Spot indole test: evaluation of four reagents

Area of Science:

• Microbiology
• Clinical Diagnostics
• Bacterial Identification

Background:

• Indole testing is crucial for identifying and differentiating bacterial species, particularly gram-negative rods.
• Several indole reagents exist, but their sensitivity and interpretability can vary.
• Accurate indole testing aids in the diagnosis and treatment of bacterial infections.

Purpose of the Study:

• To evaluate the sensitivity and interpretability of four different spot indole reagents for testing gram-negative rods.
• To determine the most effective indole reagent for routine microbiological use.

Main Methods:

• Tested 359 strains of gram-negative rods using Kovacs indole reagent, p-dimethylaminobenzaldehyde, Ehrlich indole reagent, and p-dimethylaminocinnamaldehyde.
• Inoculated bacteria on 5% sheep blood agar, Trypticase soy agar, and MacConkey agar.
• Assessed reagent sensitivity, color interpretation, and accuracy in detecting indole production.

Main Results:

• p-dimethylaminocinnamaldehyde demonstrated the highest sensitivity and easiest result interpretation among the tested reagents.
• This reagent successfully detected indole production in Providencia alcalifaciens, indicated by a unique red-violet color.
• All reagents accurately identified indole production in Enterobacteriaceae, except for P. alcalifaciens.

Conclusions:

• p-dimethylaminocinnamaldehyde is recommended as the superior spot indole reagent for routine microbiological identification of gram-negative rods.
• Its enhanced sensitivity and clear interpretability improve bacterial differentiation and diagnostic accuracy.
• This reagent provides a valuable tool for identifying specific organisms like P. alcalifaciens.