In article <noesis.6.01533DDD at cam.org>, noesis at cam.org (Noesis Vision Inc) writes:
> Does Anybody know what the Ames test is exactly? This is used to do colony
> counting on petri dishes.
>> Luc Nocente
>73514.1254 at compuserve.com
Hello Luc,
The Ames/Salmonella test is used as an assay for mutagenicity.
(mutagen = something triggering a change in the sequence of DNA)
Basically, the test involves exposure of special bacteria to the potential
mutagen (usually a chemical, but can also include radiation) and then the
bacteria are spread on an agar plate which selects for those in which
a mutation has occurred. The number of colonies that grow gives an indication
of how powerful the mutagen is (or how much is present).
The test gets its name from Prof. Bruce Ames who developed it
whilst studying histidine biosynthesis in the bacterium, Salmonella
typhimurium. Prof. Ames makes the tester strains available to just about
anyone and it is thus no surprise that the test is the most widely used short
term assay for mutagenicity. There are also a number of similar tests
(eg. trp mutation in E. coli, SOS chromotest, umu test).
The basis for the test is the monitoring of the conversion of
histidine auxotrophs of S. typhimurium to prototrophs. This rather specific
change can be detected because; a large number (millions) of bacteria are
exposed, the test (reversion) is easy to screen (positive colonies against
a negative background), the sites where the mutagens must act are at
"mutation hotspots" in the genome, the bacteria are variants that have
defective DNA repair enzymes, and the bacteria are variants that have more
permeable cell walls (there are also other enhancements).
Important things to note are that compounds that are positive in the
Ames test are not always carcinogens (carcinogen = causes cancer in animals
or man), and not all known rodent carcinogens give a positive result in
the Ames test. Also, there is no real link with how powerful a mutagen a
compound is in the Ames test and in other tests - this is due to differences
in biology. However, since the test is relatively cheap to perform it
is often used as a "screen for potential carcinogens" before embarking
on (*very* expensive) rodent carcinogen bioassays.
Many chemical mutagens require metabolic activation by mammalian
enzymes (eg. cytochromes P-450) before they can exert their effect. This
means that an enzyme extract (eg. liver microsomal fraction or liver S9)
must be incorporated in the test. The source of this enyme extract (species,
tissue, induction state etc. etc.) can have a profound effect on the test
result.
If you want more information I can point you in the direction of
some reviews.
Bernard
(A long time Ames tester)
Bernard Murray, Ph.D.
bernard at elsie.nci.nih.gov (National Cancer Institute, NIH, Bethesda MD, USA)
