Journal of Ethnopharmacology 40 (1993) I27- I30

Antimicrobial activity of Dalbergia melanoxylon extracts

Mazuru Gundidza*, Norrin Gaza

Deparrment of Pharmaev, University of Zimbabwe, P.O. BOX MP 167. Mount Pleasant. Harare, Zimbabwe

(Received 9 December 1992; accepted 5 August 1993)

Abstract

The antibacterial and antifungal properties of the methanol, citric acid, aqueous, dichloromethane and petroleum ether extracts from the bark of Dalbergiu melunoxylon were determined by using seeded agar plates with wells into which were placed the extract, and flasks of yeast extract and sucrose broth for mycelial growth of the fungi. After incubation for 24 h, the diameter of the inhibition zone was measured for the antibacterial tests and after 7 days, the dry weight of the mycelia was measured and a percentage of inhibition calculated using controls where no extracts were added. The results obtained showed that the citric acid extract exhibited strongest antimicrobial activity. The ethanol fraction showed significant antibacterial activity but was not significantly active against fungi. The dichloro- methane extract exhibited no activity against bacteria but showed notable activity against fungi. The petroleum ether fraction showed no antimicrobial activity.

Key words: Dalbergia melanoxylon; Leguminosae; Antimicrobial

1. Introduction

In many developing countries some 80% of available medicines are obtained from medicinal

plants, while in the western developed countries the plants mainly constitute raw materials for in- dustrial processing or preparation of the pure chemical derivatives (Penso, 1980). Due to ever in-

creasing prices of drugs, especially in developing countries, a need to search for cheap drugs from natural sources becomes imperative. Because the most rampant killer diseases in developing coun-

* Corresponding author.

tries are of microbiological origin, research and development of antimicrobial therapeuticals could be invaluable.

Several studies on antimicrobial substances from plants have been conducted by a number of investigators (Barnabas and Nagarajan, 1988; Aday et al., 1989; Gundidza and Manwa, 1990). Active constituents include terpenes, alkaloids, steroidal saponins, tannins, phenols, quinones and flavonoids (Ieven et al., 1979; Reinhold et al., 1981; Harborne, 1982; Bever, 1986).

Research on bioactive substances from plant sources has great scope and could lead to the pro- vision of value-added economic retdrns, the

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128 M. Gundid:a. N. Guru/J. Erhnopharmacol, 40 ( 1993) 127-130

establishment of a natural plant products industry, an increase in domestic employment, the creation of relevant technological capability, an increase in export earnings and the betterment of the rural populations in terms of health care provision. To this end the Department of Pharmacy at the Uni- versity of Zimbabwe has embarked on a broad an- timicrobial screening of Zimbabwean plants. In this study Dalbergia melanoxylon was chosen on the basis of its folklore uses that rely on an- timicrobial activity. The plant belongs to the Leguminosae family (Gelfand et al., 1985). Tradi- tionally in Zimbabwe, the bark decoction is used for cleaning wounds. A decoction of the root is drunk to alleviate abdominal pains, and is also used as an anthelmintic and a gonorrhoea remedy. The leaves are boiled in soup which is then drunk to relieve pain in the joints (Gelfand et al., 1985). In East Africa it is used to treat gonorrhoea, and the infusion of the root is used to treat stomach ache, as a mouth wash for tooth ache and by women for abdominal pains (Kokwaro, 1976). In South Africa it is used as an emetic and in Zambia for its aphrodisiac properties (Watt and Breyer- Brandwijk, 1962; Gelfand et al., 1985). Phyto- constituents of the plant include flavonoids, quinones, 4-phenyl coumarins, (S)4-methoxy dalbergiones and benzophenones (Donelly et al., 1975).

In the present investigation the solvent-solvent extraction method was used to obtain 4 extracts, the ethanol, citric acid dichloromethane and petro- leum ether fractions. Each fraction was tested against 7 bacteria and 2 fungi using the hole-plate diffusion method and the mycelial growth inhibi- tion method. The results obtained indicate that the ethanol fraction had significant overall activity whereas the citric acid fraction showed activity against bacteria and not against fungi, whereas the dichloromethane extract exhibited activity against fungi and not against bacteria. The petroleum ether showed no antimicrobial activity.

2. Materials and methods

2.1. Plant material The plant material was collected from Masv-

ingo, 267 km South of Harare, Zimbabwe, and

was identified at the National Herbarium and Botanical Gardens of Zimbabwe in collaboration with Botanists at Kew Gardens, London, UK. One voucher specimen was deposited in the Depart- ment of Pharmacy and the other at the National Herbarium and Botanical Gardens.

2.2. Extraction procedure The extraction procedure used by Ieven et al.,

(1979) was employed in this study.

2.3. Preparation of samples for testing The samples where prepared according to the

method employed by Ieven et al., (1979) in which 100 mg of lipophilic fractions (dichloromethane and petroleum ether) were dissolved in 10 ml of a mixture of polyethylene-glycol 400 and sterile physiological Tris buffer (pH 7.4) in a ratio of 2:3. Similarly, 100 mg each of the hydrophilic frac- tions, ethanol and citric acid were dissolved in 10 ml of sterile physiological Tris buffer (pH 7.4). In each case, five 2-fold serial dilutions were made to give 10, 5, 2.5, 1.25 and 0.625 mg/ml and then tested for antibacterial activity.

