Ames Mutagenicity

The Ames test employs several strains of Salmonella typhimurium, which have been selected based on their sensitivity to mutation because of: 1) an increased cell wall permeability, due to mutation which causes partial loss of the lipopolysaccharide barrier that coats the bacterial surface and results in increased permeability to large molecules (rfa mutation); 2) a mutation in the bacterial cell system to excise and repair defects in the DNA, resulting in the inability to repair damaged or mutated sections (uvrB mutation); and 3) R-factor plasmids (some strains) and a multicopy plasmid (some strains) which contain error-prone DNA repair systems. The tester strains also require histidine for growth, due to a mutation in the gene which controls production of histidine.

The Salmonella reverse mutation (Ames) tests are performed by mixing the test substance (liquid) or an extract of the test substance and the test organism together in a soft agar solution, which contains only small amounts of histidine. The histidine permits the inoculated test organism to undergo a limited number of divisions, but is insufficient to permit normal growth. If, however, the strain undergoes a reverse mutation, (spontaneous or induced by the test substance or a positive control material), the organism no longer requires histidine to grow and can produce a visible colony or revertant. Only mutations to the test organism in the region of the histidine gene will cause the test organism to undergo a reverse mutation to an organism which then no longer requires histidine. The tester strains are selected to detect various types of mutagens. The tester strains employed are TA97A, TA98, TA100, TA102 and TA1535.