Ames felt that there is a strong connection between the ability of compounds to cause cancer and their ability to cause mutations.
The metabolites of some carcinogens produced in the body are really mutagenic agents. In the body, these metabolites are produced in the liver. In bacteria, the enzymatic reactions that convert the carcinogens into bioactive metabolites did not occur.
Ames used solubilized liver enzymes to do his test. Ames treated bacteria with rat liver extract. This special strain of bacteria had a mutant allele of a gene responsible for histidine synthesis. The treated bacteria were exposed to test compound and was allowed to grow in petri plates on a medium without histidine.
Presence of revertants shows that the chemical is a mutagen and a carcinogen possibly.
Any chemical classified as a mutagen by the Ames test is considered as a carcinogen. So Ames test is a useful tool today to evaluate the safety of chemicals.