A.J. Afolayan

Assessment techniques of antimicrobial properties of natural compounds of plant origin: current methods and future trends

Medicinal plants have recently received the attention of the pharmaceutical and scientific communities and various publications have documented the therapeutic value of natural compounds in a bid to validate claims of their biological activity. Attention has been drawn to the antimicrobial activity of plants and their metabolites due to the challenge of growing incidences of drug-resistant pathogens. Some plants have shown the ability to overcome resistance in some organisms and this has led to researchers’ investigating their mechanisms of action and isolating active compounds. Particular focus is on establishing the effect of the plant(s) extracts in terms of their microstatic and microcidal action and the spectrum of organisms affected. This has enabled exploitation of plants for the treatment of microbial infections and in the development of new antimicrobial agents. This requires rigorous research and it is therefore imperative to follow standard methods to authenticate claims of antimicrobial action. Results comparability is largely dependent on the techniques employed in the investigations and conclusive results can only be obtained if methods are standardized and universal. This paper reviews the current methods used in the investigations of the efficacy of plants as antimicrobial agents and points out some of the differences in techniques employed by different authors.

Antimicrobial Activity of Solanum aculeastrum.

Abstract Solanum aculeastrum. Dunal (Solanaceae) is used in traditional medicine to treat various human and animal diseases, specifically stomach disorders and various cancers, in the Eastern Cape, South Africa. The fruit and leaf extracts of this plant were investigated for in vitro. antimicrobial activity against 10 selected bacterial and 5 fungal strains. The methanolic extracts of both the fruits and the leaves showed appreciable activity against Gram-positive and Gram-negative bacteria ranging from 4.0 to 10.0 mg/ml. Whereas the methanol extracts were the most active material, the water extracts showed the least activity against the bacteria. The methanol extracts were particularly inhibitory to the growth of the fungi with percentage inhibition ranging from 60.26% to 100% and 56.0% to 100% on Aspergillus flavus. and Pencillium notatum., respectively. The acetone extracts were active against Aspergillus flavus. (100%) and Pencillium notatum. (64.81%), and the water extract of the fruit significantly inhibited the growth of P. notatum. (69.89%). The most resistant organisms were Aspergillus niger, Candida albicans., and Fusarium oxysporum..

Extracts from the Shoots of Arctotis arctotoides Inhibit the Growth of Bacteria and Fungi

Acetone, methanol and water extracts obtained from the shoots of Arctotis arctotoides through shaking and homogenization, were investigated for their antimicrobial activities. Growth inhibition using agar dilution assays was determined against ten selected bacterial and six fungal species. Although not fungicidal, extracts from the herb showed significant growth inhibition against all the fungi tested. The homogenized water extract was particularly inhibitory to the growth of the fungi with inhibitory activity ranging from 50.7 to 95.2% on Aspergillus tamari and Penicillum digitatum, respectively. Acetone and methanol extracts were very active against the Gram positive bacteria. The Gram negative bacteria were, however, more resistant to the extracts than the Gram positive ones. None of the extracts inhibited Klebsiella pneumoniae and Pseudomonous aeruginosa, both Gram negative bacteria.

An Ethnobotanical Study of Plants Used for the Treatment of Livestock Diseases in the Eastern Cape Province, South Africa

Information on medicinal plants used by the people of the Eastern Cape for the treatment of animal diseases was collected by rapid rural appraisal techniques, including ranking, stories, transect walking, seasonal calendars and semi-structured interviews. A total of 38 species belonging to 31 families was found to be commonly used in herbal remedies against various diseases of farm animals. Out of these, 21 plant species from 17 families are used to treat gallsickness, while 13 species from 13 families are used to treat redwater disease. In the majority of preparations, 78% of the remedies are decoctions: 13% as infusions, 5% by just squeezing the plant parts, and 4% of the preparations are either prepared as infusions, or as decoctions. Leaves are the most frequently used plant part. A total of nine livestock disease conditions, namely gallsickness, redwater, heartwater, eye inflammation, retained placenta, foot rot, hastening of estrus, three-day-stiff sickness and internal parasites (helminthiasis) were reported to be treated using the plants recorded during the investigation. Generally, more than one plant species are combined for the treatment of livestock by the rural farmers of the Eastern Cape province. In most cases, the plants used are reported, in literature, to possess some pharmacological activity.

Chemical Composition and Antibacterial Activity of the Essential Oil of Centella asiatica. Growing in South Africa

ABSTRACT In South Africa, Centella asiatica. (L.) Urb is used traditionally for the treatment of various diseases. Analyses of the essential oil of this medicinal plant revealed 11 monoterpenoid hydrocarbons (20.20%), nine oxygenated monoterpenoids (5.46%), 14 sesquiterpenoid hydrocarbons (68.80%), five oxygenated sesquiterpenoids (3.90%), and one sulfide sesquiterpenoid (0.76%). α.-Humulene (21.06%), β.-caryophyllene (19.08%), bicyclogermacrene (11.22%), germacrene B (6.29%), and myrcene (6.55%) were the predominant constitutes. The essential oil extract exhibited a broad spectrum of antibacterial activities against Gram-positive (Bacillus subtilis, Staphylococcus aureus.) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa, Shigella sonnei.) organisms.

