A physician sends a stool sample to your lab, and wants to know if there are lactose fermenting microbes in the sample. How might you determine if these microbes are present or not from this mixed-microbe specimen?
A) Streak the sample for isolation on Thayer-Martin agar (which contains lactose and particular antibiotics for selectivity).
B) Streak the sample for isolation on a blood agar plate (which contains lactose and red blood cells that enrich the culture for iron).
C) Streak the sample for isolation on a MacConkey agar plate (which contains lactose and a pH indicator that turns pink when acid byproducts are present).
D) Either Thayer-Martin agar or blood agar would work, as long as the plates are incubated in an anaerobic chamber.
E) None of the answer choices would work-there's no way to reliably determine this feature from the specimen given.

Question

A physician sends a stool sample to your lab, and wants to know if there are lactose fermenting microbes in the sample. How might you determine if these microbes are present or not from this mixed-microbe specimen?
A) Streak the sample for isolation on Thayer-Martin agar (which contains lactose and particular antibiotics for selectivity).
B) Streak the sample for isolation on a blood agar plate (which contains lactose and red blood cells that enrich the culture for iron).
C) Streak the sample for isolation on a MacConkey agar plate (which contains lactose and a pH indicator that turns pink when acid byproducts are present).
D) Either Thayer-Martin agar or blood agar would work, as long as the plates are incubated in an anaerobic chamber.  
E) None of the answer choices would work-there's no way to reliably determine this feature from the specimen given.

Quizplus · Accepted Answer

Streaking the sample for isolation on a MacConkey agar plate containing lactose and a pH indicator that turns pink when acid byproducts are present provides a selective and differential method for detecting lactose-fermenting microbes. The pH indicator will turn the plate pink if lactose-fermenting microbes are present. Thayer-Martin agar and blood agar plates are not specific for lactose-fermenting microbes. Incubation in an anaerobic chamber is not necessary for the detection of lactose-fermenting microbes.

A Physician Sends a Stool Sample to Your Lab, and Wants