A Rapid and Specific Method for the Detection of Indole in Complex Biological Samples
Open accessAbstract
Indole, a bacterial product of tryptophan degradation, has a variety of important applications in the pharmaceutical industry and is a biomarker in biological and clinical specimens. Yet, specific assays to quantitate indole are complex and require expensive equipment and a high level of training. Thus, indole in biological samples is often estimated using the simple and rapid Kovács assay, which nonspecifically detects a variety of commonly occurring indole analogs. We demonstrate here a sensitive, specific, and rapid method for measuring indole in complex biological samples using a specific reaction between unsubstituted indole and hydroxylamine. We compared the hydroxylamine-based indole assay (HIA) to the Kovács assay and confirmed that the two assays are capable of detecting microgram amounts of indole. However, the HIA is specific to indole and does not detect other naturally occurring indole analogs. We further demonstrated the utility of the HIA in measuring indole levels in clinically relevant biological materials, such as fecal samples and bacterial cultures. Mean and median fecal indole concentrations from 53 healthy adults were 2.59 mM and 2.73 mM, respectively, but varied widely (0.30 mM to 6.64 mM) among individuals. We also determined that enterotoxigenic Escherichia coli strain H10407 produces 3.3 ± 0.22 mM indole during a 24-h period in the presence of 5 mM tryptophan. The sensitive and specific HIA should be of value in a variety of settings, such as the evaluation of various clinical samples and the study of indole-producing bacterial species in the gut microbiota.
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In human feces, indole is detectable at micromolar concentrations.
“In human feces, indole is detectable at micromolar concentrations (Darkoh et al., 2015)”
From The Microbiota-Gut-Brain Axis (2019) · cites this paper as Darkoh et al., 2015
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“However, Kovac’s reagent tends to react with all indole containing compounds and this nonspecific reaction limits the application of Kovac’s assay for quantitative detection.(26) On the other hand, chromatographic techniques such as high-performance liquid chromatography and gas chromatography–mass spectrometry (GC–MS…”
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“Indole Assay Indole concentrations were determined using Kovac’s reagent as previously described (Darkoh et al., 2015).”
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“hydroxylamine-based indole assay, as previously described (65).”
Antibacterial and Antibiofilm Activities of Chloroindoles Against Vibrio parahaemolyticus
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“This concentration of indole is representative of the highest concentration found in feces (Darkoh et al. 2015), thus presumably in the most distal part of the large intestine.”
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Regulation of axial and head patterning during planarian regeneration by a commensal bacterium
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- Authors
- Darkoh, C., Chappell, C., Gonzales, C., & Okhuysen, P.
- Affiliations
- The University of Texas Health Science Center at Houston (US)
- The University of Texas MD Anderson Cancer Center (US)
- Venue
- Applied and Environmental Microbiology
- Publisher
- American Society for Microbiology
- Published
- 2015-12
- Volume
- 81
- Issue
- 23
- Pages
- 8093-8097
- ISSN
- 0099-2240 (ISSN-L)
- 1098-5336
- PMC
- PMC4651089
- OpenAlex
- W2260701616
- Semantic Scholar
- c738683cc725688e7523668357b4d9797097bcfa
- Cited by
- 156
- References
- 31
- Citations by year
- 2026: 21 · 2025: 19 · 2024: 13 · 2023: 27 · 2022: 18 · 2021: 12 · 2020: 17 · 2019: 12 · 2018: 10 · 2017: 4 · 2016: 2
- FWCI
- 4.41
- Type
- journal article
- Language
- en
- Open access
- bronze
- doi.org
- Locations
- Applied and Environmental Microbiology (publishedVersion)
- Europe PMC (PubMed Central) (publishedVersion)
- PubMed Central (publishedVersion)
- PubMed (publishedVersion)
- Funders
- National Institute of Allergy and Infectious Diseases (R01AI116914)
- National Institute of Diabetes and Digestive and Kidney Diseases (DK56338, P30DK056338)
- Topics
- Life Sciences › Biochemistry, Genetics and Molecular Biology › Clinical Biochemistry › Bacterial Identification and Susceptibility Testing
- Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Polyamine Metabolism and Applications
- Physical Sciences › Chemistry › Analytical Chemistry › Analytical Methods in Pharmaceuticals
- Keywords
- tryptophan degradation, enterotoxigenic Escherichia coli, gut microbiota, biomarkers, bacterial metabolites
- MeSH terms
- Bacteriological Techniques, Environmental Microbiology, Feces, Humans, Indoles, Sensitivity and Specificity, Tryptophan, Hydroxylamine, Enterotoxigenic Escherichia coli
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