A stool culture from a child with bloody diarrhea shows SMAC straw-colored colonies, motility, MUG fluorescence, and indole positive. Which organism is suspected?

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Multiple Choice

A stool culture from a child with bloody diarrhea shows SMAC straw-colored colonies, motility, MUG fluorescence, and indole positive. Which organism is suspected?

The key idea is using culture characteristics and basic biochemistry to distinguish enteric pathogens that cause bloody diarrhea. On Sorbitol MacConkey agar, many E. coli ferment sorbitol and appear pink, but the sorbitol-nonfermenting phenotype yields pale, straw-colored colonies. That pattern strongly points to the O157:H7 serotype of Escherichia coli, which is the classic sorbitol nonfermenter associated with hemorrhagic colitis in children. Motility helps separate organisms that could appear similar on initial culture. Shigella is nonmotile, Salmonella and many other pathogens are motile, and Campylobacter has different growth requirements and colony traits. The presence of indole positivity further supports an E. coli identity, since Shigella and Salmonella are typically indole negative. The MUG fluorescence, indicating β-glucuronidase activity, also aligns with E. coli characteristics, reinforcing that the organism is an E. coli strain rather than a non-E. coli pathogen. Putting these clues together—sorbitol-nonfermenting (pale on SMAC), motile, indole positive, and reacting with MUG—best fits Escherichia coli O157:H7, a known cause of bloody diarrhea in children. In practice, confirmation would involve toxin testing and serotyping.

The key idea is using culture characteristics and basic biochemistry to distinguish enteric pathogens that cause bloody diarrhea. On Sorbitol MacConkey agar, many E. coli ferment sorbitol and appear pink, but the sorbitol-nonfermenting phenotype yields pale, straw-colored colonies. That pattern strongly points to the O157:H7 serotype of Escherichia coli, which is the classic sorbitol nonfermenter associated with hemorrhagic colitis in children.

Motility helps separate organisms that could appear similar on initial culture. Shigella is nonmotile, Salmonella and many other pathogens are motile, and Campylobacter has different growth requirements and colony traits. The presence of indole positivity further supports an E. coli identity, since Shigella and Salmonella are typically indole negative. The MUG fluorescence, indicating β-glucuronidase activity, also aligns with E. coli characteristics, reinforcing that the organism is an E. coli strain rather than a non-E. coli pathogen.

Putting these clues together—sorbitol-nonfermenting (pale on SMAC), motile, indole positive, and reacting with MUG—best fits Escherichia coli O157:H7, a known cause of bloody diarrhea in children. In practice, confirmation would involve toxin testing and serotyping.

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