Figure 35.13 Ames test. (A) A petri plate containing about 109 Salmonella bacteria that cannot synthesize histidine. (B) The same petri plate with the addition of a filter-paper disc with a mutagen, which produces a large number of revertants that can synthesize histidine. After 2 days, the revertants appear as rings of colonies around the disc. The small number of visible colonies in plate A are spontaneous revertants.