Principle
Initial screen for genotoxic and point mutation-inducing activity of test substances with different strains of Salmonella typhimurium and Escherichia coli.

Abstract
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.

Criteria
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.

Test substance
The Ames-Test is applicable for solid, soluble substances as well as liquid substances and water samples.

Principle
Determination of the genotoxic potential of water samples with the genetically engineered bacterium Salmonella typhimurium TA1535pSK1002.

Abstract
In this way, genotoxins induce the umuC-gene, which belongs to the SOS-repairing system of the cell, which counteracts any damage to the bacterial genetic material. By combining the umuC-gene with the lacZ gene coding for the ß-galactosidase, the activation of the umuC gene can be indirectly determined by means of a colour formation. The induction rate (IR), defined as the ratio between the extinction at 420 nm and the one measured in the negative control, has been set as a measure for the genotoxicity. The test is performed both with and without S9-extract for the metabolic activation of genotoxins. The growth factor determined by the increase in the optical density at 600 nm is considered for the calculation of the induction rates. An inhibition of the bacterial growth is expressed by a reduced growth factor (WF) in relation to the control. Results with growth factors below 0.5 (50% growth inhibition) are not considered.

Criteria
Test substances with an induction rate ≥ 1,5 are defined as genotoxic. For wastewater samples, the lowest value of the dilution step G (G_EU-value), with an induction rate < 1.5, will be considered as the test result.

Test substance properties
The umu-test is applicable for water and waste water samples.