Single Broth for the Differentiation of Streptococcus lactis from Streptococcus cremoris

Routinely, the Streptococcus lactis species is distinguished from Streptococcus cremoris by the following: ability to grow at 40C, production of ammonia from arginine, fermentation of maltose and dextrin, and growth in 4% NaCl broth and at a pH of 9.2 (1). This conventional differentiation is somewhat complex and cumbersome, requiring the preparation of various broths and indicators and the use of two different incubation temperatures. In contrast, the procedure herewith presented is comparatively simple and direct, utilizes only one medium and a single incubation temperature, and is based on the principle that organisms belonging to the species S. lactis will decarboxylate arginine, whereas those classified as S. cremoris will not. Briefly, the method is as follows: Decarboxylase base medium (Difco, 0872) containing brom cresol purple indicator is fortified with 0.5% l-arginine. One-tenth milliliter of an active 16-18-hr skimmilk culture is used to inoculate 5-ml quantities of arginine decarboxylase broth contained in tightly capped screwtop tubes.