Which is better for disc diffusion assays of essential oils - IsoSensitest or nutrient agar?

The disc diffusion assay is often used for screening of essential oils for antimicrobial activity. In this paper we describe an investigation into whether differences exist in the zone of inhibition obtained with two agar types; IsoSensitest and nutrient agar. Standard disc diffusion assay was performed using both agar types using three essential oils against two bacteria and one yeast. Statistically significant differences in the size of zone of inhibition were recorded for the two agar types however differences were not found consistently across essential oil or organism types. These data suggest that there is a complex interaction between bacteria, the oil being assayed and agar type and that no generalizations about the superiority of one agar or another can be made.