Ta98 and other Ames tester strains are S. typhimurium and E. coli strains which have been used for more than 40 years to detect mutagenic compounds in chemicals, pharmaceuticals, cosmetics, biocides, water and other environmental samples. They are all listed in the guideline OECD 471:Bacterial Reverse Mutation Test and in guideline ICH M7 for genotoxic impurities. Point mutations were made in the histidine (Salmonella typhimurium) or the tryptophan (Escherichia coli) operon, rendering the bacteria incapable of producing the corresponding amino acid. These mutations result in his- or trp- organisms that cannot grow unless histidine or tryptophan is supplied.

A test sample's mutagenic potential is assessed by exposing these amino acid-requiring organisms to varying concentrations of sample and selecting for the reversion event. Media lacking histidine or tryptophan are used for this selection which allow only those cells that have undergone the reversion to histidine / tryptophan prototrophy to survive and grow. A mutagenic event causing base substitutions or frameshift mutations within the gene may cause a reversion to amino acid prototrophy. These reverted bacteria will then grow in histidine- or tryptophan-deficient media, respectively, whereas non reverted bacteria will not be able to grow. The media during the growth phase can be liquid or agar based.

TA98 strains are used in agar based Ames Test in Petri dish (MacroAmes1), in miniaturized agar based MicroAmes6 or 24, and in liquid format Ames Test (Ames MPF, Ames II). The strains are used in accordance with Guidelines OECD471, ICH M7, ISO 11350:2012.