Screening of some Tanzanian medicinal plants from Bunda district for antibacterial, antifungal and antiviral activities
Introduction
Despite emphasis being put in research of synthetic drugs, a certain interest in medicinal plants has been reborn, in part due to the fact that a lot of synthetic drugs are potentially toxic and are not free of side effects on the host (Geddes, 1985). This has urged microbiologists all over the world for formulation of new antimicrobial agents and evaluation of the efficacy of natural plant products as a substitute for chemical antimicrobial agents (Pandian et al., 2006). Medicinal plants are well-known natural sources for the treatment of various diseases since antiquity. About 20,000 plant species used for medicinal purposes are reported by WHO (Gullece et al., 2006). Furthermore, natural products, either pure compounds, or as standardized plant extracts, provide unlimited opportunities for new drug leads because of the unmatched availability of chemical diversity (Cos et al., 2006). In the past the wide range of antimicrobial agents from lower organisms and synthetic drugs sufficed in the treatment or control of infectious diseases, but currently there is a problem of microbial drug resistance and there is an increase of opportunistic infections especially with AIDS patients and individuals on immunosuppressive chemotherapy. Many antifungal and antiviral drugs are of limited use due to toxicity, while other viral diseases have not yet found a cure. These problems pose a need of searching more new substances.
Tanzania is located in the tropical zone where the climate is propitious for development of infectious diseases. Like other developing countries, a major portion of its population is combating diseases by using plants with medicinal properties. The importance of using medicinal plants can be attributed to a number of reasons, including affordability and limited availability of western medicine as well as the trust in herbal medicine as an outcome from the witnessed positive results when applying herbs.
Our ethnobotanical survey in Bunda district (Maregesi et al., 2007) listed relevant references of ethnomedical surveys reported by other groups revealing the importance of herbal medicine in this country. Taking into consideration the geographical area of about 945,000 km2 with a population of over 30 million people, consisting of many ethnic groups, there remains a large area whose medicinal plants have not been documented. Even with the few documented plants through ethnopharmacological surveys, little work has been done on the screening on these plants to prove the claimed medicinal uses as described by traditional healers. Thus, after carrying out an ethnobotanical survey in Bunda district, our team took a step further to test these plants against selected microorganisms to prove their efficacy.
During interviews with traditional healers/informants, it was relatively easy to correlate the plants mentioned with bacterial/fungal infections compared to viral infections. Boscia angustifolia was the only plant claimed to treat viral infection (mumps). Knowing that viruses are responsible for many human infections, some of the mentioned diseases like respiratory infections, and diarrhoea accompanied with vomiting could be due viruses. Therefore our screening battery also included four viruses in addition to bacteria and fungi.
Section snippets
Plant collection
Interviews with traditional healers/informants on plants used to treat infectious diseases in Bunda district, Tanzania enabled the collection of 50 samples of various plant parts from 39 plants. Samples were dried under shade in open air to reduce deterioration of the plant drug material. Identification of collected plants was done at the Herbarium Unit, Botany Department of the University of Dar-Es-Salaam by Mr. Frank Mbago. Voucher specimens were deposited in the same herbarium.
Preparation of crude plant extracts
The air-dried
General
A total of 147 extracts prepared by using n-hexane, 80% methanol and water from 50 plant parts belonging to 39 plants were tested against six bacteria, two fungi and four viruses. All tested plant parts showed at least some activity to one or more tested microorganisms, even though this was sometimes only observed at the highest test concentration of 1000 μg/ml. Based on polarity, n-hexane extracts ranked highest (75%) followed by methanol extracts (70%) and the least active were water extracts
Discussion of the observed activities
The selection of plants for this study was based on their use to treat infectious diseases such as skin infections (bacterial and fungal), dysentery, diarrhoea, eye infection, and venereal diseases. Staphylococcus aureus in particular is amongst the microbes responsible for causing skin infections such as boils, abscesses, carbuncles and sepsis of wounds, which were indicated to be treated by several plants by traditional healers. Both Bacillus cereus and Staphylococcus aureus produce toxins
Conclusion
Obviously the values obtained against the microorganisms tested were higher than the positive controls, since the crude extract contains many compounds, of which only a few may be active. However, still in its crude form, the MIC value of the n-hexane extract of Elaeodendron schlechteranum root bark was comparable with that of ampicillin. Recently, Cos et al. (2006) suggested as a criterion for crude extracts to be considered active, that it should have a MIC value of 100 μg/ml or less. In this
Acknowledgements
We are very grateful to the information providers on medicinal plants from Bunda district and the cooperative support from all members of the staff in the Laboratory of Pharmacognosy and Pharmaceutical Analysis and the Laboratory of Microbiology, Parasitology and Hygiene in the Department of Pharmaceutical Sciences, University of Antwerp.
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