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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.
Main Streptococcus groups are included as "Strep." at bottom left. Diagnosis is by a swab of the affected area for laboratory testing. 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 ...
The first group of streptococci is alpha-hemolytic which comprises primarily S.pneumoniae and viridans streptococci. This group is referred to as alpha-hemolysis because the cell membrane of red blood cells is left intact. [2] When cultured, alpha-hemolysis can be deemed present when the agar gel appears greenish. [2]
The group is named after a principal member species, Streptococcus anginosus. The older name Streptococcus milleri (as well as Streptococcus milleri group, SMG) is now pseudotaxonomic, as the idea that these streptococci constituted a single species was incorrect. The anginosus group streptococci are members of the viridans streptococci group. [1]
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 .
R2A agar, a nonspecific medium, imitates water, so is used for water analysis. Tryptic (trypticase) soy agar (TSA) is a general-purpose medium produced by enzymatic digestion of soybean meal and casein. It is frequently the base medium of other agar types; for example, blood agar plates are made by enriching TSA plates with blood.