Bacteria are exposed to different concentrations of a test sample (11 mg / strain), as well as a positive and a negative control in a medium containing limited quantities of histidine (S. typhimurium) to support approximately two cell divisions. The cultures in each condition (negative control, test samples and positive controls) are poured in 6 well plates. The cells are exposed to the compound in absence and presence of S9. Rat liver post mitochondrial S9 fraction mimics the effect of the chemical after metabolization. Within three days, cells that have undergone a genetic reversion event to amino acid prototrophy will grow and form colonies. The revertant counts will be compared to those grown in the solvent (negative) control wells. A dose-dependent and significant increase in the number of revertant colonies upon exposure to test sample relative to the solvent controls indicates that the sample is mutagenic. A sterility plate for S9 and buffer is used a contamination control.
