Streptococci Flashcards
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 = necrotizing fasciitis)
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
What do superantigens produced by strep do?
superantigens bind directly with class II MHC, leads to clinical manifestations of toxic strep syndrome (releases a bunch of cytokines)
What does pyrogenic exotoxin B produced by strep do?
Pyrogenic exotoxin B degrades ECM proteins such as fibronectin & vitronectin (facilitate spread of infection)
What are common manifestations of Scarlet Fever caused by Group A strep?
- Strawberry tongue
- sandpaper rash
How do we differentiate the malar rash caused by erysipelas and SLE?
-there will be a “step up” upon palpation in erysipelas (group A strep) due to inflammation & lymphatic involvement
\What condition always precedes acute rheumatic fever and acute post-strep glomerulonephritis?
Rheumatic fever: -streptococcal pharyngitis (But not skin infections!)
Glomerulonephritis: -pharyngitis OR skin infection
What are the major manifestations of Rheumatic fever?
*Fever
JONES:
Joints (polyarthritis)
O (myocarditis)
Nodules (subcutaneous nodules)
Erythema marginatum
Sydenham’s chorea
What are the major manifestations of post-streptococcal glomerulonephritis?
- edema of face & extremeties
- hematuria
- hypertension
- decreased complement levels
- abnormal urinary sediment & RBC casts
A 65-year-old man is admitted to the hospital with an acute myocardial infarction. Emergency angiography demonstrates 95% acute occlusion of the left anterior descending coronary artery. Which of the following streptococcal components could be used therapeutically in this man?
A. DNAse B
B. Streptokinase
C. Streptolysin O
D. Hyalunonidase
B. Streptokinase
A 20-year-old boy is admitted to LDS Hospital in Salt Lake City with fever, a new heart murmur and joint swelling. His pediatrician suspects acute rheumatic fever. Which test is appropriate?
A. Anti-streptolysin O titers (ASO)
B. Blood cultures
C. Joint fluid PCR for Streptococcus pyogenes
D. Optochin susceptibility test
A. Anti-Streptolysin O Titers (ASO)
What streptococcal components is the Lancefield grouping system based on?
A. Capsular polysaccharide
B. Pyrogenic exotoxins
C. C-carbohydrates
D. Teichoic acids
C. C-carbohydrates