Question

Regarding the Ames test for the mutagenic potential of chemicals:

1) Why do we use positive and negative controls?

2) Why do we do a serial dilution?

Thank you so much in advance!

Answer 1

The Ames test is a method that uses bacteria to test whether a given chemical can cause mutations in the DNA of the test organism. It is a biological assay to assess the mutagenic potential of chemical compounds. A positive test indicates that the chemical is mutagenic and therefore may act as a carcinogen, because cancer is often linked to mutation.

1: The positive control consisted of a known mutagen ( like sodium azide) which originated a back mutation, enabling the cells to grow and reproduce (and we observe colony). The negative control was water, a substance that provided completely innocuous results to bacterial cells (i.e. no colony). Positive and negative control are nessesary for comparison, greater numbers of colonies compared to control indicates a greater mutation rate. These colonies are called revertants.

2: To determine mutant frequency, which is simply the ratio of mutants / total cells in the population, we do liquots (dilution) of a culture. By counting the number of mutants that arise and the number of cells platedplated it is possible to determine mutant frequency.