MacConkey agar is an example of a medium that is which of the following? Choose one or more: A. selective for Gram-negative bacteria B. differential based on ability to ferment mannitol C. general purpose (suitable for Gram-negatives and Gram-positives) D. enriched with blood to support selected human pathogens E. selective for Gram-positive bacteria F. differential based on ability to ferment lactose
The correct answer and explanation is:
The correct answers are:
A. selective for Gram-negative bacteria
F. differential based on ability to ferment lactose
Explanation:
MacConkey agar is a type of selective and differential medium primarily used in microbiology to isolate and differentiate Gram-negative bacteria. It is designed to suppress the growth of Gram-positive bacteria while allowing Gram-negative bacteria to grow. This selectivity is achieved by incorporating bile salts and crystal violet dye, which inhibit most Gram-positive organisms.
The medium is also differential based on the ability of bacteria to ferment lactose. It contains lactose as the primary carbohydrate source and a pH indicator called neutral red. When lactose-fermenting bacteria metabolize lactose, they produce acid, lowering the pH and causing the neutral red dye to turn pink or red. Colonies of lactose fermenters (such as Escherichia coli) appear pink or red, sometimes with a surrounding pink zone due to acid production. Non-lactose fermenters (such as Salmonella and Shigella) remain colorless or take on the natural color of the medium.
Other options are incorrect for the following reasons:
- B. differential based on ability to ferment mannitol: This is not true for MacConkey agar. Mannitol fermentation is tested using Mannitol Salt Agar (MSA), which is different from MacConkey agar.
- C. general purpose (suitable for Gram-negatives and Gram-positives): MacConkey agar is not general purpose; it is selective for Gram-negative bacteria.
- D. enriched with blood to support selected human pathogens: MacConkey agar is not blood-enriched. Blood agar contains blood to support fastidious organisms, but MacConkey agar does not.
- E. selective for Gram-positive bacteria: This is incorrect since MacConkey agar inhibits Gram-positive bacteria rather than selects for them.
In summary, MacConkey agar helps isolate Gram-negative bacteria and differentiates them by their ability to ferment lactose, making it very useful in clinical microbiology for identifying pathogens such as E. coli and Klebsiella.