To work around this problem, the Ames test is done as follows:
A petri dish is seeded with salmonella colonies, moreover, in that petri dish a little chemical is added in the center. A culture medium is also added to the plate where agar is one of the ingredients and biotin and histidine have been mixed.
Only bacteria that show some kind of mutation will grow on these agar plates. This is because histidine has the function of allowing cell divisions necessary only for cases in which mutagenesis is necessary to occur.