Antimicrobial activities of essential oils from Southern Africa against selected bacterial and fungal organisms
In the present study, essential oils from four plants including Melissa officinalis, Mentha piperita, Pelargonium graveolens and Leucosidea sericea, traditionally used to treat infectious diseases were tested for antimicrobial activity against seven Gram-positive bacteria, eight Gram-negative bacteria and six yeast species (Candida spp. and Cryptococcus neoformans) using the agar diffusion method. The minimum inhibitory concentrations (MIC) of the oils were determined by the microdilution technique. The killing kinetics of the oils was further evaluated against specific bacterial and fungal organisms. Both antifungal and antibacterial activities were observed from the essential oil of P. graveolens and M. peripeta against bacterial and fungal strains tested in the present study with the MIC values ranging from 0.95 to 7.5 mg/ml against the bacterial isolates and 0.24 to 7.50 mg/ml 'against the fungal isolates. The oils of P. graveolens were fungicidal to all the yeast isolates tested in the present study with minimum fungicidal concentration (MFC) values ranging from 0.12 to 7.50 mg/ml while the essential oil from M. piperita was fungicidal to one of the six yeast isolates tested with the smallest MFC of 0.48 mg/ml against Candida tropicalis. Essential oils from P. graveolens were able to kill 90% of the P. aeruginosa cells within three hours. The present study has revealed the antimicrobial activity of P. graveolens and M. piperita and indicated that essential oils are promising sources of natural products with potential antimicrobial activity. These results will guide the selection of some plant species for further pharmacological and phytochemical analysis. These results also support the use of essential oils to treat microbial infections and could be used as pharmaceuticals as well as preservatives in the food industry.
Key words: Medicinal plants, essential oils, antibacterial activity, antifungal activity, time-kill activity.