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Group a Streptococci
Epidemiology - streptococcal pharyngitis most common treatable cause of pharyngitis, streptococcal impetigo most common bacterial skin infection, rheumatic heart disease most common form of acquired heart disease worldwide, poststreptococcal AGN most common form of AGN
Classification
Hemolytic classes
Beta-hemolytic: clear zone of complete hemolysis around colony on blood agar, all group A strains 1. Two beta-hemolysins
Streptolysin O-oxygen-labile, subsurface hemolysis, gives rise to antibodies (ASO) that neutralize its action
Streptolysin S-oxygen-stabile, surface hemolysis
Both hemolysins can damage cell membranes other than RBCs
AIpha-hemolytic: partial hemolysis producing green zone around colony on
Gamma-hemolytic: no surface or deep hemolysis
Group a Streptococci
Classification
Serologic grouping: specific grouping (A to O) based on presence of C carbohydrate antigen, most important human pathogens are group A, extract out carbohydrate antigen and react with hyperimmune rabbit sera
Group a Streptococci
Classification
Serologic typing at GABHS:
M typing - over 70 pharyngitis, rheumatic fever, cellulitis, flesh eating bacteria, flesh-eating bacteria immunologic types that differ in their cell wall M protein, M protein extracted out and reacted with hyperimmune rabbit M protein extends out as fimbriae from surface of cell, most antibodies to M protein are protective
T typing - another cell wall protein found in a number of immunologically distinct antigenic forms, also designated by numbers group A streptococcal, pharyngitis, rheumatic fever, cellulitis, flesh eating bacteria
Metabolism
Growth - optimal at 37°C, complex nutritional requirements, promoted by blood serum
Lactic acid bacteria
Catalase - negative, nonmotile, facultative anaerobes
Group a Streptococci
Cellular Antigens
C carbohydrate, M protein, T protein
Hyaluronic acid capsule - mucoid, matt, and glossy colonies; possible virulence factor
Lipoteichoic acid - adherence to mucosal surfaces
Peptidoglycan, other surface proteins, cytoplasmic membrane
Extracellular Products
Streptolysin O and streptolysin S
Erythrogenic toxin - responsible for rash in scarlet fever, production mediated by temperate bacteriophage, at least three immunologically distinct forms (A, B, C), streptococcal TSS
Streptokinase - promotes lysis of clots by catalyzing conversion of plasminogen to plasmin, used commercially as thrombolytic agent, uncertain role as virulence factor
Deoxyribonucleases - four immunologically distinct types (A, B, C, D), antibodies to DNase B (ADB) are used in conjunction with ASO for serodiagnosis of GABHS infection
Others - NADase, hyaluronidase, proteinase
Clinical Diagnosis of GABHS
Pharyngitis
Features suggestive of GABHS as the etiology:
Sudden onset
Sore throat
Fever
Headache
Nausea, vomiting, abdominal pain
Marked inflammation of pharynx and tonsils
Patchy discrete exudate
Tender, enlarged anterior cervical nodes
Features suggestive of a viral etiology:
Conjunctivitis, coryza, cough, diarrhea
Clinical Diagnosis of GABHS
Pharyngitis
Predictive value of single findings or combinations of findings
Clinical scoring systems
Stillerman and Bernstein