Commensalism and Competition in Mixed Cultures of Lactobacillus bulgaricus and Streptococcus thermophilus1

Abstract

Cultures of Streptococcus thermophilis and Lactobacillus bulgaricus produced more acid in mixed than in single strain culture. Growth of S. thermophilus in mixed culture was enhanced during the exponential phase and reached higher numbers in the stationary phase than when grown alone. L. bulgaricus was inhibited in the exponential and stationary phases of growth in mixed culture. L. bulgaricus liberated Seitz-filterable compounds during its growth that stimulated growth and acid production of S. thermophilus. These compounds are believed to be responsible for the commensal response observed in mixed cultures. Because of its rapid growth, S. thermophilus was a better competitor than L. bulgaricus for limiting nutrients in the medium. This resulted in inhibition of the growth of L. bulgaricus. The competitive and commensal response was optimal at 37 C and at a ratio of numbers of lactobacilli to streptococci of 2:1 at inoculation.