The Selective and Differential Properties of EMB Agar

Subject : Biology

Question:

EMB agar plates contain eosin and methylene blue. Eosin is toxic to Gram-positive bacteria. Methylene blue turns black in the presence of bacteria that produce lactic acid.

What kind of medium is EMB agar?

A. Selective

B. Differential

C. Both

D. Neither

Expert Verified Solution:

The correct answer is C. Both. EMB (Eosin Methylene Blue) agar is a type of growth medium used in microbiology that serves as both selective and differential:

Selective Medium: EMB agar inhibits the growth of Gram-positive bacteria due to the presence of eosin, which is toxic to them. This selective property allows only Gram-negative bacteria to grow, making it useful for isolating these bacteria.

Differential Medium: EMB agar differentiates between types of Gram-negative bacteria based on their ability to ferment lactose and produce lactic acid. Bacteria that produce acid from lactose fermentation will cause the methylene blue to turn black or produce a metallic green sheen, distinguishing them from non-fermenters.

Thus, EMB agar is used both to select for certain bacterial species and to differentiate between them based on their metabolic properties.