Streptococci and Enterococci Flashcards
Streptococcus pyogenes
Group A strep, Beta hemolytic
Streptococcus agalactiae
Group B strep, Beta hemolytic
Appearance of streptococci
Gram positive in chains
Beta hemolysis
Blood agar plate turns clear where the strep is spread
Alpha hemolysis
Green
Gamma hemolysis
Clear
Lancefield typing
Uses an antibody test to look for clumping. Positive test shows clumping, negative test does not.
Group A beta-hemolytic streptococci and major virulence factor
S. pyogenes, sensitive to bacitracin (which is what determines group A). M protein
Pathogenesis of Strep A infection
Suppurative complication: sore throat, primarily neutrophil responses
Non-suppurative complications: manifestations of disease remote from primary site of colonization/infection.
Four suppurative (pyogenic) complications of S. pyogenes (group A beta-hemolytic)
Pharyngitis, impetigo, cellulitis, necrotizing fasciitis
Four non-suppurative complications of S. pyogenes
Acute rheumatic fever, poststreptococcal glomerulonephritis (THESE ARE BOTH Immunogenic), scarlet fever, streptococcal toxic shock syndrome.
Acute Rheumatic Fever
Occurs from ages 6-20, common in impoverished countries. Clear association with preceding throat S. pyogenes infection (not any other area). Higher incidence if strain has high levels of M-protein.
Why does the heart get messed up in ARF?
Molecular mimickry, cross reactivity of GAS antibodies with that of the endocardium.
Clinical manifestations of ARF
Migratory arthritis, carditis, sydenham’s chorea, erythema marginatum.
Why is childhood strep treated so aggressively?
To prevent progression into rheumatic fever.
Post-streptococcal glomerulonephritis
Antibodies and complement components react with streptococcal antigens to form immune complexes, which deposit in glomerulus.
Clinical manifestations of PSG
Affects children, hypertension, edema, bloody urine, may be associated with development of hypertension later in life.
Toxic shock syndrome
Caused by so-called pyrogenic exotoxins (SPEA, SPEB, SPEC), act as superantigens that nonspecifically bind MHC and T cells causing cytokine storm. Shock and multi-system organ failure ensue.
Streptococcus Agalactiae
Group B beta hemolytic strep (not sensitive to bacitracin). Associated with neonatal and puerpural sepsis, occasionally UTI and endocarditis too.
Enterococcus Fecalis, Enterococcus faecium
Bowel organisms, cause infections below the diaphragm, relatively antibiotic-resistant particularly to cephalosporins and other beta lactams. Urinary tract infections, peritonitis.
What differentiates enterococci from other streps?
Relatively antibiotic-resistant particularly to cephalosporins and other beta lactams.
Strep Viridans
Alpha-hemolytic streps (green=viridans), include S. mutans (dental caries), S. sanguis (endocarditis on previously damaged valves). Not sensitive to optochin like pneumococci are.
How to distinguish pneumococci from strep viridans?
Optochin test. Strep viridans is not sensitive to optochin, but pneumococci are.
Do strep viridans invade?
No, but can pass into bloodstream when gums are damaged. Can cause endocarditis.
