Which medium is commonly used for broad growth of gastrointestinal organisms in stool cultures?

• A. MAC O2 24h
• B. CIN O2 30C 24h
• C. SMAC O2 24h
• D. HEK O2 24h

Answer: (D)

The medium most commonly used for broad growth of gastrointestinal organisms in stool cultures is MacConkey agar. It supports growth of a wide range of Gram-negative enteric bacteria while suppressing many Gram-positive organisms, thanks to bile salts and crystal violet. The lactose fermentation indicator helps differentiate typical gut flora: lactose fermenters produce pink colonies, making it easy to spot potentially pathogenic enterics among the normal flora. This broad, general-purpose capability is why labs start with MacConkey for stool screens.

Other media in the list have more targeted purposes. CIN agar is designed to recover Yersinia species and is less inclusive for the general stool flora. SMAC differentiates E. coli O157:H7 by sorbitol fermentation, narrowing the focus to a specific toxin-producing strain. Hektoen enteric agar is tailored to help distinguish Salmonella and Shigella from other enterics, so it’s not as broad for routine stool culture.