In vitro antibacterial activity of selected plant extracts from Zimbabwe
Chitemerere, Tariro Alison
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Bacterial infections, if not properly treated, have the potential to become life-threatening in humans. The discovery of antibiotics in the 1940s made it possible for bacterial infections to be easily managed, thus, removing the threat of bacterial infections on human life. However, due to the abuse of antibiotics, many species of bacteria have developed resistance to an extent where is it feared that mankind may be on the brink of the post-antibiotic era. The once easy to treat bacterial infections are now posing as a serious threat to human life. However, all hope is not lost if antibiotics completely fail. Plants are another potential source of antimicrobial compounds that are effective against bacteria. Herbal medicines have been in use since ancient times, even before the discovery of antibiotics. Plants are known to possess many phytocompounds which have various therapeutic effects. In a bid to discover novel compounds for the development of new effective antibacterial medicines, it is therefore, important to take a fresh look at plants. Could plants from Zimbabwe be potential sources of new antibiotics effective against bacterial pathogens? This study was aimed at investigating the antibacterial activity of selected plants from Zimbabwe against 5 species of bacteria. The antibacterial activities of ethanolic extracts from 18 plants were assessed using the agar diffusion assay, minimum inhibitory concentration and minimum bactericidal concentration determination assays using ampicillin as reference. Plants were chosen on the basis of ethnomedicinal use as well as random selection. Test bacteria used in the study were Staphylococcus aureus , Pseudomonas aeruginosa , Escherichia coli , Bacillus cereus and Bacillus subtilis . Plant extracts were also tested for activity as drug efflux pump inhibitors (EPIs). The R6G accumulation assay was carried out to investigate the plant compounds that can inhibit bacterial efflux pump activity, which has been asscociated with drug resistance. The extracts that possessed antibacterial activity were used for target determining assays using the bacterial cell membrane as a potential target. Phytochemical analyses of the most potent extracts were also carried out to further isolate the antibacterial compounds. Bioautography was used to locate the antibacterial compounds of isolated phytoconstituents on a chromatogram. Of the plants tested, Syzigium cumini , Callistemon citrinus , Ficus sycomorus, Vernonia adoensis, Parinari curatellifolia , Lantana camara , Brachystegia boehmii and Mangifera indica, Faurea species and Asteraceae family exhibited antibacterial activity against all bacteria with activity ranging from bacteriostatic to bactericidal. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) determination of plant extracts ranged from 0.05 to 0.5 mg/ml and 0.06 mg/ml to > 1 mg/ml, respectively. C. citrinus and V. adoensis extracts were the most potent with bactericidal activity against all bacteria while the other extracts were bacteriostatic. Both extracts were also effective EPIs in the uptake of R6G with activity greater than the standard inhibitor reserpine. Of all the plants used in this study C. citrinus showed the most potent antibacterial activity. Phytochemical screening of C. citrinus showed the presence of tannins, cardiac glycosides, flavonoids, alkaloids and saponins in the extract. Antibacterial susceptibility tests on the isolated phytoconstituents showed the flavonoids had the most potent antibacterial activity. IC 50 values for flavonoids were in the range of 40 – 60 µg/ml and these values were comparable to the IC 50 values of reserpine, the standard plant-based EPI (40 – 80 µg/ml). Results also showed that C. citrinus and V. adoensis extracts and phytoconstituents were able to permeabilize the membrane, hence, making the bacterial membrane a potential target for the plants used in this study. The haemolysis assay showed that the sheep erythrocytes were not lysed in the presence of the crude extracts of C. citrinus and V. adoensis at concentrations lower than 2.5 mg/ml, hence drug formulations with extract concentrations less than 2.5 mg/ml can be potentially non toxic to human cells. The study concludes that C. citrinus 2 flavonoids have the highest antibacterial activity as compared to other extracts tested hence, these flavonoids have potential to serve as templates for the development of new and effective antibacterial agents as well as efflux pump inhibitors. Plants therefore, provide hope for a source of new antibacterial agents that can manage bacterial infections and may have the potential to overcome antibiotic resistance.