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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.
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]
GBS grows readily on blood agar plates as colonies surrounded by a narrow zone of β-hemolysis.GBS is characterized by the presence in the cell wall of the antigen group B of Lancefield classification (Lancefield grouping) that can be detected directly in intact bacteria using latex agglutination tests.
Streptolysin O is an oxygen-sensitive cytotoxin, secreted by most group A Streptococcus (GAS), and interacts with cholesterol in the membrane of eukaryotic cells (mainly red and white blood cells, macrophages, and platelets), and usually results in beta-hemolysis under the surface of blood agar. Streptolysin S is an oxygen-stable cytotoxin also ...
[6] [7] After incubation (18–24 hours, 35-37 °C), the enrichment broth is subcultured overnight in blood agar plates and GBS-like colonies (big colonies, 3-4 millimeters diameter, surrounded by narrow zone of hemolysis) [6] [7] are identified by the CAMP test or using latex agglutination with GBS antiserum or MALDI-TOF. [6] [7] [12] [14]
However, both Streptococcus dysgalactiae and the Streptococcus anginosus group can possess group A antigen as well. Group A streptococci, when grown on blood agar, typically produce small (2–3 mm) zones of beta-hemolysis, a complete destruction of red blood cells. The name group A (beta-hemolytic) Streptococcus is thus also used. [1]
The CAMP factor produced by S. agalactiae and the alpha toxin produced by C. perfringens act synergistically to produce enhanced hemolysis. Streaking these two organisms perpendicular to each other on a blood agar plate will yield a “bow tie” shaped zone of hemolysis which indicates a positive test. [9] [10]
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] Identifying and diagnosing alpha-hemolytic Streptococcus is done with a sputum gram stain and culture test. [5]