Image of Streptococcus, a type (genus) of spherical bacteria that can colonize the throat and back of the mouth. Streptococcus is a genus of spherical Gram-positive bacteria belonging to the phylum Firmicutes and the lactic acid bacteria group. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs. Most streptococci are oxidase- and catalase-negative, and many are facultative anaerobes. There are currently over 50 species recognised in this genus.