What limitations does the Ames test have?

The Ames test is mainly limited by the model organism it uses to evaluate the chemical compound's mutagenicity. The Ames test uses mutant strains of bacteria (e.g., his- S. typhimurium or trp- E. coli), which are prokaryotic cells, and therefore not a perfect model for eukaryotic mammalian cells. To mimic mammalian metabolism, rat liver S9 fraction is used in conjunction with the Ames test. However, there are still differences in metabolic pathways between rats and humans that can affect a compound's mutagenic classification. For example, nitrosamine, which is recognized as a carcinogen (i.e., cancer-causing mutagen) in mammals, is classified as a poor mutagen in the Ames test.