Answer to Question #129580 in Biochemistry for Khadija

Question #129580

 In a nutrient medium that lacks histidine, a thin layer of agar containing around 10^9 Salmonella typhimurium histidine auxotroph produces around 13 colonies over a two day incubation period at 37 degrees centigrade. How do these colonies arise in the absence of histidine? The experiment is repeated in the presence of 0.4 micrograms of 2-aminoanthracene. The number of colonies the number of colonies produced over 2 days exceeds 10,000. What does this indicate about 2- anthracene? What can you surmise about its carcinogenicity?

Expert's answer

These results describe evaluation of 2-anthracene carcinogenicity with the help of Ames test. About

90 % of human carcinogens also induce mutations in bacterial cells. The procedure, called the Ames test, is a widely used, relatively inexpensive, rapid, and accurate screening test. For the Ames test, an auxotrophic, histidine-requiring strain of Salmonella enterica serotype Typhimurium is used. If inoculated onto a plate of nutrient medium lacking histidine, no colonies will appear because in this auxotrophic strain the gene inducing histidine synthesis is mutated and hence not active.

The presence of 13 colonies from 109 inoculated cells can be explained by spontaneous mutations (natural revertants were generated that could again encode the enzyme needed for histidine synthesis). The same experiment with the presence of 0.4 micrograms of 2-aminoanthracene resulted in 10,000 colonies. The agent 2-aminoanthracene mutated the bacterial (his-) gene back to the wild type (his+), so visible colonies appeared on the medium. Because the agent 2-aminoanthracene is a mutagen, it is therefore a possible carcinogen in humans.

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