In addition to the few Shigella colonies that are noted, there are also abundant salmon pink colonies, around which the medium has turned yellow, losing its original blue color. You know that hektoen enteric agar must contain ________.
A) lactose
B) a pH indicator
C) a weak acid
D) malachite green
E) penicillin

Question

In addition to the few Shigella colonies that are noted, there are also abundant salmon pink colonies, around which the medium has turned yellow, losing its original blue color. You know that hektoen enteric agar must contain ________. 
A) lactose
B) a pH indicator
C) a weak acid
D) malachite green
E) penicillin

Quizplus · Accepted Answer

Hektoen enteric agar contains a pH indicator that changes color based on the pH of the medium. The original blue color of the medium changes to yellow around colonies that produce acid, indicating fermentation of lactose or other carbohydrates. Therefore, choice A (lactose) is not necessary for the color change, and choices C (a weak acid), D (malachite green), and E (penicillin) are not components of hektoen enteric agar.

In Addition to the Few Shigella Colonies That Are Noted, There Are