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''Streptococcus'' is a genus of coccus (spherical) gram-positive bacteria belonging to the phylum Firmicutes〔 and the order Lactobacillales (lactic acid bacteria). Cell division in this genus occurs along a single axis in these bacteria, and thus they grow in chains or pairs, hence the name—from Greek στρεπτός ''streptos'', meaning easily bent or twisted, like a chain (twisted chain). (Contrast this with staphylococci, which divide along multiple axes and generate grape-like clusters of cells.) Most are oxidase-negative and catalase-negative, and many are facultative anaerobes. In 1984, many bacteria formerly considered ''Streptococcus'' were separated out into the genera ''Enterococcus'' and ''Lactococcus''. Currently, over 50 species are recognised in this genus. ==Pathogenesis and classification== In addition to streptococcal pharyngitis (strep throat), certain ''Streptococcus'' species are responsible for many cases of pink eye,〔http://www.medicinenet.com/pink_eye/article.htm〕 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. Furthermore, streptococci are 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 20 described serotypes are named Lancefield groups A to V (excluding I and J). 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)〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Streptococcus」の詳細全文を読む スポンサード リンク
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