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

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)

Key idea: screening this pathogen relies on a differential medium that highlights its inability to ferment sorbitol, which sets it apart from most other E. coli. Sorbitol-MacConkey agar is designed so that most ordinary E. coli ferment sorbitol and produce a color change, making them easy to spot as colored colonies. EHEC O157:H7, however, does not ferment sorbitol quickly. On this medium, those strains form colorless (unchanged) colonies, standing out against the sorbitol-fermenting background. That color distinction is why SMAC is used for initial screening. Once a colorless colony is seen, labs perform confirmatory tests to identify O157 and its toxin genes (for example, testing for the O157 antigen and Shiga toxin genes). Other media like MacConkey, blood, or chocolate agar don’t provide this sorbitol-based differential signal, so they’re less useful for screening O157:H7.

Key idea: screening this pathogen relies on a differential medium that highlights its inability to ferment sorbitol, which sets it apart from most other E. coli.

Sorbitol-MacConkey agar is designed so that most ordinary E. coli ferment sorbitol and produce a color change, making them easy to spot as colored colonies. EHEC O157:H7, however, does not ferment sorbitol quickly. On this medium, those strains form colorless (unchanged) colonies, standing out against the sorbitol-fermenting background. That color distinction is why SMAC is used for initial screening.

Once a colorless colony is seen, labs perform confirmatory tests to identify O157 and its toxin genes (for example, testing for the O157 antigen and Shiga toxin genes). Other media like MacConkey, blood, or chocolate agar don’t provide this sorbitol-based differential signal, so they’re less useful for screening O157:H7.