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General Information About the MacConkey Agar Test
General Information About the MacConkey Agar Test
MacConkey Agar (MAC) is a selective and differential medium designed to isolate and differentiate enterics based on their ability to ferment lactose.
The Basic Chemistry of MacConkey Agar
The Basic Chemistry of MacConkey Agar
Bile salts and crystal violet inhibit the growth of Gram positive organisms. Lactose provides a source of fermentable carbohydrate, allowing for differentiation. Neutral red is a pH indicator that turns red at a pH below 6.8 and is colorless at any pH greater than 6.8. If an organism ferments lactose, it will produce an acidic environment that will appear pink.
Reading the Results
Reading the Results
Organisms that ferment lactose will appear pink because of the neutral red turning red. Bile salts may also precipitate out of the media surrounding the growth of fermenters because of the change in pH; this appears as a pink halo around the organismal growth. Non-fermenters will produce normally-colored or colorless colonies.
At the top, Escherichia coli, a vigorous fermenter of lactose, produces a pink halo around the streak of pink growth. Klebsiella pneumoniae, a lactose fermenter, produces pink colonies, and Salmonella typhimurium, which does not ferment lactose, produces tan colonies.
Application of the MacConkey Agar Test
Application of the MacConkey Agar Test
When you receive your mixed Gram positive and Gram negative unknown, one of the initial plates you streak will be a MAC plate. Given that knowledge, what should the first branch of your Gram negative flowchart be?
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