Streptococcus is a genus of non-motile, microaerophilic, gram-positive spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales. Cell division in streptococci occurs along a single axis, so as they grow, they tend to form pairs or chains that may appear bent or twisted. Most streptococci are facultative or strict anaerobes that are catalase-negative, while staphylococci are catalase-positive. Species of Streptococcus are classified based on their hemolytic properties. Alpha-hemolytic species cause oxidization of iron in hemoglobin molecules within red blood cells. Beta-hemolytic species generate a complete rupture of red blood cells, while gamma-hemolytic species are a lack of hemolysis.
There are many natural sources of Streptococci, including humans and diverse animals, where they often colonize the mucosal surfaces of the mouth, intestinal tract, nasal passages, and pharynx. Certain Streptococcus species are responsible for many cases of pink eye, meningitis, bacterial pneumonia, endocarditis, erysipelas, and necrotizing fasciitis. Although Streptococci can be potent pathogens, some species are commercially important for the production of cheese and yogurt, including S. lactis, S. cremoris, S. diacelillactis, and S. thermophilus, the latter being the most well-known.