A 6-year-old patient presents with fever, abdominal cramps, bloody diarrhea, and frequent small-volume stools. Stool culture shows Gram-negative rods; HEK yields green colonies; MAC yellow; Oxidase yellow; TSI K/A with no black precipitate; Motility non-motile; ONPG yellow; Ornithine purple. Which organism do you suspect?

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Multiple Choice

A 6-year-old patient presents with fever, abdominal cramps, bloody diarrhea, and frequent small-volume stools. Stool culture shows Gram-negative rods; HEK yields green colonies; MAC yellow; Oxidase yellow; TSI K/A with no black precipitate; Motility non-motile; ONPG yellow; Ornithine purple. Which organism do you suspect?

The main idea here is using a combination of growth media patterns and biochemical traits to distinguish Shigella from other enteric pathogens in a child with dysentery-like symptoms. Key clues point to Shigella: the organism is a Gram-negative rod that is nonmotile and does not produce hydrogen sulfide. On TSI agar you get a glucose-only fermenter pattern (K/A) with no H2S, which contrasts with Salmonella that typically shows H2S (black precipitate). The organism also yields green colonies on Hektoen enteric agar, consistent with a non-lactose fermenter, while the MacConkey result being non-lactose fermenting (colonies that don’t turn pink) further supports this. Oxidase test is negative, which fits members of the Enterobacteriaceae family, and the overall non-motile, non-H2S, non-lactose-fermenting profile is classic for Shigella among the common suspects. In comparison, Salmonella would usually be motile and produce H2S; Escherichia coli typically ferments lactose (pink on MacConkey) and is motile; Yersinia can be non-mmotile in some conditions but has other distinguishing tests that don’t align as cleanly with this pattern. The cluster of results here aligns best with Shigella, which also fits the clinical picture of bacillary dysentery in a child.

The main idea here is using a combination of growth media patterns and biochemical traits to distinguish Shigella from other enteric pathogens in a child with dysentery-like symptoms.

Key clues point to Shigella: the organism is a Gram-negative rod that is nonmotile and does not produce hydrogen sulfide. On TSI agar you get a glucose-only fermenter pattern (K/A) with no H2S, which contrasts with Salmonella that typically shows H2S (black precipitate). The organism also yields green colonies on Hektoen enteric agar, consistent with a non-lactose fermenter, while the MacConkey result being non-lactose fermenting (colonies that don’t turn pink) further supports this. Oxidase test is negative, which fits members of the Enterobacteriaceae family, and the overall non-motile, non-H2S, non-lactose-fermenting profile is classic for Shigella among the common suspects.

In comparison, Salmonella would usually be motile and produce H2S; Escherichia coli typically ferments lactose (pink on MacConkey) and is motile; Yersinia can be non-mmotile in some conditions but has other distinguishing tests that don’t align as cleanly with this pattern. The cluster of results here aligns best with Shigella, which also fits the clinical picture of bacillary dysentery in a child.

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