Indole production is characteristic of which organism?

• A. Escherichia coli
• B. Proteus mirabilis
• C. Salmonella Choleraesuis
• D. Serratia marcescens

Answer: (A)

Indole production signals the presence of tryptophanase, an enzyme that breaks down tryptophan into indole, pyruvate, and ammonia. In the indole test, bacteria capable of producing indole are incubated in a medium like tryptone broth, and after incubation a reagent such as Kovac’s is added. If indole is produced, a red color forms at the surface, indicating a positive result.

Escherichia coli is the classic indole-positive enteric bacterium, so it is the best match for this trait. The other organisms listed are typically indole negative under standard testing conditions, so they do not show the red color with Kovac’s reagent. This makes indole production a useful distinguishing feature for identifying E. coli among similar Gram-negative enterics.