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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]
Hemolyses of Streptococcus spp. (left) α-hemolysis (S. mitis); (middle) β-hemolysis (S. pyogenes); (right) γ-hemolysis (non-hemolytic, S. salivarius) Hemolysis is the breakdown of red blood cells. The ability of bacterial colonies to induce hemolysis when grown on blood agar is used to classify certain microorganisms.
Streptolysin S is an oxygen-stable cytotoxin also produced by most GAS strains which results in clearing on the surface of blood agar. SLS affects immune cells, including polymorphonuclear leukocytes and lymphocytes, and is thought to prevent the host immune system from clearing infection. Streptococcus pyogenes, or GAS, displays beta hemolysis.
Streptococcus sanguinis White and Niven 1946 S. sanguinis is a normal inhabitant of the healthy human mouth where it is particularly found in dental plaque , where it modifies the environment to make it less hospitable for other strains of Streptococcus that cause cavities , such as Streptococcus mutans .
Streptococcus thermophilus formerly known as Streptococcus salivarius subsp. thermophilus [2] [1] is a gram-positive bacterium, and a fermentative facultative anaerobe, of the viridans group. [3] It tests negative for cytochrome, oxidase, and catalase, and positive for alpha-hemolytic activity. [3] It is non-motile and does not form endospores. [3]
A Gram stain is performed to show Gram-positive cocci in chains. Then, the organism is cultured on blood agar. The rapid pyrrolidonyl arylamidase (PYR) test is commonly used, wherein a positive reaction confers a presumptive identification of group A beta-hemolytic streptococci if the appearance and clinical context is consistent.
The system, created by Rebecca Lancefield, was historically used to organize the various members of the family Streptococcaceae, which includes the genera Lactococcus and Streptococcus, but now is largely superfluous due to explosive growth in the number of streptococcal species identified since the 1970s. [2]
Streptococcus oralis is a Gram positive viridans streptococcus of the Streptococcus mitis group. [2] [3] S. oralis is one of the pioneer species associated with eubiotic dental pellicle biofilms, and can be found in high numbers on most oral surfaces. [4] [5] It has been, however, found to be an opportunistic pathogen as well. [2]