Streptococcus mitis, previously known as Streptococcus mitior, is a mesophilic alpha-hemolytic species of Streptococcus that inhabits the human mouth. It is most commonly found in the throat, nasopharynx, and mouth. It is a Gram-positive coccus, facultative anaerobe and catalase negative. It can cause infective endocarditis. It has been widely reported that this organism survived for over two years on the Surveyor 3 probe on the moon; but some NASA scientists suggest this may be a result of contamination during or after return of Surveyor parts to Earth.