For screening enterohemorrhagic E. coli in stool, which medium is best?

• A. MacConkey agar with lactose
• B. MacConkey agar with sorbitol
• C. Xylose-lysine-deoxycholate agar
• D. Brilliant-green agar

Answer: (B)

Screening enterohemorrhagic E. coli, especially the O157:H7 serotype, hinges on a phenotype difference: most E. coli ferment sorbitol, while O157:H7 does not. Sorbitol-MacConkey agar replaces lactose with sorbitol, so nonfermenters appear colorless and stand out against the pink color of sorbitol-fermenting colonies. This makes it easier to pick potential O157:H7 colonies for confirmed testing (e.g., Shiga toxin detection and O157 antigen tests). Other media either rely on lactose fermentation (which many general E. coli do) or are designed for broader enteric pathogen isolation and are not as targeted for this specific screen, so they’re less effective at highlighting EHEC O157:H7.