A medium for rapid identification and enumeration of Aspergillus flavus and related organisms.

The research reported in this note was prompted by a chance observation during a study of Clostridium perfringens (Veillon & Zuber) Holland in corn and milled corn products on Sulfide Polymyxin Sulfadiazine (SPS) agar (Difco 1 0845). Plates of this complex medium, allowed to sit on the bench during examination of corn samples, became contaminated with a fungus, later identified as A. flavus, that produced a persistent bright yellow-orange pigment, near Cadmium Yellow 2 (P1. III, Ridgway, 1912), under nonsporulating colonies. Both the pigmentation and lack of sporulation were considered desirable attributes: While the former would permit easy detection, the latter would prevent development of secondary colonies with resultant inaccuracies in counts. Experimentation showed that a simplified medium consisting of 1.5% tryptone, 1.0% yeast extract, 0.05% ferric citrate, and 1.5% agar (Aspergillus differential medium, ADM) could be substituted for SPS without changing the reverse pigmentation or inducing sporulation of the isolate. The ferric ion was found to be essential for pigment production.