Methods for detecting indole production by gram-negative nonsporeforming anaerobes

To help select the most appropriate method for detecting indole production with anaerobic gram-negative bacilli, several recommended methods were compared. Indole was measured both quantitatively and qualitatively after varying periods of incubation. Studies evaluated the results obtained in different media, the effect of adding glucose and/or tryptophan, the requirement for strict anaerobiasis, and the effects of reducing the total volume of broth. A 1-ml amount of thioglycolate broth without glucose but with 0.02% tryptophan gave optimal results after 2 to 7 days of incubation in anaerobic (Gas-Pak) jars. The majority of clinical isolates will give strong positive tests after 1 to 2 days but a few require 3 to 7 days of incubation. Prolonged incubation was required more frequently with conventional methods.