Color enhanced SEM (scanning electron micrograph) of Streptococcus thermophilus and Lactobacillus bulgaricus in yogurt. Streptococcus thermophilus is a lactic acid bacterium found in fermented milk products, used in the production of yogurt. Lactobacillus bulgaricus is a rod-shaped, gram-positive bacterium that is also used in yogurt production. It produces lactic acid, helping to preserve yogurt and acetaldehyde, a component of yogurt aroma.