The critical role of urease in yogurt fermentation with various combinations of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus

ABSTRACT The protocooperation between Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus relies on metabolite exchanges that accelerate acidification during yogurt fermentation. Conflicting results have been obtained in terms of the effect of the Strep. thermophilus urease and the NH 3 and CO 2 that it generates on the rate of acidification in yogurt fermentation. It is difficult to perform a systematic study of the effects of urease on protocooperation because it is necessary to distinguish among the direct, indirect, and strain-specific effects resulting from the combination of the strains of both species. To evaluate the direct effects of urease on protocooperation, we generated 3 urease-deficient mutants (Δ ureC ) of fast- and slow-acidifying Strep. thermophilus strains and observed the effects of NH 3 or CO 2 supplementation on acidification by the Δ ureC strains. Further, we examined 5 combinations of 3 urease-deficient Δ ureC strains with 2 CO 2 -responsive or CO 2 -unresponsive strains of L. bulgaricus . Urease deficiency induced a shortage of ammonia nitrogen and CO 2 for the fast- and slow-acidifying Strep. thermophilus and for the CO 2 -responsive L. bulgaricus , respectively. Notably, the shortage of ammonia nitrogen had more severe effects than that of CO 2 on yogurt fermentation, even if coculture with L. bulgaricus masked the effect of urease deficiency. Our work established (1) that urease deficiency inhibits the fermentative acceleration of protocooperation regardless of the Strep. thermophilus and L. bulgaricus strain combinations, and (2) that urease is an essential factor for effective yogurt acidification.