Inhibitory properties of selected South African medicinal plants against Mycobacterium tuberculosis

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Abstract

Ethnopharmacological relevance

Tuberculosis (TB), caused by Mycobacterium tuberculosis (MTB) is the most commonly notified disease and the fifth largest cause of mortality. One in 10 cases is resistant to treatment in some areas. Several plants are used locally to treat TB-related disease.

Aims of the study

The aim was to screen selected South African medicinal plants used to treat TB and related symptoms by traditional healers for antimycobacterial activity.

Materials and methods

Ethnobotanical information on these plants was obtained. Crude acetone, methanol, hexane and ethanol extracts of 21 selected medicinal plants obtained in Venda, South Africa were screened for their ability to inhibit MTB H37Ra and a clinical strain resistant to first-line drugs and one second-line drug using tetrazolium microplate assay to determine the minimum inhibitory concentration (MIC). Results were analyzed using Microsoft Excel 2007 and One way ANOVA; p < 0.05 was considered for statistical significance.

Results

Few acetone extracts were active against MTB with MIC under 100 μg/mL. Four plants showed lower MIC values; Berchemia discolor Klotzsch Hemsl 12, 5 μg/mL on H37Ra and 10.5 μg/mL on the clinical isolate, Bridelia micrantha Hochst. Baill (25 μg/mL), Warbugia salutaris Bertol. F Chiov (25 μg/mL), and Terminalia sericea Burch ex D. F (25 μg/mL) on both H37Ra and clinical isolate. However, the roots of Ximenia caffra Sond. Var. caffra, barks of Sclerocarya birrea (A Rich) Hochst, Asclepias fruticosa L, tubers of Allium sativum L, leaves of Carica papaya L, Solanum panduriforme E. Mey C, and roots of Securidaca longepedunculata Fresen gave MIC greater than 100 μg/mL.

Conclusion

The acetone extracts of Berchemia discolor, Bridelia micrantha, Terminalia sericea and Warbugia salutaris could be important sources of mycobactericidal compounds against multidrug-resistant MTB.

Graphical abstract

The aim of this study was to screen selected South African medicinal plants used to treat TB and related symptoms by traditional healers for antimycobacterial activity.

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Introduction

MTB is the aetiological agent of tuberculosis (TB), one of the most prevalent infections in the world causing high mortality in developing countries. The disease spreads more easily in overcrowded settings and in conditions of malnutrition and poverty (Pereira et al., 2005). New TB cases have been estimated at 9.2 million during 2006, with 1.7 million MTB-related deaths. This is an increase over the 9.1 million new cases reported in 2005 (World Health Organization, 2008). One-third of the world's population is estimated to have latent MTB (World Health Organization, 2008). Although the increase in new TB cases is attributed to population growth, new drug-resistant strains that threaten to increase this number, undermine efforts to eradicate this disease, and exacerbate the healthcare burden. Incidences of multidrug-resistant TB and extensive drug-resistant TB are on the rise, requiring the development of new treatment regimens, drugs, and drug targets (World Health Organization, 2008, Green et al., 2010).

Few alternative drugs are available in cases where drug resistance is a problem. Despite highly effective first-line drugs, morbidity and mortality due to MTB are still increasing (Ballell et al., 2005). It is estimated that between the years 2000 and 2020 nearly one billion people will be newly infected, 200 million will develop TB and 35 million will die from the disease (World Health Organization, 2000).

In South Africa, multidrug-resistant tuberculosis (MDR-TB) has also been identified (Green et al., 2008, Green et al., 2010) and these emerging MDR strains complicate treatment of TB (Victor et al., 2007). In 2005, it was estimated that 285,000 cases of TB developed in South Africa, representing the seventh highest total number of cases in the world and the second highest total in Africa (World Health Organization, 2007a). Second-line drugs such as kanamycin, p-aminosalicylate, ethionamide and fluoroquinolones are either toxic or less effective (Blumberg et al., 2003). In 2005, large numbers of patients with MDR-TB and XDR-TB were identified at a rural hospital in Tugela Ferry, KwaZulu-Natal (Gandhi et al., 2006).

Coinfection of MTB with the human immunodeficiency virus (HIV) and the emergence of strains resistant to available therapies (Chung et al., 1995, Ballell et al., 2005) have also been linked to morbidity and motality due to MTB. South Africa is home to one of the largest populations of HIV-infected individuals in the world and has more patients receiving antiretroviral therapy (ART) than does any other country (World Health Organization, 2007b). The increase in TB drug resistance in this context undermines the strides that the national ART rollout has made and may potentially limit its successful and continued expansion. Fifty-eight percent of patients with TB were coinfected with HIV. Of significant concern, the TB cure rate (64%) was the lowest among the 10 countries with the highest TB burden (Andrews et al., 2007). The emergence of XDR-TB is a signpost for the necessity to find new drugs for the management of TB. An alternative source of new drugs is in natural products isolated from medicinal plants.

Natural products isolated from plants have played an important role in discovery of drugs against infectious diseases. Almost 75% of the approved anti-infective drugs are derived from medicinal plants (Cragg et al., 1997). According to World Health Organization, more than 65% of the global population uses medicinal plants as a primary health care modality (Farnsworth et al., 1985). South Africa is a rich source of medicinal plants containing approximately 10% of the world's terrestrial plants, some being medicinal (Arnold et al., 2002); and members of different indigenous communities in South Africa consult traditional herbalists for the treatment of infections.

Over 350 plant species used in traditional medicine have been assessed for their antituberculosis activities (Eldeen and van Staden, 2007). Apart from the antituberculosis activities, plant extracts have also demonstrated immunomodulatory effects on different cell cultures and in experimental animals (Eloff, 2001). However, there is paucity of information and scientific validation on plants used in Venda region to cure TB and its related symptoms. The aim of the present study was to evaluate plant species for antimycobacterial activities. Plants were chosen based on ethnobotanical information, that is, they have been used in traditional medicine for the treatment of TB or symptoms of this disease in our environment.

Section snippets

Mycobacterium tuberculosis isolates

We used MTB H37Ra (American Type Culture Collection 25177) because it is sensitive to all first-line drugs, while our isolates (Green et al., 2010) are resistant to all first-line drugs.

Plant material and extraction

Two traditional healers in the Limpopo province who receive TB patients were interviewed on the type of plants they employ in treating these individuals. These individuals have either disclosed their condition confirmed by a medical report or the healers suspected TB when the patients presented with a

Results and discussion

The increase of resistance to conventional antibiotics by microorganisms has necessitated the search for new, efficient and cost effective ways for the control of infectious diseases (Samie et al., 2005, Ndip et al., 2007). A number of South African plants have been shown to contain antituberculosis activities (Mativandlela et al., 2008, Thring et al., 2007, Lall and Meyer, 1999). However, such studies were based on a very limited number of plant species with the probability of losing species

Acknowledgement

The authors are thankful to the National Research Foundation, South Africa for their financial support.

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