Ames test

The test works by taking bacteria that already have a single mutation, for example, a strain that cannot produce histidine, an amino acid that is essential for the bacterium to live. These bacteria are then exposed to the agent to be tested, and brought into an environment that is ideal for them to live and procreate, except it does not contain histidine. The more mutations the agent caused in the bacteria, the more likely it is for some of them to reverse the original mutation, that is, that the mutation occurs exactly in the right place to allow the bacteria to produce histidine again by themselves. The number of surviving bacteria colonies thus corresponds with the strength of the mutagen.