Which bacterium is traditionally used with Streptococcus thermophilus to make yogurt?

Lactobacillus delbrueckii subsp. bulgaricus is traditionally used with Streptococcus thermophilus to make yogurt.

This lactic-acid bacterium is one of yogurt’s two classic starter cultures. It ferments lactose, the sugar in milk, into lactic acid. The acid lowers the milk’s pH, causing casein proteins to coagulate and giving yogurt its thick texture and tangy flavor.

Streptococcus thermophilus works alongside it during fermentation. The two organisms support one another through exchanged nutrients and metabolic products, allowing the culture to acidify milk efficiently. Industrial yogurt production uses selected strains under controlled temperature and time conditions.

The organism has also been called Lactobacillus bulgaricus, a widely used traditional name. Modern bacterial classification places it as a subspecies of Lactobacillus delbrueckii. Yogurt may contain additional live cultures, but the defining traditional pairing is L. delbrueckii subsp. bulgaricus with S. thermophilus.