Since cancer is caused by mutations in genes, most mutagens are considered carcinogenic compounds. Thus, a simple test to see whether or not a compound is mutagenic is highly useful. A variation on the mutation rate assay shown below provides a simple assessment of the mutagenic ability of agents. In this variation, two histidine-requiring E. coli cultures are grown in parallel. Both are inoculated into histidine-containing medium. Culture B medium also contains a potential mutagenic agent, while culture A does not. Later, each culture is spread on its own plate on a medium lacking histidine, and cells that can produce their own histidine grow into visible colonies. If culture B is exposed to a mutagen, what is the expected result?
A. Culture B will have more colonies on the histidine-minus plate than culture A.
B. Culture B will have the same number of colonies as culture A.
C. Culture B will not have any colonies at all due to the detrimental nature of mutagens.
D. Culture B will have fewer colonies on the histidine-minus plate than culture A.

In this example, Culture B is a culture of E. coli that has been exposed to a mutagenic agent, therefore it has the potential of developing bacteria that by chance, is able to live in a medium lacking histidine. On the other hand, Culture A is the same E. coli culture but without the exposure to the mutagenic agent. The expected result is that Culture B will have more colonies on the histidine-minus plate than culture A, illustrating that a mutagenic agent can produce E. coli able to reproduce in a histidine-minus plate.