Which organism is non-lactose fermenting, hydrogen sulfide positive, citrate positive, motile, PAD negative, and indole negative?

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Multiple Choice

Which organism is non-lactose fermenting, hydrogen sulfide positive, citrate positive, motile, PAD negative, and indole negative?

This question tests recognizing a bacterial species by a specific biochemical fingerprint. The described profile—non-lactose fermenting, hydrogen sulfide positive, citrate positive, motile, phenylalanine deaminase negative, and indole negative—fits Edwardsiella tarda very well. Non-lactose fermentation narrows to organisms that don’t convert lactose on MacConkey. Hydrogen sulfide production places the organism with others like Salmonella, Proteus, or Edwardsiella. Motility confirms it’s not a non-motile clue like Haemophilus parainfluenzae. The citrate test helps differentiate: Salmonella is typically citrate negative, while Edwardsiella tarda can be citrate positive, so citrate positivity argues against Salmonella and toward Edwardsiella tarda. The lack of phenylalanine deaminase (PAD negative) excludes PAD-positive relatives such as Proteus penneri. Indole being negative further aligns with Edwardsiella tarda in this combination, helping separate it from organisms that are indole-positive. Putting it all together, the full set of traits is most consistent with Edwardsiella tarda. Haemophilus parainfluenzae would not produce hydrogen sulfide and isn’t a motile Enterobacteriaceae member, so it doesn’t fit this profile.

This question tests recognizing a bacterial species by a specific biochemical fingerprint. The described profile—non-lactose fermenting, hydrogen sulfide positive, citrate positive, motile, phenylalanine deaminase negative, and indole negative—fits Edwardsiella tarda very well.

Non-lactose fermentation narrows to organisms that don’t convert lactose on MacConkey. Hydrogen sulfide production places the organism with others like Salmonella, Proteus, or Edwardsiella. Motility confirms it’s not a non-motile clue like Haemophilus parainfluenzae. The citrate test helps differentiate: Salmonella is typically citrate negative, while Edwardsiella tarda can be citrate positive, so citrate positivity argues against Salmonella and toward Edwardsiella tarda. The lack of phenylalanine deaminase (PAD negative) excludes PAD-positive relatives such as Proteus penneri. Indole being negative further aligns with Edwardsiella tarda in this combination, helping separate it from organisms that are indole-positive.

Putting it all together, the full set of traits is most consistent with Edwardsiella tarda. Haemophilus parainfluenzae would not produce hydrogen sulfide and isn’t a motile Enterobacteriaceae member, so it doesn’t fit this profile.

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