Streptococci (Gram-Positive)

Alpha and Beta haemolytic streptococci by GrahamColm

Streptococci, coccus meaning "sphere", appear as strips of balls. Unlike staphylococci, streptococci does not have enzyme catalase.

Classification
By the ability to lyse red blood cells: Beta, completely hemolytic. Alpha, partially hemolytic and leaves a greenish color on the culture medium around the colony. Gamma, not hemolytic.

By Lancefield antigens found in the cell wall, which differentiate species: Group A (beta), B (beta) and D (alpha). Include Streptococcus pneumoniae (alpha) and the Viridans group (alpha), which make up the five pathogenic streptococci species in humans.

Group A Streptococcus pyogenes

• Beta-hemolytic.
• Cell wall antigens:
• M protein prevents complement activation, but can be targeted by antibodies.
• Pathogenic enzymes:
• Streptolysin O destroys RBCs and white blood cells. Oxygen labile (inactivated by oxygen).
• Streptolysin S destroys RBCs and white blood cells. Oxygen stabile.
• Pyrogenic exotoxin are "super" antigenic, causes scarlet fever and toxic shock. Only present in a few strains.
• Streptokinase activates plasmin, an enzyme that breaks up blood clots.
• Diseases: 
• Strep throat.
• Skin infections, including necrotizing fasciitis.
• Scarlet fever, symptoms caused by pyrogenic exotoxin.
• Toxic shock caused by pyrogenic exotoxin. 
• Rheumatic fever, an autoimmune disease caused by antibody cross-reaction to heart tissue. 
• Acute post-streptococcal glomerulonephritis, inflammation in the glomerulus when complement cascade is activated by antibody-antigen complexes. Follows strep throat or strep skin infection. Causes brown urine (hematuria).  

Group B Streptococcus agalactiae

• Beta-hemolytic. 
• Diseases (of babies):
• Meningitis, pneumonia, sepsis by infection from the vagina after birth.

Group D Enterococcus faecalis, Enterococcus faecium, Non-enterococci (Streptococcus bovis, Streptococcus equinus, ...)

• Alpha-hemolytic. 
• Strains resist most drugs, live in the GI tract, and cause infections of the urinary tract or biliary tract. 

Streptococcus pneumoniae

• Alpha-hemolytic. 
• Antigens:
• Polysaccharide capsule prevents bacterium from being phagocytized.
•  Diseases:
• Meningitis, pneumonia in adults.
• Middle ear infection in children. 

Viridans Streptococcus salivarius, Streptococcus sanguis, Streptococcus mitis, ...

• Alpha-hemolytic.
• Diseases:
• Dental infections and cavities.
• Endocarditis by dental infection leading to bacterial growth in the heart valves via bloodstream.
• Abscesses in the abdomen or brain, caused by Streptococcus intermedius.