Hello Ramiz Raja

Ames test is usually used to detect the revert mutations present in strains.

Because of the simplicity of the test in which Salmonella typhimurium (His-) regains its ability to synthesize histidine (His+) as they undergo reversion to histidine prototrophy to survive and grow, when cultured in medium lacking histidine and containing chemicals that are mutagenic which bring about the reversion (His-) to (His+).Thus such mutant strains become most favorable to be used in Ames test.

Moreover, it also makes this test rapid and can be performed at low cost to screen a number of substances for possible carcinogenic effects.

I hope this explanation is helpful.

Regards.

Malcolm Nobre Thanks for your positive inputs but How can be sure that test chemical cause only histidine revertant? test chemical could target another gene or amino acid. I would like to know why negative His strain and the same bacteria ? because test chemical don't know specific thing and so could cause anything!

Hi Ramiz -

you are correct. The test chemical may target other genes. However, the his genes in the Ames Salmonella strains have hot spots in them making them more prone to genetic mutations. It is possible to observe, when exposed to high levels of test chemical or high concentrations of S9, a decrease in revertant numbers due to cell death caused by essential genes being mutated by the test chemical.

Regards,

Hello Ramiz

Why use Salmonella bacteria?

Because Salmonella strain has different mutations in various genes of ‘his’ operon; each of these mutations is designed to respond to the mutagen that act via different mechanisms. Moreover, in addition to the above mutations more mutations are engineered in these strains to make them more sensitive to a variety of mutagens.

Why use histidine-ve strain?

Because the genes responsible for histidine biosynthesis showed a number of spontaneous, radiation- and chemical-induced mutations not common in genes responsible for the biosynthesis of other amino acids. These mutations were single base changes (base pair substitution) and addition or deletion of one or more bases (frameshift). Above all this, it was realized that some of these his mutant strains had the ability to revert to wild type in the presence of mutagens.

So the genes in 'his' operon are more susceptible to the mutagenic effect of the chemical than any other genes (or other amino acids).

For more detailed explanation to your questions, I would like to refer you to the research article given below.

10.1016/s0027-5107(00)00064-6

I hope this information is helpful

Best Wishes.