Evaluation of the medicinal potentials of the methanol extracts of the leaves and stems of Halleria lucida.

The medicinal potentials of the methanol extracts of the leaves and stems of Halleria lucida (Scrophulariaceae) were evaluated by assessing their antibacterial and antioxidant properties in vitro using standard procedures. The antioxidant activities of methanol extract of the leaves as determined by the ABTS, DPPH, proanthocyanidins and total flavonoids were higher than that of the stem. On the other hand, the total phenols, the flavonoids and the FRAP contents of the stem were higher than that of the leaves. The extracts however showed poor activity against both Gram-positive and Gram-negative bacteria. The methanol extract of the stem showed activities against Bacillus cereus and Staphylococcus epidermidis at MIC of 1.0 mg/ml. The methanol extract of the leaves did not show activity against any of the organisms used in this study. This study has to some extent validated the medicinal potential of the leaves and stems of H. lucida.

Evaluation of Antimicrobial Activity of Extracts from South African Usnea barbata

Usnea barbata is a lichen of the Usneaceae family that grows epiphytically on trunks and branches of trees. It is widespread in Hogsback, South Africa. Extracts of the lichen were investigated for antimicrobial activity against 10 bacterial and 5 fungal strains by dilution method on solid agar medium. The extracts showed significant activity against the Gram-positive bacteria with minimum inhibitory concentration as low as 0.1 mg/ml on Bacillus subtilis, Enterococcus faecalis, Micrococcus viradans and Staphylococcus aureus. The acetone extract was the most active while the water extract showed the least activity against the microbes. Our findings have validated the use of the lichens for the treatment of various infections in man and livestock.

COMPARISON OF THE NUTRITIONAL VALUE AND BIOLOGICAL ACTIVITIES OF THE ACETONE, METHANOL AND WATER EXTRACTS OF THE LEAVES OF SOLANUM NIGRUM AND LEONOTIS LEONORUS

The nutritional, phytochemical, antioxidant and antibacterial activities of the acetone, methanol and water extracts of the leaves of Solanum nigrum and Leonotis leonorus were investigated using standard analytical methods in order to assess the numerous potential of the leaves of these plants. The proximate analysis showed the that the leaves of the two plants were rich in moisture content, ash content, crude protein, crude lipid, crude fibre and carbohydrate. Elemental analysis in mg/100g (DW) indicated that the leaves contained sodium, potassium, calcium, magnesium, iron, zinc, phosphorus, copper, manganese, and nitrogen. The chemical composition in mg/100g (DW) for alkaloid, saponins, and phytate were moderate. The plants were also rich in polyphenols and had good antioxidant activities. The different extracts of the plants had activities against some of the organisms used in this study. Comparing the nutrient and chemical constituents with recommended dietary allowance (RDA) values, the results reveal that the leaves contain an appreciable amount of nutrients, minerals, and phytochemicals and low levels of toxicants.

Anticancer Activity of Steroid Alkaloids Isolated from Solanum aculeastrum.

Abstract Solanum aculeastrum. Dunal is a medicinal plant that has long been used to treat various cancers and many other conditions in the Eastern Cape Province of South Africa. In this study, two steroid glycosides were isolated from the berries of this plant, which were identified as tomatidine and solasodine by spectroscopic techniques. Effects on cell growth of these compounds were investigated with HeLa, MCF7, and HT29 cancer cell lines. The IC50 values confirmed that tomatidine and solasodine had the highest inhibitory effect on HeLa cells and the IC50 of the combined compounds was lower than the value for solasodine and unchanged from that of tomatidine. However, the IC50 values of the two compounds combined was also lower in HT29 and MCF7 cells than for the individual compounds. Both tomatidine and solasodine inhibited cell growth by blocking the cell cycle in the G0/G1 phase after 24-h exposure with an increase from 55.6% to 64.2% and 66.8%, respectively. Using annexin V–FITC/PI staining by flow cytometry, the compounds showed very low apoptotic indices.

Antioxidant Constituents in Vernonia amygdalina. Leaves

Abstract Vernonia amygdalina. Del. (Asteraceae) is a plant widely used both for nutritional and medicinal purposes throughout the tropical Africa. The health-promoting ability of this plant species might be related to the antioxidative effect of its constituents. In this study,the antioxidant activity of this plant was evaluated by comparing the reducing capacity and the DPPH radical scavenging of two previously isolated sesquiterpene lactones (vernolide and vernodalol) with the ethanol extract from which the two compounds were isolated. Results indicated that vernolide had a higher reducing power than vernodalol and the ethanol extract. At 0.25 mg/mL, vernolide had an absorbance value of 0.15 while vernolide and the ethanol extract had absorbencies of 0.042 and 0.144, respectively. Catechin (a standard antioxidant compound), however, exhibited a higher reducing power than all the three samples. In the DPPH radical scavenging, both the sesquiterpene lactones and the ethanol extract exhibited appreciable activity. At 0.25 mg/mL, the activity order was ethanol extract > vernodalol > vernolide. At all concentrations, the ethanol extract had higher radical scavenging activity than the sesquiterpene lactones, suggesting a possible synergistic effect of these and any other antioxidant constituents.