The Bacterial Ames Test The Ames Salmonella/microsome mutagenicity assay (Salmonella Test; Ames test) is a shortterm bacterial reverse mutation assay specifically designed to detect a wide range of chemical substances that can produce genetic damage that leads to gene mutations. The test employs several histidine dependent Salmonella strains each carrying different mutations in various genes in the histidine operon. These mutations act as hot spots for mutagens that cause DNA damage via different mechanisms. The Ames test is the most widely and validated genotoxicity test. When the Salmonella tester strains are grown on a minimal media agar plate containing a trace of histidine, only those bacteria that revert to histidine independence (his+) are able to form colonies. The number of spontaneously induced revertant colonies per plate is relatively constant. However, when a mutagen (the compound) is added to the plate, the number of revertant colonies per plate is increased, usually in a dose-related manner (166). The sensitivity of the test is enhanced by the use of particular mutant strains preventing, e.g., adequate DNA repair or increasing resistance to toxic compounds. Bacteria are grown on a selective medium in the presence of the tested compound. After 48 h of incubation at 37 °C, mutant colonies are counted and compared to the number of colonies formed in unexposed cultures (spontaneous back mutatons). A compound is considered genotoxic when the mean number of revertants is the double of that found in the solvent control culture (+ dose effect relationship). Negative controls are normally between approximatively 5-50 revertants for TA98 and 100-150 for TA100. Screening of the Bioactivity of the Isolated Natural Products. Positive controls must show a clear increase over the solvent controls.