Ames test

The Ames test is a clever bacterial experiment that asks a simple question: can this chemical cause mutations? It uses strains of Salmonella bacteria that have been crippled so they cannot grow without a specific nutrient. If a test chemical mutates their DNA in just the right way, it can repair the crippling defect, and those bacteria suddenly regain the ability to grow.

In practice, scientists spread the bacteria on plates lacking the nutrient, add the test compound, and count the colonies that appear. More colonies than the background means more mutations, flagging the compound as mutagenic. Crucially, the test is often run with a liver-enzyme extract (called S9) so that compounds which only become dangerous after metabolism are also caught.

Named after Bruce Ames, who devised it in the 1970s, this assay is fast, cheap, and a standard early genotoxicity screen. A positive result is a serious warning but not a final verdict: it does not perfectly predict whether a compound causes cancer in humans, so follow-up mammalian-cell and whole-animal tests are used to confirm or refute the risk.