Streptococcus salivarius subsp. thermophilus (previous name Streptococcus thermophilus) is a gram-positive bacteria and a homofermentative facultative anaerobe, of the viridans group.It tests negative for cytochrome, oxidase and catalase, and positive for alpha-hemolytic activity.. 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.