Are you using petri dishes? You could cut out very small squares of an absorbent paper, like water color paper, label them A, B, C, etc. with a waterproof pen, put a drop of each essential oil you’re testing and put the paper in the petri dish. If the essential oil stops bacteria growth, then stuff will grow around it, in some distance away from the little square, and if the oil doesn’t stop it, it will grow right over it.