Ames test is a widely used test for determining if a chemical is a mutagen. In other words, it is a test used to see whether certain thing caused mutations. The test was developed in 1975 by Bruce Ames and his colleagues at the University of California at Berkeley. The Ames test is based on the assumption that substances that are mutagenic for a strain of bacterium (Salmonella typhimurium) might also be carcinogenic in man. Some substances that cause cancer in laboratory a animals (e.g. dioxin) do not give a positive Ames test (and vice versa). But However, the test is rather cheap and easily performed, and has proved extremely useful in rapidly screening substances in our environment for possible carcinogenicity.
The bacterium used in Ames test carries a gene mutation making it unable to synthesize the essential acid histidine from the culture medium. However, this mutation can be reversed, a back mutation, with the gene regaining its function. These revertants are able to grow on a medium lacking histidine. If cultures of these bacteria are exposed to mutagenic chemicals then increased numbers of bacteria might regain the ability to grow without histidine, forming visible colonies (above those spontaneously reverting in control dishes). Many chemicals are not mutagenic (or carcinogenic) by themselves, but become converted into mutagens (and carcinogens) as they are metabolized by the body. In this reason the Ames test includes a mixture of liver enzymes. The Ames test yield a number, specifically the number of growing bacterial colonies, which is a measure of the mutagenic activity (potency) of a treatment chemical. This value is often expressed as the number of revertants per microgram of a pure chemical (mutagen) or per gram of food containing that mutagen.
Btw, in some Ames assays an S-9 mix is added. The S-9 mix contains liver enzymes, generally from a rat. These enzymes can metabolize the agent being tested in order to predict the mutagenic properties within a living system, specifically the source of the S-9 enzymes.
Here's the standard procedures of Ames Test :
Read this journal for the details of Ames test.
An application for the usage of Ames test can be seen (read : explained) in this video.
"Unless they're mutated, they wouldn't survive". (they refers to: bacteria grown on petridishes)
"If you eat natural foods do it because you like the flavor, don't do it because you think you're avoiding carcinogens, if anything it's just the opposite."
Anyhow, there's also some cons regarding to the application of Ames test, especially in the usage of Salmonella. Some people think that Salmonella is not a perfect model for human body. So, some also developed another test using animals as their object. Read here for details.