Question

Describe how the Ames test can be used as a test for screening potential carcinogenic compounds. Include in your answer what an auxotroph is, why you need an auxotrophic mutant, limits of the test. Why is meant by spontaneous reversion?

Answer 1

In 1970, Bruce Ames developed a test for determining whether a chemical is a mutagen or not, based on the reverse or back mutation (also known as the bacterial reverse mutation assay) and thus used this test for screening potential carcinogenic compounds. An auxotroph is a mutant organism which requires a particular nutrient that tits strain lacks. An auxotroph mutant’s requirement can best be understood using the following example. Salmonella typhimurium carries the mutation in genes involved in histidine synthesis. These strains are auxotrophic mutants which require histidine for their growth but cannot produce it. The Ames tests uses such strains (that lack the ability to produce histidine) in a medium with chemicals that cause mutations in the genes of the Salmonella typhimurium so that they are able to synthesise histidine. This is called reverse mutation and such chemicals are called mutagens. Limits of this test are that Salmonella typhimurium is a prokaryote and therefore not an ideal model for humans. Sometimes while performing the Ames tests using strains of Salmonella typhimurium lacking histidine synthesising ability, a few strains grow without the addition of chemicals or mutagens in the control plates. These are strains that have gone spontaneous reverse mutations as opposed to the reverse mutations which are caused due to addition of a mutagen. The Ames tests measures mutagenicity as the difference between induced and spontaneous rates of reverse mutation.