Indole

Indole is produced when certain bacteria metabolize the amino acid tryptophan using the enzyme tryptophanase, converting it into indole, pyruvate, and ammonia. The indole test (using Kovac's reagent, which turns red/pink in the presence of indole) is a classic microbiology laboratory test used for bacterial identification.

Clinically important indole-positive organisms include Escherichia coli, Proteus vulgaris, and Klebsiella (variable), while indole-negative organisms include Klebsiella pneumoniae, Enterobacter, Salmonella, Shigella, and Proteus mirabilis. This distinction is high-yield for differentiating E. coli (indole-positive) from Klebsiella (indole-negative) and Proteus vulgaris (indole-positive) from Proteus mirabilis (indole-negative), both common Step 1 microbiology testing points.

The mnemonic 'EK CaP' or similar memory aids help students remember indole-positive organisms (Escherichia coli, Klebsiella oxytoca, Citrobacter, Proteus vulgaris). Indole production is part of the IMViC test series (Indole, Methyl red, Voges-Proskauer, Citrate) used to differentiate Enterobacteriaceae members, particularly distinguishing E. coli from other coliforms in water and clinical microbiology.

Clinically, this matters because E. coli is the most common cause of urinary tract infections and a leading cause of traveler's diarrhea and neonatal meningitis, while distinguishing it from Klebsiella (which causes pneumonia in alcoholics and diabetics with 'currant jelly' sputum) has therapeutic implications since these organisms may have different antibiotic susceptibility patterns.