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The Production of a Growth Inhibitor by Lactic Streptococci^{1, 2,}

Department of Bacteriology, University of Wisconsin, Madison

ABSTRACT

Nine strains of Streptococcus lactis and Streptococcus cremoris and three commercial starter cultures were found to produce a heat stable substance that inhibited growth of Lactobacillus casei. The material also inhibited certain strains of lactic streptococci, although to a lesser extent than it restrained growth of the rods. The lactic streptococci varied considerably in their ability to produce the inhibitor; also, strains of L. casei differed markedly in their susceptibility to inhibition. When given sufficient time, the rod cultures overcame the effect of the inhibitor.

Most of the inhibitor was formed by the lactic streptococci during the first 24 hr. of their growth in broth, although its concentration continued to increase slowly for as long as 60 hr. The culture medium used for growing the streptococci had a marked effect on the amount of inhibitor produced.

Whey and curd obtained during the manufacture of ceddar cheese was. inhibitory to the one strain of L. casei tested. Thus, the production of an inhibitory substance by starter organisms may delay growth of lactobacilli in cheese during ripening. It also may exert a selective action that favors the predominance of less susceptible strains of lactic rods in the flora that develops.

¹ Published with the approval of the Director of the Wisconsin Agricultural Experiment Station.

² Supported in part from funds supplied by the Wisconsin Alumni Research Foundation.

³ Current address: The Western Condensing Co., Appleton, Wisconsin.