Master Streptococcus pyogenes Characteristics with Picmonic for Medicine

Group A Streptococci, commonly referred to as GAS, are infections caused by the bacterium Streptococcus pyogenes. Diseases caused by Streptococcus pyogenes infections include skin conditions such as impetigo and cellulitis, as well as pharyngitis, scarlet fever, and toxic shock-like syndrome. S. pyogenes infections can also lead to episodes of rheumatic fever and acute glomerulonephritis.

This organism stains positive on a Gram stain due to its thick peptidoglycan layer, which absorbs crystal violet.

This bacterium has a spherical shape and appears on a histological slide as cocci arranged in chains.

S. pyogenes typically causes large zones of beta hemolysis, which is the complete lysis of red blood cells in the culture media. Streptolysin O, an exotoxin produced by the bacteria, causes complete lysis of red blood cells by interacting with the cholesterol membrane.

Characteristically, S. pyogenes is catalase-negative, which helps distinguish it from Staphylococcal species that are catalase-positive.

Bacitracin can be used to differentiate Streptococcus pyogenes from other beta-hemolytic Streptococci, such as Streptococcus agalactiae. Streptococcus pyogenes is bacitracin-sensitive, whereas Streptococcus agalactiae is resistant.

Many strains of Streptococcus pyogenes have a hyaluronic acid capsule, which aids this organism in resisting phagocytosis. Another defensive factor against phagocytosis is M protein.

Streptolysin O is an oxygen-labile exotoxin that S. pyogenes releases. It is often tested for using an ASO, or antistreptolysin O titer.

Streptococcus pyogenes produces DNase, an enzyme that cleaves the DNA backbone, degrading DNA. This allows the organism to infect pharyngeal tissues and skin, while also degrading the DNA of neutrophil extracellular traps, which would normally kill these bacteria.

Streptokinase is an enzyme that inhibits the coagulation cascade in humans. S. pyogenes produces this enzyme, causing blood clots to dissolve, enabling the bacteria to spread easily 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.