Which organism is non-lactose fermenting with hydrogen sulfide production, indole positive, PAD negative, and urea negative?

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Multiple Choice

Which organism is non-lactose fermenting with hydrogen sulfide production, indole positive, PAD negative, and urea negative?

Focusing on how a small panel of biochemical tests narrows down an Enterobacteriaceae identification helps you see why this pattern points to a specific organism. The key is matching multiple traits together rather than relying on a single test. - Non-lactose fermenting means the organism does not turn lactose into acid quickly, so typical lactose fermenters like E. coli or Klebsiella are out; you’re looking at other enteric bacteria such as Salmonella, Edwardsiella, Proteus, or Vibrio. - Hydrogen sulfide production is a hallmark of several non-lactose fermenters, including Salmonella, Proteus, and Edwardsiella. This places the organism in that subset rather than Vibrio, which usually does not produce H2S. - Indole positivity shows the organism can convert tryptophan to indole. Among the H2S producers, Edwardsiella tarda is indole positive, whereas Salmonella is typically indole negative. - Phenylalanine deaminase (PAD) negative helps separate Edwardsiella and Salmonella from Proteus, since Proteus species usually have PAD activity (PAD positive). - Urease status provides another split: Proteus species are urease positive, causing rapid alkaline change in the medium, whereas Edwardsiella tarda is urease negative. Putting all these results together—non-lactose fermenting, H2S positive, indole positive, PAD negative, and urease negative—the combination fits Edwardsiella tarda best. This helps distinguish it from Proteus penneri, which would typically be PAD positive and urease positive, not aligning with all the given results.

Focusing on how a small panel of biochemical tests narrows down an Enterobacteriaceae identification helps you see why this pattern points to a specific organism. The key is matching multiple traits together rather than relying on a single test.

  • Non-lactose fermenting means the organism does not turn lactose into acid quickly, so typical lactose fermenters like E. coli or Klebsiella are out; you’re looking at other enteric bacteria such as Salmonella, Edwardsiella, Proteus, or Vibrio.

  • Hydrogen sulfide production is a hallmark of several non-lactose fermenters, including Salmonella, Proteus, and Edwardsiella. This places the organism in that subset rather than Vibrio, which usually does not produce H2S.

  • Indole positivity shows the organism can convert tryptophan to indole. Among the H2S producers, Edwardsiella tarda is indole positive, whereas Salmonella is typically indole negative.

  • Phenylalanine deaminase (PAD) negative helps separate Edwardsiella and Salmonella from Proteus, since Proteus species usually have PAD activity (PAD positive).

  • Urease status provides another split: Proteus species are urease positive, causing rapid alkaline change in the medium, whereas Edwardsiella tarda is urease negative.

Putting all these results together—non-lactose fermenting, H2S positive, indole positive, PAD negative, and urease negative—the combination fits Edwardsiella tarda best. This helps distinguish it from Proteus penneri, which would typically be PAD positive and urease positive, not aligning with all the given results.

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