Yoghurt bacteria. Coloured scanning electron micrograph (SEM) of the bacteria Streptococcus thermophilus (spherical) and Lactobacillus bulgar- icus (rod-shaped) in live yoghurt. S. thermophilus are Gram-positive bacteria that often form chains as seen here. These thermophile bacteria are capable of living at the high temperatures of 50- 65 degrees Celsius. L. bulgaricus are Gram- positive bacteria that grow in acid media and produce lactic acid from the fermentation of carbohydrates. Both of these bacteria are used in the manufacture of yoghurt. They are added to milk and incubated at a high temperature to produce yoghurt. Magnification: x4,375 at 6x7cm size.