title: A rapid and specific method for the detection of indole in complex biological samples.
abstract: 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.
authors.affiliation: The University of Texas Health Science Center, School of Public Health, Department of Epidemiology, Human Genetics, and Environmental Sciences, Center for Infectious Diseases, Houston, Texas, USA charles.darkoh@uth.tmc.edu.
authors.forename: Charles
authors.identifier: http://orcid.org/0000-0003-0063-7856
authors.initials: C
authors.lastname: Darkoh
authors.affiliation: The University of Texas Health Science Center, School of Public Health, Department of Epidemiology, Human Genetics, and Environmental Sciences, Center for Infectious Diseases, Houston, Texas, USA.
authors.forename: Cynthia
authors.initials: C
authors.lastname: Chappell
authors.affiliation: The University of Texas Health Science Center, School of Public Health, Department of Epidemiology, Human Genetics, and Environmental Sciences, Center for Infectious Diseases, Houston, Texas, USA.
authors.forename: Christopher
authors.initials: C
authors.lastname: Gonzales
authors.affiliation: The University of Texas Health Science Center, School of Public Health, Department of Epidemiology, Human Genetics, and Environmental Sciences, Center for Infectious Diseases, Houston, Texas, USA The University of Texas M. D. Anderson Cancer Center, Department of Infectious Diseases, Infection Control and Employee Health, Houston, Texas, USA.
authors.forename: Pablo
authors.initials: P
authors.lastname: Okhuysen
mesh_terms: D001431:Bacteriological Techniques / Q000379:methods*; D054307:Enterotoxigenic Escherichia coli / Q000378:metabolism; D004783:Environmental Microbiology; D005243:Feces / Q000737:chemistry / Q000382:microbiology; D006801:Humans; D019811:Hydroxylamine / Q000737:chemistry*; D007211:Indoles / Q000032:analysis*; D012680:Sensitivity and Specificity; D014364:Tryptophan / Q000032:analysis
chemical_list: D007211:Indoles; D019811:Hydroxylamine; C030374:indole; D014364:Tryptophan