Category:MacConkey Agar

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Purpose

MacConkey agar is a differential fermentation indicator used to determine whether bacteria can metabolize sugars, commonly lactose.^[1] It also acts as a selective media because the bile salts and crystal violet present prevent Gram-positive bacteria from forming colonies.

Technical Details

Bacteria grown on this medium may either use peptone, or the designated sugar as its energy source. Bacteria that can metabolize the sugar produce acid that turn the colonies red. The acid is also secreted into the medium, lowering the pH around the colony. In response to the lowered pH, bile salts in the agar precipitate, causing the surrounding medium to become opaque.^[2] Bacteria that cannot metabolize the sugar will utilize peptone instead; the ammonia that results will rai se the pH of the surrounding medium, leading to white colonies and transparent agar.

Materials Needed

• MacConkey base
• lactose or other fermentable sugar
• distilled water

Methods

Examples

• Colonies that are able to metabolize the added sugar appear dark red while those that cannot appear white. There are intermediate levels of color (e.g. light red) which coincide with the rate or level of fermentation.
• Improved specificity identifying Shigella^[3]
• Microbe Library MacConkey agar sample plates

Notes

• Red Colonies: E. coli, Enterobacter, and Klebsiella
• White Colonies: Salmonella, Proteus, P. aeruginosa, and Shigella

References

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