In the Ames test, the bacteria are exposed to the test substance and then plated on a medium lacking histidine. If the substance is mutagenic, it will induce mutations that allow some bacteria to grow without histidine. The number of colonies formed on the plate is counted to assess the mutagenic potential. Often, a rat liver extract (S9 mix) is added to mimic the metabolic activation of the compound in mammalian systems.
