Question

Urinary tract infections are commonly caused by Staphylococcus saprophyticus and Escherichia coli, less commonly caused by Proteus mirabilis. You have a mixed culture of these pathogens, which you inoculate onto both MacConkey agar and nutrient agar, and then you incubate the plates at optimal growth conditions. On the MacConkey agar, E. coli appears pink, P. mirabilis appears colorless, and S. saprophyticus does not grow. All three microorganisms appear cream on the nutrient agar plate. What is the best explanation for this data?A. MacConkey agar is a selective medium whereas nutrient agar is a differential medium.
B. MacConkey is both a differential and a selective medium.
C. Nutrient agar is both a selective and differential medium.
D. MacConkey agar is a differential medium, whereas nutrient agar is a selective medium.

Answer 1

Answer:

B. MacConkey is both a differential and a selective medium.

Explanation:

Selective media contain the ingredients that allows for the growing of specific microbes while inhibiting the others.

For example MacConkey media facilitate the growth of gram negative bacteria and inhibit the growth of gram positive bacteria. That why E. coli and P. mirabilis grew on the media because these are gram negative and S. saprophyticus does not grew being gram positive.

Differential media allow to differentiate between selected microbes by visual observation.

For Example MacConkey media differentiate between organisms that can ferment lactose to the organisms that can't ferment it. That why E. coli appeared pink on the media because it can ferment lactose while P. mirabilis appears colorless because it usually cannot ferment lactose.