A Dissertation Submitted in Partial Fulfilment of the Requirements for the Degree of
Master’s in Life Sciences of the Nelson Mandela African Institution of Science and Technology
The aim of this study was to evaluate antimicrobial, synergistic and cytotoxicity activities of Conyza bonariensis, Tribulus terrestris and Rubia cordifolia growing in Tanzania. Two fungal strains, four Gram-negative bacteria and one Gram-positive bacterium all being human pathogens were tested using 96 well microdilution method. The minimum inhibitory concentrations (MICs) was used to determine antimicrobial and synergistic activity of the extracts whereas LC50 values on brine shrimp larvae (Artemia salina) were used to determine cytotoxicity of the plants extracts. About 14% of all uncombined extracts (224) demonstrated moderate antimicrobial activity with MIC value range of 0.7825 mg/mL - 1.5625 mg/mL. Synergistic effect was evaluated by treating the microbes by mixture of extracts. A binary combination of extracts had synergistic effect in all combinations and MIC for each microbe was lowered. About 44% of mixed extracts showed moderate antimicrobial activity with MIC value range of 0.7825 mg/mL - 1.5625 mg/mL. Strong antimicrobial activity of 0.3906 mg/mL was recorded for CBLC-CBLE, CBLC-CBRC and CBLC-TLC against E. coli and S. typhi. The comparison between MIC of plant extracts was done using analysis of variance (ANOVA) and differences among the means were determined for significance at P = 0.05 using statistical package for social science. The order of antimicrobial effectiveness of the plants was C. bonariensis ˃ T.terrestris ˃ R. cordifolia with average MIC 7.49, 10.2 and 13.6, respectively for all microbes. Root extracts and extracts extracted by ethyl acetate were the most effective plant part and solvent, respectively. Candida albicans and Salmonella typhimurium were the least sensitive fungus and bacterium to most of unmixed extracts. All extracts were non-toxic to brine shrimps except TTC, RCA and CBM with mildly toxicity of LC50 52.5069, 75.8198 and 79.0076, respectively. Pharmacologists and herbalists should consider synergism of plant extracts for prescription of herbal medicines.
Key words: Antimicrobial, extract, MIC, synergistic effect, toxicity.