the Ames test

The Ames test is a cheap, fast laboratory way to ask a practical question: does this chemical damage DNA enough to cause mutations? Devised by Bruce Ames in the 1970s, it cleverly uses bacteria as living mutation detectors, turning an invisible molecular event into a count of colonies you can see on a dish.

It works by starting with a special strain of Salmonella that carries a mutation crippling its ability to make the amino acid histidine, so it cannot grow on a plate that lacks histidine. The test then asks whether the suspect chemical can cause a second mutation that reverses the first one — a back-mutation that restores the gene and lets the bacterium grow again. You spread the bacteria on histidine-free plates, add the chemical, and count the colonies that appear. A mutagen produces many more revertant colonies than the handful that arise spontaneously; the more colonies, the stronger the mutagen. Crucially, the chemical is usually first mixed with a rat liver extract, because many substances are harmless until our own liver enzymes convert them into reactive mutagens.

The Ames test matters because it gives a rapid first-line screen for mutagenicity, and because mutagens are very often carcinogens, it doubles as an early warning for cancer-causing chemicals — used widely to vet drugs, cosmetics, food additives, and industrial compounds. It is not the final word, though: it tests one effect in bacteria, not the full complexity of a human body, so a positive result is a red flag prompting further study rather than proof of human cancer risk, and a negative result does not guarantee safety.