Which medium is used to detect Escherichia coli O157 in stool cultures?

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Multiple Choice

Which medium is used to detect Escherichia coli O157 in stool cultures?

Sorbitol MacConkey (SMAC) is used to screen for Escherichia coli O157:H7 because this strain typically does not ferment sorbitol. SMAC replaces lactose with sorbitol, so most E. coli colonies that ferment sorbitol turn pink, while sorbitol-nonfermenting organisms like O157:H7 form colorless (or pale) colonies. This visible difference makes SMAC a practical tool to identify suspected O157:H7 colonies for further confirmation, usually after about 24 hours of aerobic incubation. The other media are designed for different enteric pathogens and do not specifically differentiate E. coli O157:H7 in stool: Hektoen enteric agar helps differentiate Salmonella and Shigella; CIN agar targets Yersinia; and CAMP-based/microaerophilic setups are used for Campylobacter.

Sorbitol MacConkey (SMAC) is used to screen for Escherichia coli O157:H7 because this strain typically does not ferment sorbitol. SMAC replaces lactose with sorbitol, so most E. coli colonies that ferment sorbitol turn pink, while sorbitol-nonfermenting organisms like O157:H7 form colorless (or pale) colonies. This visible difference makes SMAC a practical tool to identify suspected O157:H7 colonies for further confirmation, usually after about 24 hours of aerobic incubation.

The other media are designed for different enteric pathogens and do not specifically differentiate E. coli O157:H7 in stool: Hektoen enteric agar helps differentiate Salmonella and Shigella; CIN agar targets Yersinia; and CAMP-based/microaerophilic setups are used for Campylobacter.

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