Nguimbous, Simone Pierrette2026-03-262026-03-262025-09https://ir-library.ku.ac.ke/handle/123456789/32860A Thesis Submitted in Partial Fulfilment of the Requirements for the Award of the Degree of Master of Science (Microbiology) in the School of Pure and Applied Sciences of Kenyatta University, September 2025Globally, and particularly in less-developed countries, one of the principal factors associated with morbidity and mortality is infectious diseases. Over the years, the abuse and misuse of pharmaceutical products have caused an increase in resistant microbes. Today, the rate of infectious disease cases continues to increase to dangerously high levels as most pharmaceutical products have lost their efficacy. It is worth noting that close to 80% of the African population utilizes medicinal plants, and more than 70% of Kenyans rely on traditional remedies as a primary source of curatives. However, issues such as scarcity of information concerning their active compounds and pharmacological and toxicological properties considerably affect their usage in modern medicine. Therefore, this study assessed the presence or absence of major phytochemicals in stem bark and/or roots of Carissa edulis, Acanthus ebracteatus, Albizia gummifera, Prunus africana, Combretum molle, Warbugia ugandensis, and Cuscuta spp., evaluated their in vitro inhibitory activities against C. albicans (ATTC 10231), E. coli (ATTC 25922), and S. aureus (ATCC 25923) and tested for possible toxic effects in Swiss albino mice (only for highly potent extracts). Selected plants were collected from the Mt. Kenya and Elgon regions. Crude extracts were made by macerating powdery plant samples in methanol. Each plant was then screened for major phytoconstituents using standard methods. Polar and nonpolar extracts were obtained via sequential solvent–solvent partitioning using hexane, dichloromethane, ethyl acetate, and methanol at room temperature. Sterile dimethyl sulfoxide solution (DMSO; 5% in water) was used to dissolve the extracts, and each was tested for inhibitory activity in vitro using the agar disk diffusion (Kirby-Bauer) method. MICs were determined for active plant extracts by means of 96-well microtiter plates (broth microdilution method). MBCs and MFCs were obtained by subculturing the contents of the last wells. Extracts with significant bactericidal/fungicidal activities at concentrations ≥ 250 mg/ml were further tested for toxicity using a total of 50 Swiss albino mice. Acute toxicity was investigated for a period of 14 days at concentrations of 500, 866, and 1500 mg/kg body weight. Mice were kept under careful observation throughout the study and were euthanized on the 15th day. Blood collected was used for biochemical and hematological tests. The data obtained were analyzed using SPSS software and ANOVA (p>0.05). Phytochemical screening revealed that each tested plant contained a range of different secondary metabolites. Preliminary assessment using the Kirby-Bauer method showed that the W. ugadensis DCM extract had the highest activity against C. albicans, E. coli, and S. aureus, with mean inhibition zones of 21.00 ± 0.58, 10.00 ± 0.57, and 15.67 ± 0.33 mm, respectively. MIC testing demonstrated that E. coli had the lowest susceptibility, whereas S. aureus had the highest susceptibility to the various extracts. Findings from the lethality assay demonstrated that selected plant extracts (A. gummifera ethyl acetate, P. africana methanol, and W. ugadensis DCM) did not cause significant alterations in the mean body weights, relative organ weights, or behavioral, hematological and biochemical parameters of mice at the tested concentrations. Although mice treated with W. ugadensis DCM at 866 and 1500 mg/kg were observed to have lose weight during the first week after intake of the extract, these 3 extracts were deemed antimicrobials and safe for administration, with LD50 >1500 mg/kg. Nonetheless, additional research to confirm their safety, particularly in situations of repeated and prolonged therapy, is needed. Also, there is a need to identify specific bioactive compound(s) responsible for their antibacterial and antifungal effects.enThesis