Hi Phil,
Some carcinogens can cause mutations in bacteria, however not
all human carcinogens score posistive in the ames test: they
need to be biotransformed (i.e. activated) by enzymes that are
not present in the bacteria used for the ames test. This
disadvantage can often be avoided by adding liver homogenate
(usually rat). Liver is the main organ involved in
biotransformation, but most others also have some activity.
The principle of the ames test is as follows: the ames tester
strain "salmonella typhimurium" is unable to grow in a medium
devoid of histidine, because of a mutation in either one of the
genes belonging to the histidine biosythesis pathway. Back
mutations caused by the testing agent can restore the original
DNA-sequence, thus enabling the cell to synthesize
histidine and allowing the cell to grow in a histidine-free
medium.
In practise you count the amount of colonies the tester strain
forms in histidine-free medium after treatment with the
substance of interest (the possible carcinogen). This value
needs to be compared with a blank, because spontaneous
back-mutations also occur.
Succes!
Arne
