1. Dextrose is not in itself injurious but rather favorable to toxin production. When added in quantities not exceeding 0.2 per cent to peptone bouillon freed from fermentable acid-producing substances (muscle sugar) it leads to a maximum accumulation of toxin by utilizing the available peptone to the best advantage.
2. The different courses taken by cultures of diphtheria bacilli in ordinary unfermented peptone bouillon containing muscle sugar and in peptone bouillon made from fermented infusion to which 0.1 to 0.2 per cent dextrose has been added are manifested by an increased production of toxin in the latter as well as by a rapid return from an acid to an alkaline reaction. In the former an acid reaction may prevail even under most favorable conditions.
3. These differences may be explained by assuming either that the acid products of the muscle sugar are different from those of dextrose and non-utilizable, or else that the bouillon contains certain other unknown inhibitory substances removed during fermentation. The use of synthesized media and an analysis of the acid products in fermented bouillon plus dextrose and in unfermented bouillon would aid in explaining the differences.
4. Among the accessory conditions which favor the toxin production in unfermented bouillon, as pointed out by Park and Williams, are increased quantities of peptone, well developed surface growth of the diphtheria bacilli, and a low initial acid reaction (phenolphthalein). In fermented bouillon these accessory conditions are also favoring, though of legs importance.