Studies on antimicrobial activities of crude extracts of some higher plants on selected pathogenic microorganisms of veterinary importance.

Antimicrobial activities of 19 plant parts from 12 different plant species, (Appendix 1), were studied with the objective to contribute to the worldwide interest to explore alternative sources of effective drugs from plant resources. Extraction of plant crude extracts using organic solvents, screening for antimicrobial activity of plant crude extracts by using plate agar diffusion technique and broth (tube) dilution method, and cytotoxicity tests by using Brine Shrimp Test (BST) by Meyer et al. (1982) are described. Crude extracts from 11 out of the 19 plant parts, namely, Isolona cauliflora leaf, Mkilua fragrans stembark, M. frttgrans rootbark, Melodorum gracile leaf, M. gracile rootbark, Stephanostema stenocarpum leaf, S. stenocarpum stembark, S. stenocarpum rootbark, Commiphora spanthulata stem exudate, Raphidiocystis chrysocoma fruit and Hugonia easteneifolia stembark showed growth inhibitory activity against different bacterial strains (Streptococcus agalactiae, Staphylococcus aureus, Escherichia coli and Salmonella gallinarum) of veterinary importance in vitro. Minimum Inhibition Concentration (MIC) ranged from 7.8 ug/ml to 62.5 ug/ml which fall within the standard MIC range (0.01 - 100 ug/ml) far the most effective clinical antimicrobial agent far fully susceptible microorganisms reported by Sherris et al., (1984). There was significant difference in the sensitivity between disc technique and agar well method (P < 0.001), with agar well method showing higher sensitivity than disc technique. This points to the dependency on concentration for the antimicrobial diffusion rate and bacteria killing process. There was a significant difference in strength of the antimicrobial activity among root, stem and leaf parts of the same plant. Some of the active crude extracts exhibited significantly better effects than the active standard test antibiotics, this could be related to a combination of different compounds in the crude extracts which may lead to stronger effects as compared to the standard test antibiotics. Some of the plants studied may also contain new and stronger antimicrobial substances as compared to the presently used antibiotics. On the other hand, different substances in crude extracts may have activity inhibitory effects among themselves, and/or, crude extract may contain only very small proportion of the active substances as compared to the standard test antibiotics, that's why in this study some of the standard test antibiotics exhibited significantly better effects than the active crude extracts. There was no activity shown by the studied antimicrobial agents against growth of test fungal strains (Microsporum sp. and Trichophyton sp.) of veterinary importance. Fifty percent of the studied plants exhibited cytotoxicity in the BST (ICS50 < 240 ug/ml, with the highest shown by D. mossambicensis with ICS50 < 8 ug/ml. This could be related to the high plant species diversity and abundance in the tropical ecosystems which necessitate the development of strategies by species for survival. The plants have developed chemical weapons for defence against competing organisms, such chemicals might be lethal to the host animals) to be treated, thus rendering the plants not good choices for chemotherapy. The crude extracts from S. stenocarpum (leaf, stem, and root), R. chrysocoma (fruit), and H. casteneifolia (stem) had shown no cytotoxic effects. It is thus positive to assume that these plants are promising alternative sources of antimicrobial agents for the test bacterial strains of veterinary importance and can play a role in searching for new agents in the therapeutical treatment of diseases in veterinary medicine.