Initial screen for genotoxic and point mutation-inducing activity of test substances with different strains of Salmonella typhimurium and Escherichia coli.
Due to certain genetic modifications Salmonella strains used in the Ames-Test are not able to grow on agar plates without the amino acid histidine (his-). In addition, these strains contain mutations that increase the sensitivity of the test strains: the rfa-mutation increases the permeability of the cell membrane with regard to large molecules, and the DuvrB-mutation inhibits the repair of mutations by a particular DNA excision system.
The test principle is based on the fact that increasing back-mutations occur to the his+ genotype in the presence of mutagenic substances. These "revertants" are growing in colonies on agar plates that contain only traces of histidine, and that can be subsequently counted. An increase in the number of revertants in relation to the strain-specific spontaneous mutation rates (solvent control) is a direct measure for the mutagenic effect of a test substance.
Particular mutagens are only activated or inactivated by the liver metabolism of mammals. By adding a special extract taken from the liver of rats (S9), the liver enzymes become also available for the bacterial test.
There are several criteria for determining a positive result, such as a concentration-related increase over the range tested and/or a reproducible increase at one or more concentrations in the number of revertant colonies per plate in at least one strain with or without metabolic activation system.
The Ames-Test is applicable for solid, soluble substances as well as liquid substances and water samples.