Abstract:
Many microorganisms have the potential to cause diseases for which the human body is prone. The discovery of antibiotics has, however, provided important tools to combat bacterial diseases. But due to the indiscriminate use of commercial antimicrobial drugs in the treatment of infectious diseases multiple drug resistance has developed among some pathogens. This creates a need for searching new effective and safe antimicrobial therapeutic agents. The aim of this study was to test the antibacterial and antifungal activities of crude extracts of leaves of Psidium guajava, Occimum lamifolium, Euclea schimperi and aerial parts of Euphorbia candelabrum against four pathogenic bacterial species, i.e. Staphylococcus aureus, Streptococcus agalactiae, Escherichia coli and Salmonella Typhi and two species of fungal pathogenic species, i.e. Aspergillus niger and Aspergillus flavus. The leaves of the three-plant species and the aerial parts of the remaining plants were shade dried and ground to powder and the bioactive compounds were extracted using (99.8%) methanol, absolute ethanol and distilled water. In the present study, the results showed that the percentage yield of the methanol, absolute ethanol and distilled water extracts varied among the plant species. The highest percentage yield (22.34 ±.59%) was recorded for methanol extract of P. guajava and the lowest yield (6.28±.1.21%) was obtained for the distilled water extract of E. schimperi. The antimicrobial activities of the extracts were also evaluated using the disc diffusion method. In addition, the Minimum Inhibitory Concentrations and Minimum Lethal Concentrations of the plant extracts against the test pathogens were assessed using broth dilution method. Chloramphenicol and Tilt were used as positive controls against bacterial and fungal pathogens, respectively, while Dimethyl sulphoxide was used as a negative control against both groups of pathogens. Moreover, the present study results revealed that the crude extracts of all solvents had antimicrobial activities against all test pathogens except A. flavus. The distilled water extracts of O. lamifolium had the highest growth inhibitory activity at a concentration of 20 µl/disc with zonal inhibitions ranging from 14.67± .67- 37.67±.88 mm against S. aureus and E. coli, respectively. At the same time, P. guajava and E. schimperi leaves and extracts from aerial parts of E. candelabrum at a concentration of 20 µl/disc showed 15.5± .57, 24.0±.57and 17.0±.1.0 mm of zone of inhibitions against E. coli and S. aureus, respectively. S. aureus and S. agalactiae were found to be the most affected by the distilled water extract of O. lamifolium showed MIC and MLC of 31.25 mg/ml and 62.5 mg/ml, respectively, whereas E. coli and S. Typhi were the least inhibited by all of the crude extracts. Similarly, of the two-fungal species, A. niger was more strongly inhibited than A. flavus by all of the crude extracts at a concentration of 50 µl/disc. The findings of this current study suggested that the selected medicinal plants have the potential to be used as a source of alternative antimicrobial agents in the treatment of diseases resulting from infection with the studied test pathogens as well as in combating problems of drug resistance. At last the present study suggested that specific antimicrobial agents with specified recommendations on the reductions of bacterial and fungal pathogenic activities. However, further extensive studies have to be undertaken for developing concrete recommendations for antimicrobial agents to reduce the effects of bacterial and fungal pathogenic activities.