The effect of 3-indoleacetic acid on the response of Lactobacillus arabinosus 17-5 to nicotinamide.

Lactobacillus arabinosus 17-5 has been widely used as an assay organism for nicotinic acid (NA) since the development of the method by Snell and Wright (1). Although it has been realized that other substances present in tissue extracts may interfere with the bioassay, the nature of such substances has not been elucidated. During an investigation of tryptophan metabolism in the pea plant, we studied the possible conversion of this compound to nicotinic acid, since such a transformation has been demonstrated to occur in numerous organisms (2-4). The method involved infiltration of tissue with large quantities of tryptophan and subsequent bioassay for nicotinic acid with Lactobacillus arabinosus 17-5. Certain anomalous results led us to believe that other metabolites of tryptophan were interfering with the assay. Because 3indoleacetic acid (IAA) is a known plant metabolite of tryptophan (5), we tested it for possible interference with the assay, and, as described below, found that such interference may occur under certain circumstances.