Question:hard
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?
Show Hint
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)
Show Solution
The Correct Option is A
Solution and Explanation
Biochemical-key approach. The single fact that cracks this question is that E. coli O157:H7 is a non-sorbitol fermenter, unlike the bulk of commensal E. coli. Sorbitol MacConkey agar is engineered around exactly that difference: it uses sorbitol in place of lactose, so sorbitol-positive organisms make acid, take up neutral red and grow pink-red, while O157:H7 produces pale, colourless colonies that stand out. Eliminate the rest by their real targets, Wilson and Blair (bismuth sulphite) and deoxycholate citrate agar are enteric media aimed at Salmonella and Shigella, and potassium tellurite in McLeod's medium is the classic selective plate for Corynebacterium diphtheriae, where colonies turn black. None of those keys on sorbitol, so they cannot single out O157:H7. Answer: SMAC.
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