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

question 63

Multiple Choice

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.

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