Screening for antimicrobial activity of some essential oils by the agar overlay technique

Fifty-three essential oils were tested against five micro-organisms (Bacillus subtilis, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Candida albicans) using the agar overlay technique. The essential oils were randomly selected and not on the basis of a supposed activity. It was found that all oils showed an activity against at least one micro-organism, and that substantial activities againstP. aeruginosa were scarce. Combined activities againstC. albicans, the Gram-positive bacteria andE. coli, and an activity againstC. albicans were most often observed. Secondly a combined activity againstC. albicans, B. subtilis andS. aureus was found. The differences between the inhibition zones were too small for a differentiation of the antimicrobial activities of the essential oils. A correlation matrix shows the relationships of the micro-organisms as to the activity patterns of the essential oils. High correlations were found for all the micro-organisms, except forP. aeruginosa.