Science Project

Arne van Schanke schanke at nioz.nl
Thu Nov 7 11:51:06 EST 1996

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 
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.



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