What is the most pathogenic group of streptococcus?
Group A (Streptococcus pyogenes)
What is the microscopic morphology of streptococci?
-Spherical cells, tend to form pairs or chains of cocci -Gram positive (blurple)
What are the various hemolysis patterns of streptococci types on 5% sheep blood agar?
-alpha = green (some intact red cells) -beta = clear (no intact red cells) -gamma = no hemolysis (intact red cells)
Which hemolysis type does Group A strep have?
-Beta (clear)
What hemolysis type does strep pneumoniae have?
-alpha (Green)
What does the bacitracin susceptibility test reveal about a strep culture, and which hemolytic pattern is it used on?
-used on beta-hemolysis (clear) -Strep pyrogenes is susceptible (group A), the other groups are resistant
What is the important Group D strep species?
-S. gallolyticus [bovis]
What is Lancefield classification?
-developed for strep -immunologic classification based on differences of cell wall carbs -used for beta-hemolytic and non-hemolytic streptococci
How are Group D streptococci identified?
-either alpha or gamma hemolytic -can grow in presence of 40% bile -can hydrolyze esculin (turns black on bile esculin agar) **very sensitive to penicillin
What is the antigenic structure of streptococci?
-capsule (non-antigenic, antiphagocytic) -M-proteins (antiphagocytic) -C-carbohydrate (lancefield groupings) -Peptidoglycan -lipoteichoic acid
How is the capsule of strep non-antigenic?
-probably because it is chemically indistinguishable from hyaluronate in ground substance of connective tissue
How are M-proteins of strep antiphagocytic?
-prevents interaction of organism with complement **antibody to M-protein is protective and imparts long-lasting immunity
What are the extracellular products of Group A strep?
-Pyrogenic exotoxins A -streptolysin O (hemolysin) -Streptokinase -Hyaluronidase -DNAse
Which extracellular product of Group A strep is responsible for a number of the clinical manifestations of toxic strep syndrome?
-Pyrogenic exotoxins
What do pyrogenic exotoxins of strep do?
-superantigens bind directly with class II MHC, leads to clinical manifestations of toxic strep syndrome (releases a bunch of cytokines) -Pyrogenic exotoxin B degrades ECM proteins such as fibronectin & vitronectin (facilitate spread of infection)
What does streptokinase do?
-activates plasminogen to disrupt blood clots -can be used to lyse coronary occlusions *Hypersensitivity reactions occur in patients previously exposed to this antigen.
Why are anti-DNAse titers more useful than Antistreptolysin O (ASO) titer in diagnosing strep A/Rheumatic fever?
-DNAse is not neutralized by cholesterol during skin infections
How does Group A strep cause disease?
-direct damage due to infection itself (acute suppurative disease) -toxin production -immune response (post-streptococcal sequelae)
What is the pathogenesis of acute suppurative diseases caused by Group A strep?
-adherence to skin/mucous membrane by lipoteichoic acid -tendency for local spread, due to enzymes -other enzymes = "thin dishwater pus"
What are common examples of acute suppurative diseases caused by Group A strep?
-Pharyngitis/tonsillitis (strep throat) -Pyoderma (impetigo) -cellulitis/Lymphangitis -erysipelas
What is pyoderma?
-impetigo -discrete skin lesions with a papule-vesicle-pustule crusting sequence
What is erysipelas?
-form of cellulitis -diffuse lymphangitis of skin, infection spreads along lymphatics -well demarcated
What are some common examples of toxin-associated disease caused by Group A strep?
-scarlet fever -Toxic strep syndrome
What is toxic strep/shock syndrome?
-"flesh-eating bacteria" Group A strep (pyrogenic exotoxins) -severe infections, soft tissue, with bacteremia -high mortality -hypotension with multiple organ impairment
What is scarlet fever?
-caused by group A strep -Streptococcal pharyngitis/tonsillitis accompanied by a diffuse, bright scarlet erythema of skin and mucous membranes -rash begins on trunk and neck, then spreads to extremities, sparing the face -due to production of erythrogenic (scarlatiniform) toxin
What are some common post-streptococcal sequelae caused by group A strep?
-acute rheumatic fever, from cross-reactivity between strep M-proteins & cardiac sarcolemmal proteins -acute glomerulonephritis, from deposition of immune complexes in glomeruli -guttate psoriasis, from superantigenic stimulation of T-cells by pyrogenic exotoxin C
What are the first line treatments for Group A strep infections?
-penicillins & cephalosporins *resistance to these has not yet been described *macrolide can be used in pts with penicillin allergy
How should patients with a history of rheumatic fever be managed long-term? Why?
-long-term penicillin prophylaxis -because they are at an increased risk to develop repeated attacks
What are the two important Strep strains that do alpha-hemolysis?
-Viridans strept -strept pneumo
How do we differentiate viridans strep from strept pneumo?
-Viridans strept is not susceptible to Optochin, strep pneumo are -strep neumo is soluble in bile salts, viridans is not
