Postgenomic Analysis of Streptococcus thermophilus Cocultivated in Milk with Lactobacillus delbrueckii subsp. bulgaricus: Involvement of Nitrogen, Purine, and Iron Metabolism

Streptococcus thermophilus is one of the most widely used lactic acid bacteria in the dairy industry, in particular in yogurt manufacture where it is associated with Lactobacillus delbrueckii ssp. bulgaricus. This bacterial association, known as a proto-cooperation, is poorly documented at the molecular and the regulatory levels. We thus investigate the kinetics of the transcriptomic and proteomic modifications of S. thermophilus LMG18311 in response to the presence of L. bulgaricus ATCC11842 during growth in milk at 2 growth stages. Seventy-seven different genes or proteins (4.1 % of total CDS), mainly implicated in the metabolism of nitrogen (24%), nucleotide base (21%), and iron (20%), varied specifically in co-culture. One of the most unpredicted results was a significant decrease of most of the transcripts and enzymes involved in purine biosynthesis. Interestingly, the expression of nearly all genes potentially coding iron transporters of S. thermophilus decreased whereas that of iron-chelating dpr as well as fur/perR regulator genes increased, suggesting a reduction in the intracellular iron concentration probably in response to H2O2 production by L. bulgaricus. The present study reveals undocumented nutritional exchanges and regulatory relationships between the two yoghurt bacteria, which provide new molecular clues for the understanding of their associative behaviour.