Antimicrobial activities and time kill profiles of five essential oils from Southern Africa against selected bacterial and fungal organisms

Essential oils are complex mixtures of volatile secondary metabolites from plants. In this study, essential oils from five plants 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 species 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 Conyza scabrida, Eriocephallus punculatus and Artemisia afra 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 Adansonia digitata and A. afra were fungicidal to all the yeast isolates tested with minimum fungicidal concentration (MFC) values ranging from 0.12 to 7.50 mg/ml, while the essential oil from C. scabrida was fungicidal to 4 of the 6 yeast isolates tested with the smallest MFC of 0.48 mg/ml against Candida tropicalis. Essential oils from A. afra were able to kill 90% of the Pseudomonas aeruginosa cells within 3 h. This study revealed the antimicrobial activity of C. scabrida and Helichrysum foetidum. The results of this study indicate 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.