Chemical metabolite profiling and in vitro antibacterial activities of selected medicinal plants of Kakamega tropical forest, Kenya

Abstract

Plants play a major role in the management of diseases, either exclusively or in combination with conventional medicines. This study investigates crude extracts of Conyza floribunda, Zanthoxyllum gilletii, Olea capensis, Warburgia ugandensis, and Lantana trifolia for antibacterial phytochemicals. Extraction of the phytochemicals was done by cold percolation in hexane, followed by 50% methanol in dichloromethane. The chemical composition of the non-polar hexane extracts was established using gas chromatography –mass spectrometry (GC-MS). The major chemical components were found to be; 1H-cycloprop[e]azulen-7-ol (8.41%, C. floribunda), 2,4-decadienamide (38.89%, Z. gilletii) isopimarol (29.08%, O. capensis), 2,2,5,7,8-pentamethyl-6-hydroxychroman (8.54%, W. ugandensis leaf), (1R,4aS,6R,8aS)-8a,9,9-trimethyl-1,2,4a,5,6,7,8,8a-octahydro-1,6-methanonaphthalen-1-ol (10.17%, W. ugandensis bark) and tetrapentacontane (13.63%, L. trifolia). The in vitro antibacterial activities of the plant extracts were evaluated by agar well diffusion. Although each extract showed selective inhibition towards tested microbes, W. ugandensis stem-bark hexane extract showed the highest activities with zones of inhibition (ZI) of 16.0, 13.0, 11.0, 11.0, 10.0, and 9.0 mm against S. aureus, E. coli, methicillin-resistant S. aureus (MRSA), P. aeruginosa, P. aeruginosa (7533), and S. aureus (29213), respectively. This activity could be attributed to the major constituent (1R,4aS,6R,8aS)-8a,9,9-trimethyl-1,2,4a,5,6,7,8,8a-octahydro-1,6-methanonaphthalen-1-ol in addition to the synergistic effects of sesquiterpenes, which accounted for 80% of the total phytochemicals in the extract.  This supports the ethno-medicinal use of W. ugandensis in the management of bacterial infections, further recommending its full utilization for the development of antimicrobial drugs.