2.4. Antibacterial testing The methods suggested by Hoges (1988) for the

MIC and MBC determination were employed. Tetracycline hydrochloride and benzyl penicillin sodium were used as references for Gram-negative bacteria (Escherichia coli, Pseudomonas aerugi- nosa, Salmonella typhimurium and Yersinia pestis) and Gram-positive bacteria Bacillus subtilis, Kleb- siella pneumoniae, and Staphylococcus aureus), respectively.

2.5. Antifungal testing The method described by Deans and Ritchie

(1987) was used for the antifungal testing.

3. Results

The antibacterial activity results are shown in Table 1 and the antifungal activity results in Table 2. Negative results from dichloromethane and pe- troleum ether are not shown in Table 1.

Generally the above results show that the citric acid fraction exhibited significant activity against

M. Gundidza, N. Ga:a/J. Erhnopharmacol. 40 (1993) 127-130

Table 1

Antibacterial activities of ethanol, citric acid extracts and tetracycline hydrochloride and benzyl penicillin sodium

129

Bacteria Source Ethanol

MIC

(&ml)

MBC

(f&ml)

Citric acid

MIC

(&ml)

MBC

(r&ml)

Tetracycline hydro-

chloride/benzyl

penicillin sodium

MIC MBC

(&ml) (&ml)

Bacillus subrilis NCIB 3610 48 49 2.81 5.12 3.31 13.1

Escherichia coli NCIB 8879 100 120 6.25 12.5 12.5 25.0

Klebsiella pneumoniae NCIB 418 132 138 11.83 24.0 13.3 51.1

Pseudomonas aeruginosa NCIB 950 100 130 6.25 12.5 100 135

Salmonella typhimurium NCTC 1074 125 140 50 65 12.5 50.0

Staphylococcus auerus NCIB 6571 49 101 3.35 12.6 103 139

Yersinia pesris NCTC 10460 125 142 50 65 12.5 100

bacteria followed by the ethanol fraction with the dichloromethane and petroleum ether fractions showing no antibacterial activity (results not shown in Table 1). The citric acid exhibited higher antifungal activity than the ethanol extract. The dichloromethane fraction exhibited significant an- tifungal activity with the petroleum ether extract exhibiting no antifungal activity at the given con- centrations.

4. Discussion

This investigation has revealed that the plant ex- tracts can be effective antibiotics. The citric acid fraction showed more antibacterial activity than the tetracycline hydrochloride and benzyl penicil- lin sodium at the concentration chosen. The MIC

Table 2

Antifungal properties of Dalbergia melanoxylon extracts

Fraction Candida albicans Aspergillus niger (0.625 mg/ml) (Source: IMI 15954) (Source: IMI 17454)

Ethanol 8.2 11.2

Citric acid 59. I 55.7

Dichloromethane 62.8 61.0

Petroleum ether 0.0 0.0

Nystatin 70.3 75.4

Inhibition is expressed as a percentage relative to the control

flasks.

and MBC results showed that the extracts ex- hibited bacteriostatic activity at lower concentra- tions and bactericidal activity at higher concen- trations. Practical application of such results in- volves comparing the MIC with the concentration active in vivo in order to work out the mechanism of action which could be either bacteriostatic or bactericidal (Bever, 1986). Generally, antibiotics require an in vivo concentration in the tissues that is equal to or greater than their minimum inhibi- tion concentration in vitro (Bever, 1986). There- fore, the MIC determination is important in giving a guideline to the choice of an appropriate and ef- fective concentration of a therapeutic substance. The active constituents of this plant may be flavo- noids which Barnabas and Nagarajan (1988) have shown to be active against Gram-positive bacteria. Barnabas and Nagarajan (1988) indicated that fla- vonoids exert antimicrobial activity in the healing of wounds and in the treatment of skin diseases. This correlates well with local herbalist beliefs that a decoction of the bark is useful in the cleaning of wounds (Gelfand et al., 1985).

The citric acid and dichloromethane fractions exhibited comparable antifungal activity with nystatin. This implies that the plant extracts can well be used to control fungal infections or food spoilage by fungi.

However, it is important to point out that crude extracts such as these need to be further processed

130 M. Gundid:a. N. Ga:a /J. Erhnopharmacol. 40 (1993) 127-130

to pure compounds which can then be tested for antimicrobial activity. This forms the basis of the next generation of experiments in which the crude extracts will be purified and in turn tested for an- timicrobial activity as well as for toxicity with the view of formulating them into crude drugs if the therapeutic threshold is acceptable.

5. Acknowledgements

The authors are greatful to the University of Zimbabwe for providing facilities and financial as- sistance for this study, to the technicians in the De- partment of Pharmacy for their technical help and to Miss Grater Mukondorongwe for typing this paper.

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