Interactions between microorganisms in a simple ecosystem: yogurt bacteria as a study model

Yogurt is a simple ecosystem whose successful manufacture relies on interactions between two thermophilic lactic acid bacteria, Streptococcus thermophilus and Lactobacillus delbrueekii subsp. bulgaricus (Lb. bulgaricus). The present work Studied the impact of co-culturing S. thermophilus with Lb. bulgaricus on bacterial growth in milk. To achieve this, we considered both sides of the bacterial association, by characterising proteolysis and quantifying formic acid in pure and mixed Cultures of S. thermophilus and Lb. bulgaricus. In pure cultures, S. thermophilus CNRZ385 exhibited better growth than the two Lb. bulgaricus strains (1038 and CNRZ397). The S. thermophilus/Lb. bulgaricus association was positive for S. thermophilus growth with strain 1038, contrary to the association with strain 397. These effects were correlated with the different proteolytic capacities of the two Lb. bulgaricus strains. In contrast, the bacterial association had no significant effect on Lb. bulgaricus growth, possibly because of an insufficient production of formic acid by the S. thermophilus strain. We also showed that the S. thermophilus/Lb. bulgarius association affected the production of volatile molecules involved in flavour development.