How does EMB agar select Gram-negative bacteria?

How does EMB agar select Gram-negative bacteria?

• A. it uses eosin and methylene blue dyes to inhibit the growth of gram positive bacteria
• B. it uses blue salts and crystal violet dyes to inhibit the growth of gram positive bacteria
• C. it relies on pH indicators to differentiate lactose fermentation
• D. it uses high salt to suppress Gram-positive organisms

Answer: (A)

EMB agar is selective for Gram-negative bacteria because the dyes it contains, eosin Y and methylene blue, are inhibitory to many Gram-positive organisms. This toxicity suppresses Gram-positive growth, allowing Gram-negatives to thrive on the medium. The lactose in the medium plus a pH indicator also provides differentiation: lactose fermenters produce acid, which darkens the colonies (and can give distinctive looks, such as the metallic-green sheen seen with some E. coli). Nonfermenters stay colorless or pale.

EMB agar is selective for Gram-negative bacteria because the dyes it contains, eosin Y and methylene blue, are inhibitory to many Gram-positive organisms. This toxicity suppresses Gram-positive growth, allowing Gram-negatives to thrive on the medium. The lactose in the medium plus a pH indicator also provides differentiation: lactose fermenters produce acid, which darkens the colonies (and can give distinctive looks, such as the metallic-green sheen seen with some E. coli). Nonfermenters stay colorless or pale.