How does McConkey agar select Gram-negative bacteria?

How does McConkey 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 inhibit non-halophiles

Answer: (B)

MacConkey agar uses selective agents to favor Gram-negative bacteria and suppress Gram-positives. Bile salts and crystal violet in the medium inhibit the growth of many Gram-positive organisms, so only Gram-negative bacteria can grow well. It also differentiates lactose fermentation: the medium contains lactose and a pH indicator, so lactose-fermenting Gram-negatives produce acid that lowers the pH and turns the colonies pink/red, while nonfermenters remain pale. That combination—selective inhibition of Gram-positives by bile salts and crystal violet, plus lactose-based differential signaling by a pH indicator—explains how the medium selects for Gram-negative bacteria.

MacConkey agar uses selective agents to favor Gram-negative bacteria and suppress Gram-positives. Bile salts and crystal violet in the medium inhibit the growth of many Gram-positive organisms, so only Gram-negative bacteria can grow well. It also differentiates lactose fermentation: the medium contains lactose and a pH indicator, so lactose-fermenting Gram-negatives produce acid that lowers the pH and turns the colonies pink/red, while nonfermenters remain pale. That combination—selective inhibition of Gram-positives by bile salts and crystal violet, plus lactose-based differential signaling by a pH indicator—explains how the medium selects for Gram-negative bacteria.