In microbiology, which medium is commonly used to differentiate lactose fermenters among Gram-negative enteric bacteria?

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Multiple Choice

In microbiology, which medium is commonly used to differentiate lactose fermenters among Gram-negative enteric bacteria?

MacConkey agar is both selective and differential for studying Gram-negative enteric bacteria. Its composition inhibits most Gram-positive organisms and many non-enteric bacteria, so it helps isolate enterics. It also contains lactose as the fermentable sugar and a pH indicator, neutral red. If a bacterium ferments lactose, it produces acid, lowering the pH and turning the colonies (and often the surrounding medium) pink to red. Non-fermenters don’t acidify the medium, so their colonies stay colorless or pale. This makes it easy to distinguish lactose fermenters (like many E. coli) from non-fermenters (like Salmonella and Shigella) directly on the plate. Other media don’t serve this differentiating purpose: blood agar is non-selective and won’t separate lactose fermenters; chocolate agar is enriched for fastidious organisms but not designed to differentiate lactose fermentation; mannitol salt agar selects for Staphylococcus species and differentiates by mannitol fermentation, not lactose fermentation.

MacConkey agar is both selective and differential for studying Gram-negative enteric bacteria. Its composition inhibits most Gram-positive organisms and many non-enteric bacteria, so it helps isolate enterics. It also contains lactose as the fermentable sugar and a pH indicator, neutral red. If a bacterium ferments lactose, it produces acid, lowering the pH and turning the colonies (and often the surrounding medium) pink to red. Non-fermenters don’t acidify the medium, so their colonies stay colorless or pale. This makes it easy to distinguish lactose fermenters (like many E. coli) from non-fermenters (like Salmonella and Shigella) directly on the plate.

Other media don’t serve this differentiating purpose: blood agar is non-selective and won’t separate lactose fermenters; chocolate agar is enriched for fastidious organisms but not designed to differentiate lactose fermentation; mannitol salt agar selects for Staphylococcus species and differentiates by mannitol fermentation, not lactose fermentation.

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