the salmonella mutagenesis test (ames test) uses multiple strains of "salmonella typhimurium" that are auxotrophic to histidine to test whether a given chemical can cause mutations by observing its auxotrophic property in response to an added chemical compound. the mutation a chemical substance or compound causes is measured by applying it to the bacteria on a plate containing histidine then moving the bacteria to a new plate without sufficient histidine for continual growth. if the substance does not mutate the genome of the bacteria from auxotrophic to histidine back to prototrophic to histidine, then the bacteria would not show growth on the new plate. so by comparing the ratio of the bacteria on the new plate to the old plate and the same ratio for the control group, it is possible to quantify how mutagenic a substance is, or rather, how likely it is to cause mutations in dna. a chemical is considered positive for ames test if it causes mutations increasing the observed reversion rate and negative if presents similar to the control group. there is a normal, but small, number of revertant colonies expected when an auxotrophic bacteria is plated on a media without the metabolite it needs because it could mutate back to prototrophy. the chances of this are low and therefore cause very small colonies to be formed. if a mutagenic substance is added, however, the number of revertants would be visibly higher than without the mutagenic substance. the ames test, basically, is considered positive if a substance increases chance of mutation in the dna of the bacteria enough to cause a quantifiable difference in the revertants of the mutagen plate and the control group plate. negative ames test means the possible mutagen did not cause increase in revertants and positive ames test signifies that the possible mutagen did increase the chance of mutation. these mutagenic effects on bacteria are researched as a possible indicator of the same effects on larger organisms, like humans. it is suggested that if a mutation can arise in bacterial dna under presence of a mutagen then the same effect would occur for larger organisms causing cancer. a negative ames test result could suggest that the substance is not a mutagen and would not cause tumor formation in living organisms. however only few of the positive ames test resulting chemicals were considered insignificant when tested in larger organisms but the positive ames test for bacteria still could not be conclusively linked to expression of cancer in larger organisms. while it can be a possible determinant of tumors for living organisms, humans, animals, and so on, more studies must be completed to come to a conclusion.