Are your cultures all equivalent and of the same strains?
ok now lets do the following
1Make a solution of 2% benzene in water with a 10% alcohol solvent. Make another solution , benzene free.
2Mark your cultures
In one set of dishes aspirate like one fine squirt iof the benzene mix and , in the other set, just spry the alcohol water solvent mix.
Develop your cultures and sample after 24 48 and 72 hours. Benzene should give you some usable data on environmental effects of toxicants at low levels.
For a neat presentation , use some stains in your agar.