A 22-year-old patient has a high fever, abdominal pain, headache and constipation. A stool culture on Cary-Blair medium yields Gram-negative rods. HEK shows black colonies; TSI is K/A with black precipitate; Indole is red; VP yellow; Citrate blue; Phenylalanine yellow; Motility motile; Urea yellow. Which organism do you suspect?

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Multiple Choice

A 22-year-old patient has a high fever, abdominal pain, headache and constipation. A stool culture on Cary-Blair medium yields Gram-negative rods. HEK shows black colonies; TSI is K/A with black precipitate; Indole is red; VP yellow; Citrate blue; Phenylalanine yellow; Motility motile; Urea yellow. Which organism do you suspect?

The main idea here is recognizing a stool isolate that makes hydrogen sulfide (H2S) and ferments glucose with an H2S-bearing pattern, along with motility and lack of urease activity. On Hektoen Enteric agar the appearance of black-centered colonies signals H2S production. The corresponding Triple Sugar Iron test result, showing a red slant and yellow butt with a black precipitate, confirms glucose fermentation with H2S formation. Motility is noted as positive, and urease activity is negative (no color change), which helps distinguish among enteric pathogens. Salmonella enterica fits this combination best: it typically produces H2S, is motile, and does not hydrolyze urea. The black H2S production on HEK and the TSI black precipitate are classic hallmarks. Shigella would usually be non-motile and not produce H2S, and E. coli generally does not generate H2S and often yields different lactose fermentation patterns (A/A on TSI). Proteus could be H2S-producing as well, but it is usually urease-positive and often demonstrates swarming motility, not seen here. The overall biochemical fingerprint points to Salmonella enterica. Cary-Blair medium is used to isolate enteric pathogens from stool, reinforcing the likelihood of Salmonella in this scenario.

The main idea here is recognizing a stool isolate that makes hydrogen sulfide (H2S) and ferments glucose with an H2S-bearing pattern, along with motility and lack of urease activity. On Hektoen Enteric agar the appearance of black-centered colonies signals H2S production. The corresponding Triple Sugar Iron test result, showing a red slant and yellow butt with a black precipitate, confirms glucose fermentation with H2S formation. Motility is noted as positive, and urease activity is negative (no color change), which helps distinguish among enteric pathogens.

Salmonella enterica fits this combination best: it typically produces H2S, is motile, and does not hydrolyze urea. The black H2S production on HEK and the TSI black precipitate are classic hallmarks. Shigella would usually be non-motile and not produce H2S, and E. coli generally does not generate H2S and often yields different lactose fermentation patterns (A/A on TSI). Proteus could be H2S-producing as well, but it is usually urease-positive and often demonstrates swarming motility, not seen here. The overall biochemical fingerprint points to Salmonella enterica. Cary-Blair medium is used to isolate enteric pathogens from stool, reinforcing the likelihood of Salmonella in this scenario.

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