This image depicts acute group A streptococcal pharyngitis in an 8-year-old female patient. Note the acute inflammation of the right tonsil. It is enlarged with adherent plaque. The image was acquired by digital camera 48 h after initiation of symptoms.

Streptococcus pyogenes (Group A Strep) is a Gram positive coccus that forms chains, and is an important pathogen in humans that causes disease. It contains the Group A Lancefield polysaccharide antigen on the surface, and many strains are encapsulated. The capsule, the M protein and lipoteichoic acids are virulence factors associated with the cell wall. The capsule and the M protein prevent phagocytosis in the host immune response. The M protein is also a superantigen, which elicits an overwhelming immune response in the host. Lipoteichoic acids facilitate the adherence of the bacteria to the epithelium of the host respiratory tract. There are also several toxins and enzymes associated with S. pyogenes virulence. The Spe toxin causes erythrogenic effects seen with scarlet fever, and may enhance the effects of other toxins. The exotoxin produced by S. pyogenes is pyrogenic (causes fever) and causes necrotizing effects to tissue. The pyrotoxin is also a superantigen. Streptolysin O is an enzyme that lyses many human cells, including red blood cells, platelets, and neutrophils. On a blood agar plate, the enzyme would produce beta-hemolysis, a complete clearing of the red blood cells. Other enzymes produced by S. pyogenes are DNase, which depolymerizes host DNA, and hyaluronidase, which breaks down hyaluronic acid, causing tissue damage, that facilitates the spread of the infection. Finally the bacteria possess a streptokinase that dissolves fibrin clots formed in the body; fibrin clots prevent the spread of infection.

Numerous suppurative diseases are caused by S. pyogenes, including strep throat (pharyngitis) pneumoniae, scarlet fever, and necrotizing fascitis (flesh-eating disease). An invasive disease, called toxic-strep syndrome can occur because the bacterium possesses superantigens, that will induce shock. These infections are generally treated with penicillin or erythromycin. There are also many complications that can arise post-infection, that are induced by an auto-immune response. Human antibodies made to Group A antigens bind to myosin in the heart and heart valves and lead to endocarditis. This disease is known as rheumatic fever. The antibodies can also damage in the kidneys by binding to basement glomerular membrane, resulting in glomerulonephritis.

Legend written by:
Julia A. Frey
Colorado State University
Fort Collins, Colorado