Variations in the Coagulation and Proteolysis of Milk by Streptococcus Lactis

Abstract

The variations occurring among the organisms that are included in a species and also among the cultures secured by plating what is believed to be a pure culture greatly complicate the classificaton of the bacteria. Variety and type designations have been introduced with some species to point out differences that appear to be important without over-emphasizing them, but the significance to be attached to variations is often difficult to determine. The streptococci found in dairy products present a classification problem that is of special importance because of their extensive development in these materials, the variety of changes which they produce and the lack of well defined boundaries between the types. Even with the streptococci that produce considerable quantities of lactic acid in milk, variations in the type and rate of the changes produced, as well as in the morphology, make the development of a logical scheme of grouping very difficult. When a supposedly pure culture of Streptococcus lactis is plated and colonies picked into litmus milk the cultures secured sometimes show a wide variation in their rates of coagulation. A similar variation may occur among the cultures other than the citrate fermenters that are secured when a butter culture developed by combining a S./avt/s culture with a citrate fermenter is plated and colonies picked into litmus milk. This variation among organisms which presumably came from the same original culture but which differ so widely in the rates at which they produce changes in milk brings up the question of the relationship of these types.