Bacterial Strains Used in the Muta-ChromoPlate Kit (Ames Test)

The Ames test uses a mutant strain of Salmonella typhimurium that cannot grow without the amino acid histidine being supplied in the media in which it is grown, as it has a mutation in a gene that encodes one of the several enzymes necessary for the bacteria to synthesize it itself.  This means that the mutants can only grow if histidine is supplied in the growth medium.

These auxotrophic (mutant phenotype) mutants are called histidine-dependent or his-  because they require histidine to be supplied.

Prototrophic (wild-type phenotype) bacteria, on the other hand, are of the same species of bacteria, but can synthesize their own histidine and thus grow in media without histidine being supplied.

Since all test bacteria used cannot survive in a plate with minimal histidine, the only way the bacteria can grow and form colonies is if the bacteria undergo mutations in the gene that codes for histidine biosynthesis.

There are several different strains of S. typhimurium that are used for the Ames test, as they have different mutations in their histidine biosynthesis gene, making them sensitive to different types of mutagens, allowing the test to be overall more sensitive to different types of chemicals.

In addition to these unique mutations, all strains share a few common introduced mutations:

• (urvB) Indicates a lack a DNA excision-repair mechanism that exists in wild-type bacteria and would normally repair any new mutations in the DNA that are caused by exposure to mutagens. This means DNA is repaired at a reduced rate, leading to 100-1000 times higher effective mutation rate

• (rfa) Indicates a defective lipopolysaccharide layer that allows chemicals to penetrate more easily into the cell than is true with wild-type bacteria. This allows for the testing of chemicals which may not be able to pass through bacterial cell walls, but could in human cells.

• • The plasmid PKM 101 increases the resistance of both Escherichia coli and Salmonella typhimurium to the lethal effects of UV radiation and also increases the susceptibility of cells to mutagenesis by UV and a wide variety of chemicals, which greatly increases the range and sensitivity of the test.

The Muta-ChromoPlate Bacterial Strains Quick Guide:

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