AN AGAR MEDIUM FOR THE DIFFERENTIAL ENUMERATION OF YOGURT STARTER BACTERIA1,2

Abstract

Generally, Streptococcus thermophilus ferments sucrose, whereas Lactobacillus bulgaricus fails to utilize this disaccharide. When both sucrose and lactose were added to a basal medium, S. thermophilus fermented both carbon sources, produced sufficient acid to change the color of an acid-base indicator (bromcresol purple), and hence formed yellow colonies. On the same medium, most L. bulgaricus strains grew more slowly, produced less acid, and yielded white colonies. Acid diffusion around the S. thermophilus colonies was localized by incorporation of CaCO3 into the medium. To test the efficacy of this medium when known strains of starters are used, the effect of freezing with liquid nitrogen on mixed cultures of S. thermophilus and L. bulgaricus was studied.