Group A Streptococci, commonly called GAS, infections are caused by the bacteria Streptococcus pyogenes. Diseases caused by Streptococcus pyogenes include skin infections like impetigo, cellulitis, pharyngitis, scarlet fever, and toxic shock-like syndrome. Streptococcus pyogenes infections can also precipitate episodes of rheumatic fever and acute glomerulonephritis.
This organism stains positive on Gram stain, due to thick peptidoglycan layer which absorbs crystal violet.
This bacterium has a spherical shape.
Strep pyogenes typically produces large zones of beta hemolysis, which is complete lysis of red cells in the blood culture media. An exotoxin called streptolysin O is the enzyme produced by the bacteria, which causes complete lysis of red blood cells via interaction with the cholesterol membrane.
Characteristically, Streptococcus pyogenes is catalase-negative, which is helpful in distinguishing Streptococcus from Staphylococcal species that are catalase- positive.
Bacitracin can be used to distinguish Streptococcus pyogenes from other beta- hemolytic Streptococci, like Strep agalactiae. Streptococcus pyogenes is bacitracin- sensitive, while Streptococcus agalactiae is bacitracin-resistant.
Many strains of Streptococcus pyogenes have a hyaluronic acid capsule, which aids this organism in resisting phagocytosis.
Streptolysin O is an oxygen-labile exotoxin released by Streptococcus pyogenes. This is often tested for with an ASO, or antistreptolysin O titer.
Streptococcus pyogenes species produce DNase, an enzyme which cleaves the DNA backbone, degrading DNA. This allows this organism to infect pharyngeal tissues and skin, while degrading DNA of neutrophil extracellular traps, which would normally kill these bacteria.
Streptokinase is an enzyme which inhibits the coagulation cascade in humans. These bacteria produce this enzyme, causing blood clots to dissolve so the bacteria can easily spread throughout the body.
Exotoxin A produced by S. pyogenes aids in virulence by decreasing the production of antibodies and potentiating the likelihood for necrotizing fasciitis and streptococcal toxic shock syndrome. The expression is highly variable among different strains of this bacteria.
The pyrrolidonyl arylamidase (PYR) test is a rapid test, which has largely replaced the bacitracin test. Streptococcus pyogenes is a positive control, and is well known to be positive for PYR, which can be tested for in 10-15 minutes.
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