Comparison of three reagents for detecting indole production by anaerobic bacteria in microtest systems

Three reagents for detecting indole, Kovac, Ehrlich, and p-dimethylaminocinnamaldehyde (DMCA), were evaluated with commercial microtest systems for characterizing and identifying anaerobic bacteria. The DMCA reagent, the most sensitive of the three reagents, gave a positive reaction with 445 of 449 strains of various indole-producing anaerobic bacteria. There was 99.6% agreement between the results obtained with the DMCA in the microtest systems and results using the conventional tube test to detect indole by using xylene extraction and Ehrlich reagent. Ehrlich reagent detected indole in 163 of 176 (92.6%) indole-positive strains when the inoculum was overlaid with mineral oil before incubation. Kovac reagent was the least sensitive of the reagents tested. When the inoculum was overlaid with mineral oil, Kovac reagent detected only 80 of 108 (74.0%) of indole-positive strains. In addition to being the most sensitive reagent for detection indole, DMCA also allowed detection of indole derivatives (skatole, 3-indolepropionic acid, and 3-indolebutyric acid) produced by some clostridia.