Streptococcus thermophilus
Streptococcus thermophilus formerly known as Streptococcus salivarius subsp. thermophilus is a gram-positive bacterium, and a fermentative facultative anaerobe, of the viridans group. It tests negative for cytochrome, oxidase, and catalase, and positive for alpha-hemolytic activity. It is non-motile and does not form endospores. S. thermophilus is fimbriated. It is also classified as a lactic acid bacterium. S. thermophilus is found in fermented milk products and is generally used in the production of yogurt, alongside Lactobacillus delbrueckii subsp. bulgaricus. The two species are synergistic, and S. thermophilus probably provides L. d. bulgaricus with folic acid and formic acid, which it uses for purine synthesis. S. thermophilus has an optimal growth temperature range of , while L. d. bulgaricus has an optimal range of . At least 26 strains of S. thermophilus have been identified and had their genomes sequenced. S. thermophilus is one of the most widely used bacteria in the dairy industry. USDA statistics from 1998 showed that more than 1.02 billion kilograms of mozzarella cheese and 621 million kilograms of yogurt were produced from S. thermophilus. Although its genus, Streptococcus, includes some pathogenic species, food industries consider S. thermophilus a safer bacterium than many other Streptococcus species. In fact, yogurt and cheese that contain live cultures of S. thermophilus are thought to be beneficial to health. Live cultures of S. thermophilus make it easier for people who are lactose-intolerant to digest dairy products. The bacteria break down lactose, the sugar in milk, that lactose-intolerant people find difficult to digest. As early as the 1900s, S. thermophilus was used to make yogurt. Its purpose is to turn lactose, the sugar in milk, into lactic acid. The increase in lactic acid turns milk into the gel-like structure characteristic of yogurt. "Streptococcus" derives from a Greek term meaning "twisted kernel" and refers to the way the bacterium is grouped in chains that resemble a string of beads.
