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Gamma-hemolytic species cause no hemolysis. [11] Beta-hemolytic streptococci are further classified by Lancefield grouping, a serotype classification (that is, describing specific carbohydrates present on the bacterial cell wall). [6] The 21 described serotypes are named Lancefield groups A to W (excluding E, I and J).
Similarly, group B streptococcus typically denotes Streptococcus agalactiae, although minor beta-hemolytic group B streptococci like S. troglodytidis exist. [15] While most streptococcal illnesses in humans originate from species adapted to humans, such as S. pneumoniae or S. pyogenes, there are zoonotic species capable of causing infections. [15]
If an organism does not induce hemolysis, the agar under and around the colony is unchanged and the organism is called non-hemolytic or said to display gamma-hemolysis (γ-hemolysis). Enterococcus faecalis (formerly called "Group D Strep"), Staphylococcus saprophyticus, and Staphylococcus epidermidis display gamma-hemolysis.
The viridans streptococci are a large group of commensal streptococcal Gram-positive bacteria species that are α-hemolytic, producing a green coloration on blood agar plates (hence the name "viridans", from Latin "vĭrĭdis", green), although some species in this group are actually γ-hemolytic, meaning they produce no change on blood agar. [1]
Streptococcus agalactiae (also known as group B streptococcus or GBS) is a gram-positive coccus (round bacterium) with a tendency to form chains (as reflected by the genus name Streptococcus). It is a beta- hemolytic , catalase -negative, and facultative anaerobe .
Streptococcus bovis is a group of strains of Gram-positive bacteria, originally described as a species, [5] [6] that in humans is associated with urinary tract infections, endocarditis, sepsis, [7] and colorectal cancer. [8] S. bovis is commonly found in the alimentary tract of cattle, sheep, and other ruminants, [9] and may cause ruminal acidosis.
Bacteria of the genus Enterococcus, formerly known as group D streptococci, were classified as members of the genus Streptococcus until 1984 and are included in the original Lancefield grouping. [4] Many—but not all—species of streptococcus are beta-hemolytic .
Hemolysis by Streptococcus seen on a plate. Not only are the erythrocytes affected by hemolysins, but there are also some effects among other blood cells, such as leucocytes (white blood cells). Escherichia coli hemolysin is potentially cytotoxic to monocytes, lymphocytes and macrophages, leading them to autolysis and death.