Answer to Question #157504 in General Chemistry for JAM

Question

In a nutrient medium that lacks histidine, a thin layer of agar containing ~109 Salmonella typhimurium histidine auxotrophs (mutant cells that require histidine to survive) produces ~13 colonies over a two-day incubation period at 37 ° C. How do these colonies arise in the absence of histidine?

Answer 1

Approximately 90 percent of human carcinogens 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 nutrient medium plate 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).