Diplococcin, an anti-bacterial protein elaborated by certain milk streptococci

Abstract

Two strains of milk streptococci which inhibit cheese starters possess a cellular protein, called diplococcin, which powerfully inhibits the growth of Streptococcus cremoris, & closely related organism. Other Gram-positive cocci are less susceptible, and Escherichia coli is unaffected. Diplococcin occurs in the medium, if the latter becomes acid through carbohydrate fermentation, but is more easily extracted from the cells by the action of 0.4% acetic acid at 24[degree] C or better at 100[degree] C. It is insoluble in alcohol, soluble in water, contains no sulfur or P, and appears to be of small molecular size, possibly a proteose. It is stable at pH 4 at 100[degree] C, but not at pH 6 or higher. A method for its isolation is described, the yield being about 20 mg./g. of dried bacteria separated from 5 1. of a semi-synthetic medium.