streptococcus UK /ˌstɹɛptəˈkɒkəs/US /ˌstɹɛptəˈkɑkəs/ noun1 •A spherical, gram-positive bacterium of the genus Streptococcus. Although commonly found benignly in the human mouth and gut, and though many species are non-pathogenic, other species can cause diseases including strep throat and more serious conditions. formsstreptococci