the present invention relates to a non - toxic dipolar solvent for chromogenic substrate for detecting presence of lacz gene and / or gene activity , which comprises a stabilizing amount of a solubilizing agent . the present invention also relates to a method for inducing lac operon in screening assay , comprising the step of contacting an agar plate with at least one essential oil in a concentration sufficient to induce the lac operon . the present invention further relates to a method for detecting the presence of bacteria , comprising the step of contacting an agar plate with at least one essential oil in a concentration sufficient to induce detection of the bacteria .