tuberculosis. Combinations of 7-methyljuglone with isoniazid or rifampicin resulted in a four to six-fold reduction in the minimum inhibitory concentration of each compound. Fractional inhibitory concentration (FIC) indexes obtained were 0.2 and 0.5, respectively, for rifampicin and isoniazid, suggesting a synergistic interaction between 7-methyljuglone and these anti-TB drugs. The ability of 7-methyljuglone to enhance the activity of isoniazid and rifampicin against both extracellular and intracellular organisms suggests that 7-methyljuglone may serve as a promising compound for development as an anti-tuberculous agent.

Aegicerin, the first oleanane triterpene with wide-ranging antimycobacterial activity, isolated from Clavija procera

An ethanol extract of the Peruvian plant Clavija procera, a member of the rare Theophrastaceae family, was fractionated using a colorimetric bioassay-guided protocol against Mycobacterium tuberculosis (MTB), yielding the oleanane triterpenoid aegicerin (1) as the active constituent. Its MIC values ranged between 1.6 and 3.12 microg/mL against 37 different sensitive and resistant MTB strains (1 H37Rv, 21 susceptible clinical isolates, 2 INH-resistant clinical isolates, and 13 MDR clinical isolates).

New perspectives on natural products in TB drug research

The challenge of discovering new, urgently needed anti-TB drugs from natural sources requires a truly interdisciplinary research. Cutting-edge mycobacteriology and innovative natural products chemistry tools have to be developed and employed in tandem, in order to meet these demands. The present review provides cross-linkage to the most recent literature on anti-TB active natural products and summarizes the recent developments in both fields and their potential to impact the early steps of the TB drug discovery process.