How do bacteria help change milk into yogurt?

Bacteria help change milk into yogurt through a fermentation process. Specific strains of bacteria, primarily Lactobacillus bulgaricus and Streptococcus thermophilus, are added to the milk, where they ferment lactose (the sugar in milk) into lactic acid. The production of lactic acid lowers the pH, causing the milk proteins to coagulate and thicken, resulting in the creamy texture of yogurt. This process also enhances the flavor and nutritional profile of the yogurt.