Indole

Sample type:

Pus, tissue, or other material properly obtained from an abscess, biopsy, aspirate, drainage, exudate, lesion, or wound.

Uses:

1) To differentiate Proteus mirabilis (indole negative) from all other Proteus species (indole positive).

2) To differentiate Klebssiella pneumoniae (indole negative) from Klebsiella oxytoca (indole positive).

3) To differentiate Citrobacter freundii (indole negative) from Citrobacter koseri (indole positive).

Precautions:

1) Take a sterilized test tubes containing 4 ml of tryptophan broth.

2) Inoculate the tube aseptically by taking the growth from 18 to 24 hrs culture.

3) Incubate the tube at 37°C for 24-28 hours.

4) Add 0.5 ml of Kovac’s reagent to the broth culture.

5) Media containing dye, such as MacConkey and EMB, are unsuitable sources of inoculum due to possible carryover of dye and subsequent interference of indole color interpretation.

6) Media containing glucose should not be used for indole testing due to the formation of acid end products which have been shown to reduce indole production.

Interfering Factors:

Indole-positive colonies have been reported to cause adjacent indole-negative colonies to appear false-positive due to diffusion of indole into the media.

Pre analytical errors:

1) Using an unsterilized test tubes.

2) Using expired Indole Kovacs Reagent.

3) Using expired Indole Spot Reagent.

Corrective action:

1) Using a sterilized test tubes.

2) Using unexpired indole Kovacs Reagent.

3) Using unexpired indole Spot Reagent.

Post analytical errors:

1) Improper reporting and interpreting.

Corrective action:

1) Proper reporting and interpreting.