Streptococcus is a genus of gram-positive (plural ) or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. 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. This differs from staphylococci, which divide along multiple axes, thereby generating irregular, grape-like clusters of cells. Most streptococci are oxidase-negative and catalase-negative, and many are facultative anaerobes (capable of growth both aerobically and anaerobically). The term was coined in 1877 by Viennese surgeon Albert Theodor Billroth (1829–1894), by combining the prefix "strepto-" (from ), together with the suffix "-coccus" (from Modern , from .) In 1984, many bacteria formerly grouped in the genus Streptococcus were separated out into the genera Enterococcus and Lactococcus. Currently, over 50 species are recognised in this genus. This genus has been found to be part of the salivary microbiome. Pathogenesis and classification In addition to streptococcal pharyngitis (strep throat), certain Streptococcus species are responsible for many cases of pink eye, meningitis, bacterial pneumonia, endocarditis, erysipelas, and necrotizing fasciitis (the 'flesh-eating' bacterial infections). However, many streptococcal species are not pathogenic, and form part of the commensal human microbiota of the mouth, skin, intestine, and upper respiratory tract. Streptococci are also a necessary ingredient in producing Emmentaler ("Swiss") cheese. Species of Streptococcus are classified based on their hemolytic properties. Alpha-hemolytic species cause oxidization of iron in hemoglobin molecules within red blood cells, giving it a greenish color on blood agar. Beta-hemolytic species cause complete rupture of red blood cells. On blood agar, this appears as wide areas clear of blood cells surrounding bacterial colonies. Gamma-hemolytic species cause no hemolysis. Beta-hemolytic streptococci are further classified by Lancefield grouping, a serotype classification (that is, describing specific carbohydrates present on the bacterial cell wall). The 21 described serotypes are named Lancefield groups A to W (excluding I and J). This system of classification was developed by Rebecca Lancefield, a scientist at Rockefeller University. In the medical setting, the most important groups are the alpha-hemolytic streptococci S. pneumoniae and Streptococcus viridans group, and the beta-hemolytic streptococci of Lancefield groups A and B (also known as “group A strep” and “group B strep”). Table: Medically relevant streptococci (not all are alpha-hemolytic) Alpha-hemolytic When alpha-hemolysis (α-hemolysis) is present, the agar under the colony will appear dark and greenish due