streptococci - medsyllabus.org · 2 key words • lancefield groups • hemolysis (alpha, beta,...

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STREPTOCOCCI

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Key Words• Lancefield groups• Hemolysis (alpha, beta, gamma)• Group A streptococcus (S. pyogenes)• Bacitracin susceptibility test • M, T, R proteins• Streptolysins O and S• F protein/lipoteichoic acid• Rheumatic fever/carditis/arthritis• Glomerulonephritis• Scarlet fever• Toxic shock-like

syndrome/bacteremia• “Flesh-eating bacteria”• Pyrogenic toxin• Erythrogenic toxin

• Group B streptococcus (S.agalactiae)• Neonatal septicemia/meningitis • CAMP test• Hippurate hydrolysis test• Group D streptococcus• Urinary tract infection/ endocarditis • Bile-esculin test• Enterococci • Non-enterococci• Large colony• Minute colony • Viridans streptococci• Dental caries/endocarditis

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• Streptococci – facultative anaerobe– Gram-positive– usually chains (sometimes pairs) – catalase negative

(staphylococci are catalase positive)

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Streptococcus in chains (Gram stain)

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Streptococcus pneumoniae (diplococcus). Fluorescent stain

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Identification : Lancefield groups- carbohydrate antigens

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groupable streptococci • A, B and D

– frequent• C, G, F

– less frequent

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Non-groupable

• S. pneumoniae–pneumonia

• viridans streptococci–e.g. S. mutans

*dental caries

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hemolysis reaction - sheep blood agar

• α (alpha)– partial hemolysis – green color

• β (beta) – complete clearing

• γ (gamma) - no lysis

White colonies

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Hemolysis

• Groups A an B –β

• Group D – α or γ

• S. pneumoniae and viridans –α

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Identification: hemolysis reaction + one biochemical characteristic

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Group A streptococcus (S. pyogenes)

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Group A streptococcal infections affect all agespeak incidence at 5-15 years of age

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S. pyogenes -suppurative

• non-invasive – pharyngitis – skin infection, impetigo

• invasive bacteremia – toxic shock-like syndrome – "flesh eating" bacteria– pyrogenic toxin

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Pyrogenic toxin• superantigen• T cell mitogen • activates immune system

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Scarlet fever

• rash • erythrogenic toxin

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non-suppurative• rheumatic fever

– inflammatory disease– life threatening– chronic sequalae

• fever • heart • joints

• rheumatic NOT rheumatoid arthritis

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Rheumatic fever -etiology

• M protein – cross-reacts heart myosin – autoimmunity

• Cell wall antigens – poorly digested in vivo– persist indefinitely

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Rheumatic fever

• penicillin• terminates pharyngitis• decreases carditis

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Acute glomerulonephritis

• immune complex disease of kidney

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Major pathogenesis factors

• lipoteichoic acid/F protein– fimbriae– binds to epithelial cells

• M protein – anti-phagocytic

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S. pyogenes

fibronectin

lipoteichoic acidF-protein

epithelial cells

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M protein

M protein

fibrinogen

rrr

peptidoglycan

rrr

IgG

Complement IMMUNE

NON-IMMUNE

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M protein

• major target – natural immunity

• strain variation– antigenicity

• re-infection– occurs with different strain

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Capsules

• Anti-phagocytic– mucoid strains

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Isolation and identification

• β hemolytic colonies – bacitracin inhibits growth

• β hemolytic colonies– group A antigen

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β hemolysis

• hemolysin O – sensitive oxygen

• hemolysin S– insensitive oxygen

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Modern Rapid “Strep” TestThroat swab extract (+/- streptococcal antigen)

Antibody

Liposome

+

-

Streptococcal antigen

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Post-infectious diagnosis (serology)

• antibodies to streptolysin O • important if delayed clinical

sequelae occur

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Traditional serotyping of proteins:- M - T - R

Typing

Current:- Sequencing of M protein gene

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Group B streptococcus

• neonatal meningitis• septicemia• transmission

– vaginal flora

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Group B streptococcus - identification

• β hemolysis• hippurate hydrolysis• CAMP reaction

– increases β hemolysis of S. aureus

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Group D streptococcus• Growth on bile esculin agar

– black precipitate

• 6.5% saline• grow

– enterococci• no growth

– non-enterococci

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Enterococci

• distantly related to other streptococci• genus Enterococcus • gut flora

– urinary tract infection • fecal contamination

– opportunistic infections• particularly endocarditis

• most common E. (S.) faecalis

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Enterococci

• resistant to many antibiotics – including vancomycin

• terminal D-ala replaced by D-lactate

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Minute colony streptococciVarious groups/hemolysis (e.g. group A)

– genetically distinct from large colony (e.g. S. pyogenes)– no rheumatic fever

Large colony Minute colony

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Viridans streptococci• diverse species • oral • dental caries• α hemolytic and negative for other tests • non-groupable.• includes S. mutans

– occassional endocarditis after tooth extraction