Which medium is used to screen for EHEC O157:H7, and how is O157:H7 typically distinguished?

• A. MacConkey agar; O157:H7 ferments sorbitol
• B. Blood agar; O157:H7 forms beta-hemolysis
• C. SMAC agar; O157:H7 does not ferment sorbitol
• D. Chocolate agar; O157:H7 grows at 42°C

Answer: (C)

Screening relies on a medium that reveals differences in sugar fermentation to flag potential O157:H7 quickly. Sorbitol-MacConkey agar uses sorbitol instead of lactose; most E. coli ferment sorbitol and appear pink, but EHEC O157:H7 typically does not ferment sorbitol within the usual incubation time, so its colonies stay colorless on this medium. That colorless appearance stands out against the pink background and makes SMAC a practical first screen for this pathogen. If a sorbitol-nonfermenting colony is found, it is then sent for confirmatory testing for the O157 antigen or toxin genes to verify identity.

Other media don’t provide this differential for O157:H7. A regular MacConkey plate doesn’t assess sorbitol fermentation specifically, blood agar doesn’t distinguish this strain, and chocolate agar or growth at 42°C is not used as a screening basis for EHEC O157:H7.