An Autoinhibitory Substance Produced by Platydorina caudata Kofoid.

An inhibitory substance was obtained from culture filtrates of Platydorina caudata Kofoid, (a colonial, green flagellate), in which death of the culture had occurred. The cessation of growth occurs before, and is seemingly independent of, medium depletion. The substance can be detected as early as the 15th day, and by the 25th day growth is completely suppressed. A bioassay method for quantitating the activity of the substance was devised. The substance is heat-labile, nondialyzable, acid-labile, and apparently specific for Platydorina. The substance resists short periods of freezing and bacterial contamination. Preliminary results suggest that trypsin destroys activity of the substance.