Shiga toxin-producing E. coli (STEC/EHEC O157:H7) characteristically does NOT ferment which sugar, distinguishing it from most other E. coli on a selective medium?
- A Glucose
- B Sorbitol (on sorbitol-MacConkey agar) ✓
- C Lactose
- D Mannitol
Correct answer: B. Sorbitol (on sorbitol-MacConkey agar)
Explanation
On sorbitol-MacConkey (SMAC) agar, most E. coli strains ferment sorbitol and appear pink, whereas E. coli O157:H7 does NOT ferment sorbitol, producing colourless (sorbitol-negative) colonies, facilitating its identification. STEC O157 causes haemorrhagic colitis and haemolytic uraemic syndrome (HUS). Antibiotics are contraindicated in STEC as they may precipitate HUS by releasing more Shiga toxin. All E. coli ferment glucose, lactose, and mannitol.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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