I have been doing ames test with TA102 strain and having problems with plates having metabolic activation with S9 (rat mix). Since my test chemicals are biological in nature I have been using 45 pre-incubation step instead of 20 min incubation. I once had large number of revertants in with meatbolic activation. When I try to reproduce it all my plates except one or 2 are turning out to be with very little or without colonies. On the other hand plates with out S9 are perfectly normal in terms of spontaneous revertants and do not get any effect.
I would be thankful for your response.