Influence of S9 mix on the expression of mutants in the L-arabinose resistance test of Salmonella typhimurium.

S9 mix produces an effect similar to that of D-glucose in the L-arabinose resistance test of Salmonella typhimurium, releasing the growth inhibition exerted by L-arabinose. Two elements are responsible for this effect: the glucose-6-phosphate present in the cofactors of the S9 mix and the S9 fraction itself. UV light was used as a mutagen to compare the efficiency of S9 mix and D-glucose in allowing phenotypic expression of mutants in selective plates with L-arabinose; 0.5 ml of S9 mix per plate showed and efficacy similar to that of 0.5 mg of D-glucose per plate. To verify that the S9 mix is equivalent to D-glucose traces in selective plates with respect to the number of induced mutants in compounds requiring metabolic activation, we utilized 2 direct-acting nitrofurans. Our conclusion is that activation of agents could be erroneously attributed to the S9 mix, when plates with 0.5 mg of D-glucose are compared to plates with 0.5 ml of S9 mix plus 0.5 mg of D-glucose. Our results suggest that D-glucose traces be omitted in experiments requiring the presence of the S9 mixture.