Antimicrobial Activity And Phytochemical Profiles Of Warburgia Ugandensis Sprague (Canellaceae) Extracts From Different Populations Across The Kenyan Rift Valley

ABSTRACT

Warburgia ugandensis Sprague is a highly valued medicinal plant which is over-exploited for its medicinal use among many communities. Encroachment and fragmentation of the species habitat has led to a notable decrease in its population size and distribution to the level that warrant some conservation efforts. Moreover, information on diversity in its antimicrobial activity and phytochemical profiles is lacking. The aim of this study was to evaluate variations in antimicrobial activity and phytochemical profiles of leaf and stem bark extracts of W. ugandensis from different populations across the Kenyan Rift using different extraction solvents. The plant materials were collected, washed, dried at room temperature, milled into fine powder and sequentially extracted with dichloromethane (DCM) and methanol (MeOH). The antimicrobial activity tests against Staphylococcus aureus (ATCC 25923), Escherichia coli (ATCC 25922) and Candida albicans (ATCC 90028) were carried out using disk diffusion and ninety six well microtitre plate assays. Antimicrobial activities were qualitatively and quantitatively assessed by the presence or absence of inhibition zones and minimum inhibitory concentration (MIC) values. The MIC values were determined by correlating the minimum diameter of zone of inhibition with the lowest concentration of the extract at which no visible microbial growth was noted. Phytochemical profiles in the extracts were determined using GC-MS system. Staphylococcus aureus and Candida albicans were susceptible to the plant extracts while Esherichia coli was resistant.The stem bark extracts displayed the highest antimicrobial activity compared to the leaf extracts. The DCM extracts exhibited stronger antimicrobial activity compared to MeOH extracts. Preliminary bioassay screening using disk diffusion method showed that DCM stem bark extracts from Kitale and Rumuruti were the most active against S. aureus and C. albicans with mean zone of inhibition of 19.75mm, respectively. The MIC analyses with ninety six well microtitre plate assay revealed that DCM stem bark extracts from Londiani, Rumuruti, Kitale and Kinale were the most active against C. albicans with mean MICs of 0.11 ± 0.03, 0.32 ± 0.10, 0.47 ± 0.12 and 0.56 ± 0.20mg/ml, respectively. The DCM stem bark extracts from Rumuruti and Karura were the most active against S. aureus with mean MICs of 1.43 ± 0.28 and 1.47 ± 0.36mg/ml, respectively. This study revealed significant differences in antimicrobial activity among extracts of W. ugandensis from different parts and regions (populations) (p < 0.05). The plant sites, samples, parts, solvent types and their interactions had significant effects on antibacterial and antifungal activities. Quantitative and qualitative differences were observed in the phytochemical profiles of the Kenyan populations of W. ugandensis. Sesquiterpenoids (30.25-56.76%), fatty acid derivatives (9.48-22.50%) and monoterpenoids (0.83-15.97%) were the most dominant classes of compounds. Other classes of compounds identified included diterpenoids, triterpenoids, phytosterols, tocopherols, ketones, aldehydes, carbohydrate derivatives, phenolics, furans, furanones, pyrans, coumarins, benzene derivatives, alkaloids, flavonoids, anthracenes, phenanthracenes and naphthalene derivatives. The observed variation in the antimicrobial activity could be linked to the diversity in the phytochemical profiles of this plant species. This study recommends sustainable utilization of W. ugandensis as an antimicrobial agent. There is also need to develop efficient conservation strategies for the Kenyan populations of W. ugandensis.