12 - Streptococci Flashcards
(33 cards)
What are key characteristics of streptococci? What is it’s shape? What type of metabolism does it do?
Gram + cocci arranged in pairs or chains. Catalase -
Most species are facultative anaerobes. Requires blood or serum for culturing.
Ferment carbs, resulting in lactic acid production.
What are the 3 schemes that can be used to classify streptococci?
- Cell wall carb antigens recognized by Abs aka Lancefield typing.
- group A - S. pyogenes
- group B - S agalactiae - Hemolytic pattern on agar containing blood cells
- alpha - partial hemolysis (green)
- beta - complete clearing
- gamma - no change in RBS - Biochemical properties
Which streptococci are beta hemolytic?
Group A strep: strep pyogenes
Group B strep: strep agalactiae
Group C strep: strep dysgalactiae
Group F strep: strep anginosus
Which streptococci are alpha and gamma hemolytic?
S. pneumoniae
S. mutans
S. bovis
What are the surface proteins of Streptococcus pyogenes?
Group specific antigen: lancefield group A carb.
Type specific antigen: M proteins encoded by emm genes
M-like surface proteins
Lipoteichoic acid and F protein mediate adherence to fibronectin.
HA capsule
C5s peptidase.
What is the mechanism of pathogenesis of strep pyogenes?
- Avoidance of opsonization and phagocytosis via virulence factors
- Adherence to host cells- lipoteichoic acid, M proteins, F protein
- Invasion of host cells - M proteins and F proteins
- Toxins and enzymes
How does SPE toxin mediate several clinical manifestations of S. pyogenes infections?
- Cytokine release may be key to severity of nec fasciitis and streptococcal toxic shock syndrome
- responsible for rash in pts with scarlet fever
Name the toxins and enzymes produced by S. pyogenes?
Streptolysin A: causes lysis of RBCs (B hemolysis) and kills PMNs and macrophages
Streptolysin O - oxygen labile hemolysin
Streptokinase - cleavage of plasminogen to release plasmin and then cleavage of fibrin and fibrinogen
DNAses - depolymerize DNA to spread infection
Who is protected from Strep pyogenes? How is it spread?
Patients with antibodies to M proteins are protected.
Spread by droplets.
What clinical diseases are associated with strep pyogenes?
- Strep pharyngitis
- Scarlet fever
- Impetigo or pyoderma
- Erysipelas - of skin and subQ tissue. Distinct, advancing borders
- Streptococcal toxic shock
- Endocarditis
- Necrotizing fasciitis
What are late sequelae associated with Group A strep?
Rheumatic fever: follows resp infections, hypersensitivity to strep antigens that cross react with human heart tissue antigens; fever, poly arthritis, carditis.
Glomerulonephritis: can follow pharyngeal or cuteanous infectoins, antigen-ab complexes in glomerular BM; fever, blood in urine, edema, HTN, increased BUN.
How would you diagnose strep pyogenes in the lab?
Gram stain from infected tissues for soft tissue infections.
Antigen detection - rapid tests for group A carb.
Nucleic acid amplification - pharyngeal specimens.
Culture from throat swab
Gram stain blood bottles; culture draining pustules.
How is strep pyogenes pharyngitis treated? What if someone is allergic to the first option? How would you prevent strep pyogenes?
Penicillin, penicillin V amoxicillin
Allergy to penicillin: cephalosporin or a macrolide.
Prevent spread by prevention of droplet transmission (wash hands and good hygiene).
How is severe of systemic strep pyogenes infections treated?
Surgical debridement and antibiotics.
What is the pathogenesis of strep agalactiae?
Expresses group B carbohydrate antigen.
Avoids phagocytosis by expressiv a capsule.
What is the epidemiology of strep agalactiae?
Asymptomatic colonization of lower GI tract and genitourinary tract; risk for neonates increases with prolonged labor, premature membrane rupture, premature birth, of mother has disseminated group B disease.
What is the clinical disease associated with strep agalactia (group B)?
Neonatal disease, early and late onset of meningitis, pneumonia bacteremia; infections in pregnant women, infections in other adults (bacteremia, pneumonia, bone and joint infections, soft tissue infections).
How would you diagnose strep agalactiae in lab? How is it treated?
Gram stain of CSR for meningitis, pneumonia, and wound infections.
Culture, PCR, and group specific antigen test for vaginal carriage.
Penecillin G for serious infections. Penicillin and aminoglycoside.
What are characteristics of strep pneumonia?
Encapsulated gram +, elongated or oval coccus arranged in pairs or chains.
Alpha- hemolytic on blood agar.
Capsular polysaccharides bassis for classification.
What is unique about the cell wall composition of strep pneumonia?
Contains phosphorylcholine + specific teichoic acids:
- C polysaccharide binds to serum CRP, a marker for acute inflamm.
- F antigen cross reacts with forssman surface antigens on mammalian cells
What are the mechanisms of pathogenesis of streptococcus pneumonia?
- colonization: surface protein adhesions allow binding to epithelial cells of oropharynx
- Resistance to phagocytosis: via secretion of IgA proteases that cleave Fc portion
- Release of toxic cell wall components that trigger inflamm response (IL-1 and TNFa production).
How does the pathogenesis of step pneumonia differ from S. pyogenes?
S. pneumonia infection is due mostly due to host reaction from it being in the tissue rather than the expression of bacterial toxins seen in S. pyogenes.
What is the epidemiology of strep pneumonia? What is the disease associated with?
Colonizes healthy ppl. Pneumonia can occur when endogenous oral organism is aspirated into lower airways.
Disease associated with breakdown of natrual defense mechanisms.
Pneumococcal pneumonia associated with antecedent viral resp. disease.
What does strep pneumo commonly cause in children?
Otitis media.
