STREPTECOCCUS PYOGENES: (GROUP A ) :
Gram + cocci in pairs or chains
Beta-hemolytic
Pyrrolidonyl arylamidase (PYR) +
Bacitracin sensitive
lancifield grouping according to their cell wall antigens : A :group specific
group-specific carbohydrate (A antigen) and type-specific proteins (M protein) in cell wall
Pharyngitis and soft-tissue infections typically caused by strains with different M proteins
normally colonize peacefully : skin ,mucous, pharynx,vagina
grows on enriched blood agar
Person-to-person spread by respiratory droplets (pharyngitis) or through breaks in skin after direct contact with infected person, fomite, or arthropod vector
Hyaluronic acid capsule: inhibits phagocytosis
which is a common component of human connective tissue.Thus, Streptococcus pyogenes can evade host detection as the capsule is non-immunogenic
Protein M: Main virulence factor for strep. pyogenes,Inhibits phagocytosis and activation of complement, type specific cell wall antigen
Streptolysin (Hemolysin) O: oxygen-labile hemolysin , Lyses RBCs (causing beta-hemolysis)
cutaneous infections do not develop ASO antibodies
Streptolysin S: oxygen-stable hemolysin, can lyse erythrocytes, leukocytes, and platelets. responsible for the characteristic β-hemolysis seen on blood agar media.
streptokinase: virulence factor that converts plasminogen to plasmin which in turn degrades fibrin, preventing development of a fibrin barrier at the site of infection,
Enables more spread from initial site of infection
Antibodies directed against
these enzymes (anti-streptokinase antibodies) are a useful marker for infection
DNase: not cytolytic but can depolymerize free deoxyribonucleic
acid (DNA) present in pus.reduces the viscosity of the abscess material and facilitates spread. Antibodies developed against DNase B are an important marker of S. pyogenes infections
particularly for patients with cutaneous infections, because
they fail to make antibodies against streptolysin O
(anti-DNase B test)
SUPPURATİVE İNFECTİONS
Pharyngitis (bacterial strep throat): most common cause of bacterial pharyngitis
Scarlet Fever : complication of streptococcal pharyngitis; occurs when the infecting strain is lysogenized by
a bacteriophage that mediates production of a pyrogenic exotoxin; strawberry tongue
Impetigo
Erysipelas
Cellulitis
Necrotizing fasciitis
Streptococcal Toxic Shock Syndrome: production of pyrogenic exotoxins; this toxin act as a superantigen
Bacterimia
NON SUPPURATİVE
Rheumatic fever: is a nonsuppurative complication of S.
pyogenes pharyngitis not skin infections, characterized by inflammatory changes involving the heart, joints, blood vessels, and subcutaneous tissues.
mitral valve generally infected
antibodies against M protein or other virulence factors (e.g. streptolysin O) can attack self-tissues
Similarity of M protein to self-cells is due to molecular mimicry
Type ıı sensitivity reaction
Poststreptococcal Glomerulonephritis
Usually occurs 1-2 weeks after GAS infection(e.g. skin, pharynx)
Type III Hypersensitivity Reaction
TREATMENT : penicilin
DIAGNOSIS:
MICROSCOPY :
Gram stains of affected tissue can be used to make a rapid preliminary diagnosis of S. pyogenes soft-tissue infections
but not pharyngitis or nonsuppurative complications
many species of streptococci are part of the
normal oropharyngeal fora, so observation of streptococci
in a respiratory specimen from a patient with pharyngitis has
no diagnostic significance.
ANTIGEN DETECTION:
antibodies that react with the group-specific carbohydrate in the bacterial cell wall can be used to detect group A streptococci directly in throat swabs.These tests are rapid, inexpensive, and specific.
Antigen tests are not used for cutaneous or nonsuppurative
diseases.
Direct tests for the group A antigen are useful for the diagnosis of streptococcal pharyngitis
NUCLEIC ACID BASED:
Commercial nucleic acid probe assay and nucleic acid amplification assays are available for the detection of S. pyogenes in pharyngeal specimens.
CULTURE:
IDENTIFICATION:
Differentiation of S. pyogenes from other species of streptococci with the group-specific A antigen can be determined by their susceptibility to bacitracin or the presence of the enzyme L-pyrrolidonyl arylamidase (PYR).
ANTIBODY DETECTION:
measurement of antibodies against streptolysin O (ASO test) is useful for confirming rheumatic fever or acute glomerulonephritis resulting from a recent streptococcal pharyngeal infection.
anti-DNase B test should be performed pharyngitis or soft tissue infection