The study investigated the antibacterial activity of crude extracts of C. edwardsii, Combretum krausii, and Maytenus nemorosa as well as their interactions with selected antibiotics against drug resistant bacterial strains. Using the rapid p-iodonitrotetrazolium chloride colorimetric assay, minimum inhibitory concentration values of plant extracts and antibiotics were determined. The interactions of plant extracts and antibiotics were studied using a checkerboard method. The MICs of the plant extracts and antibiotics were in the range of 0.037-6.25 and 0.001-2.5 mg/ml, respectively. The plant fractions tested in the present study displayed varying levels of antibacterial activity depending on the bacterial strains. Generally, Staphylococcus aureus was the most susceptible of the three strains of bacteria while the other two beta-lactamase producing Gram-negative bacteria were the most resistant. The hexane leaf extract of M. nemorosa was the most active (MIC = 37 μg/ml) against S. aureus. Ethyl acetate leaf extract of C. krausii was the most active against Klebsiella pneumoniae and ethyl acetate leaf extract of C. edwardsii was the most active against Escherichia coli. Synergistic interactions were detected in 13% of the combinations against E. coli, 27% of the combinations against K. pneumoniae and 80% of the combinations against S. aureus. The few synergistic interactions observed in the present study suggest that the crude extracts of the leaves of M. nemorosa, C. edwardsii, and C. krausii could be potential sources of broad spectrum antibiotic resistance modifying compounds.
Keywords: Combretum edwardsii; Combretum krausii; Maytenus nemorosa; antibacterial activity; synergistic effects.