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Group A Streptococcal Infections

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 a viral etiology:

Conjunctivitis, coryza, cough, diarrhea

Clinical Diagnosis of GABHS

Pharyngitis