All enterics ferment glucose What would be some consequences

All enterics ferment glucose. What would be some consequences of replacing the sugars in HE agar with glucose? What color combinations would you expect to see?

Solution

Ans. HE media consist of lactose (or, sucrose, salicin in some variants).

I. Lactose in not fermented by Salmonella and Shigella. So, the color of these colonies remains the same as that of the medium.

II. Most enteric ferment lactose in the medium, produce acid (lowers pH) and cause the color of the medium to change to red (color of phenolphthalein indicator in acidic medium).

III. So, lactose non-fermenters (Salmonella and Shigella) with media-colored colonies can be differentiated from lactose-fermenters (pink color colonies).

Conclusion: Since glucose can be fermented by all enteric equivalently, the Salmonella and Shigella can’t be differentiated from other enteric.