Question:
Which culture medium is used for the isolation of E. coli O157:H7?
Which culture medium is used for the isolation of E. coli O157:H7?
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The medium swaps lactose for a sugar this strain cannot ferment, so its colonies stay colourless.
Updated On: Jun 23, 2026
- Sorbitol MacConkey agar (SMAC)
- Wilson and Blair medium
- Potassium tellurite in McLeod's medium
- Deoxycholate citrate agar (DCA)
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The Correct Option is A
Solution and Explanation
Answer: Sorbitol MacConkey agar (SMAC).
Step 1: E. coli O157:H7 is the enterohaemorrhagic strain that causes bloody diarrhoea and haemolytic uraemic syndrome. To pick it out from ordinary gut E. coli, labs exploit one biochemical quirk.
Step 2: Most E. coli ferment sorbitol, but O157:H7 does NOT (or does so very slowly). SMAC replaces lactose with sorbitol, so sorbitol-fermenting strains turn pink-red while O157:H7 stays colourless. Those colourless colonies flag the pathogen.
Step 3: The distractors are for other bugs: Wilson and Blair (brilliant green bismuth sulphite) and DCA are for Salmonella and Shigella, while potassium tellurite in McLeod's medium is for Corynebacterium diphtheriae.
Ref: Standard microbiology selective-media references.
Step 1: E. coli O157:H7 is the enterohaemorrhagic strain that causes bloody diarrhoea and haemolytic uraemic syndrome. To pick it out from ordinary gut E. coli, labs exploit one biochemical quirk.
Step 2: Most E. coli ferment sorbitol, but O157:H7 does NOT (or does so very slowly). SMAC replaces lactose with sorbitol, so sorbitol-fermenting strains turn pink-red while O157:H7 stays colourless. Those colourless colonies flag the pathogen.
Step 3: The distractors are for other bugs: Wilson and Blair (brilliant green bismuth sulphite) and DCA are for Salmonella and Shigella, while potassium tellurite in McLeod's medium is for Corynebacterium diphtheriae.
Ref: Standard microbiology selective-media references.
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