Indolepositive

Indolepositive refers to organisms that produce indole from the amino acid tryptophan, a trait detected by the indole test in bacteriology. The ability to generate indole is due to the enzyme tryptophanase, which cleaves tryptophan into indole, pyruvate, and ammonia.

In the standard indole test, a culture is grown in a medium such as tryptone broth. After incubation, Kovac’s reagent (p-dimethylaminobenzaldehyde in amyl alcohol) is added. If indole is present, a red or cherry-colored layer forms at the surface, indicating a positive result. Some variations use Ehrlich’s reagent or other indole-detecting reagents, but red coloration remains the typical positive sign. A negative result shows little or no color change.

Indole positivity is a useful differentiator in clinical microbiology, especially within the Enterobacterales. Classic examples include Escherichia coli (indole-positive). Within related genera, there are both positive and negative representatives: Klebsiella oxytoca is indole-positive, while Klebsiella pneumoniae is typically indole-negative. In the genus Proteus, Proteus vulgaris is indole-positive and Proteus mirabilis is indole-negative. Such patterns aid in organism identification when used alongside other biochemical tests (for example, the IMViC suite).

Limitations include variation among strains, incubation time and temperature effects, and medium composition, which can affect indole production or detection. False negatives may occur with insufficient incubation or unstable tryptophan levels, while false positives can arise from contamination or reagent issues. Therefore, the indole test is most reliable when interpreted within a broader phenotypic profile.

See also Indole test, Tryptophanase, Kovac’s reagent.