Efficacy of Essential Oils From Some African Spices Against Two Strains of Bacillus cereus Isolated From Vegetable Salad

Chemical preservatives have been used to preserve our foods against spoilage and pathogenic microorganisms over the years. Consumers now frown at this hence the need to source for preservatives from natural sources. In this investigation fifteen samples of vegetable salad were collected from retail outlets in Lagos, Nigeria dilutions of which were plated on Brilliance Bacillus cereus agar. Isolates were identified based on morphological, biochemical characteristics and reactions to API 20E and API 50 CHB/E test kit. Two strains of Bacillus cereus; B. cereus 1 and B. cereus 2 were isolated. The organism was spread evenly on Mueller -Hinton agar and wells of 5mm in diameter were made on the inoculated agar with sterile cork borer. Different dilutions (1, 0.1, 0.01, 0.001 mg/ml) of essential oils extracted from three African spices; Aframomum melegueta, Xylopia aethiopica and Piper guineense by hydro distillation method were introduced into the wells in triplicate. After incubation diameter of zones of inhibition were measured. The essential oils of the spices inhibited growth of the two strains. A. melegueta produced the greatest zone of inhibition (14 to 24 mm) and with the lowest minimum inhibitory concentration (MIC) of 31.25 mg/ml, followed by X. aethiopica (10 to 18 mm) with MIC of 62.5 mg/ml and P. guineense (11 to 15 mm) and MIC of 125 and 250 mg/ml against B. cereus 1 and B. cereus 2 respectively. P. guineense was the least inhibitory. However, B. cereus 1 was more sensitive (inhibition zone of 12 to 24 mm) to the essential oils of the spices than B. cereus 2 (inhibition zone of 10 to 17 mm). The higher the concentration of the essential oils the greater the resultant zones of inhibition. The spices especially A. melegueta have proved very efficient in the inhibition of B. cereus a pathogen obtained from vegetable salad. The spices can therefore help in ensuring food safety.