Lactobacillus bulgaricus is a rod-shaped bacterium that is widely used in the production of yogurt. It is a part of the lactic acid bacteria group and plays a crucial role in the fermentation process by converting lactose into lactic acid. This bacterium is known for its ability to thrive in milk and its symbiotic relationship with Streptococcus thermophilus, another lactic acid bacterium used in yogurt production.