Strep Flashcards
Classification of strep:
- gram positive found in pairs or chains
- Catalase NEGATIVE
THREE WAYS TO CLASSIFY:
1) RBC hemolysis
Gamma: Garbage (no hemolytic activity).
Alpha: Almost (almost lyse, but incomplete).
Beta: Best (complete lysis).
2) beta hemolytic divided into groups based on carbohydrate in cell wall (serologic properties)
3) non-hemolytic (gamma) cannot be grouped but identified by physiological properties
Strep physiology:
- facultative anaerobe (used ATP or fermentation to make energy)
- complex growth requirements (need media)
- ONLY in human host
- PYR can be used to identify
Sensitivity to bacitracin:
- distinguishes group A from B
- GAS = group A strep
GROUP A is sensitive, B is resistant
Group A:
- pyogenes
- many strain
- varying virulence factors
Characteristics that promote Group A virulence:
- ability to adhere to epithelial cell surface
- ability to avoid opsonization
- ability to cause tissue damage and inhibit clearance
Group A binding to cell surface influenced by:
- lipteichoic acid
- M protein
- F protein
Group A ability to invade host cell:
-m and f protein
Group A mechanisms to avoid opsonization:
- M protein binds factor H, this downregulated C3b preventing phagocytosis
- prevented if patient has antibodies to strains M protein
- C5a may also be inactivated blocking neutrophil chemotaxis
Common virulence factors of GAS:
- exotoxins, big contributor
- m protein
- capsule
- hyaluronidase
- C5a peptidase
- streptokinase
- streptodornase (DNase)
- lipteichoic acid/F protein
- WHEN YOU THINK OF GAS think of cytokine release
GAS M protein:
- immunodominant protein
- 2 polypeptide chains that anchor in the bacterial cell membrane and extend through the cell wall
- external portion is high variable, thats why you can develop an immunity to GAS but then get an infection
GAS M protein antiphagocytic activity:
- prevents strep from being phagocytosed and destroyed by neutrophils
- if strain doesnt have M protein it is phagocytksed
Strep epidemiology;
- upper respiratory tract or skin
- oropharynx colonization (pharyngitic in children 5-15)
- throat strains may also cause skin infection
- spread person to person or through breaks in skin
RISK FACTORS: winter = pharyngeal summer = cutaneous, crowds
Localized infection GAS:
- pharyngitis/tonsilitis
- cutaenous
Cutaneous/soft tissue GAS:
- pyoderma: young children, yellow crust
- erysipelas: young children, older adults - similar to cellulitis
- cellulitis: deeper subcutaneous tissue
- necrotizing faciitis: cant just treat with antibiotics
- scarlet fever: chest rash due to eryhtrogenic toxin
dussenubated (invasive) GAS
- pneumonia
- bacteremia
- septicemia (sepsis)
post infectious complications:
- rheumatic fever
- acute glomerulonephritis
Acute Nonsuppurative (non-pus forming)
- rheumatic fever: inflammatory disease with carditis, polyarthritis, chorea, erythema, subcutaneous nodules
- acute glomrulonephritis
Rheumatic fever:
- autoimmune disease (induction of immune response to M protein)
- heart valve damage
- major factor in damaged heart valves
- occurs 20 days after strep throat or scarlet fever
- mitral stenosis, aortic stenosis
acute glomrulonephritis
- acute inflammation of renal glomeruli
- autoimmune disease
- latent period
GAS toxic shock syndrome:
- multisystem toxicity
- nonspecific stimulation of t cells by superantigen rather than effects of a toxin
- many t cells are activated to express cytokines
Diagnosis of GAS:
- microscopy
- direct antigen test (rapid strep): agglutination of antigen in the presence of a specific antibody
- culture
- ASO test: detection of antibodies against conserved toxin O confirms recent GAS infection (can also used DNase B antibodies)
GAS treatment:
- Pharyngitis: penicillin, GAS not resistant but neighboring oral flora may secrete beta lactamases in which case erythromycin or cephalosporin used
- Rheumatic fever: salicylates/corticosteroids
- Glomerulonephritis: sodium restriction, diuretics, anticonvulsants
*****NO VACCINE
Group B strep differences:
- similar to strep BUT niche is lower GI and urinary tract
- also has animal reservoir
- distinguish from GAS due to inherent resistance to bacitracin
- leading cause of neonatal infection due to maternal fetal transmission
- neonatal infection decreased by screening an prophylactic treatment late in preganancy
GBS:
-capsule contains sialic acid which makes it antiphagocyic and evades clearance
