You have conducted an Ames test on a given compound. Which of the following would be classified as a positive result on the Ames test?

(A) His + strain grows on an his - plate.

(B) His - strain grows on an his + plate.

(C) His + strain grows on either an his - or an his + plate.

(D) His - strain grows on an his - plate.

(E) His + strain grows on an his + plate

The correct answer is option (D) "His - strain grows on an his - plate".

Explanation:

The Ames test is a procedure that allows to detect if a given compound can cause mutations on a tested organism by growing strains of bacteria that require histidine (His-) to growth, since they are not able to produce its own histidine. In the Ames test, the bacteria is exposed to a given compound and is grown in a medium with small amount of histidine (his - plate). If the His - strain grows on a his - plate, it means that the bacteria has mutated to synthesize its own histidine and it is considered as a positive result on a the Ames test.

(D) His - strain grows on an his - plate.