37. Streptococcus Flashcards
streptococci are what type of bacteria under microscope?
gram+ cocci in pairs and chains
catalase -
classified by hemolysis and serological then biochemical (or now genetic)
alpha hemolysis
partial hemolysis : green discoloration
vidirans streptococci
beta hemolysis:
complete hemolysis (clear zone around colony)
B-hemolytic streptococci
gamma hemolysis:
no hemolysis
vidirans streptococci
what is the only clincally important GAS?
strep. pyogenes
suppurative vs non-suppurative? (re: GAS)
suppurative: pharyngitis and skin infections (pus)
non-suppurative: rheumatic fever and rheumatic heart disease, acute glomerulonephritis (no pus)
M protein?
GAS virulence factor: strongly atiphagocytic, binds serum proteins (like factor H) that inhibit activation of alternative complement components - evades immune system!
but elicits opsonic Abs
> 100 types conferring specific immunity (opsonic abs directed at distal epitopes so can get sick multiple times)
adhesive: binds to numerous serum proteins and CD46 on keratinocytes
certain types can generate Abs that react w/cardiac myosin and sarcolemma
somatic virulence factors of GAS?
capsule (hyaluronic acid looks like us and is thus antiphagocytic)
surface adhesins:
- lipoteichoic acid (binds host cells)
- M protein (binds keratinocytes)
- Protein F (binds fibronectin and mucosal cells)
toxins from GAS?
hemolysins:
- streptolysin O (Abs to this suggests previous infection)
- streptolysin S
streptococcal Pyrogenic Exotoxins (Spe)
- SpeA, SpeC
- encoded by bacteriophages
- superantigens (cause release of CYTOKINES)
- responsible for characteristic features of SCARLET FEVER and STREPTOCOCCAL TOXIC SHOCK SYNDROME
- HLA dependent response (not everyone reacts in same way)
GAS enzymes?
play a role in rapid spread through tissues
DNAses
Hyaluronidase
Streptokinase (degrades fibrin, used as a Rx)
GAS avoidance of innate immune response?
C5a peptidase (disrupts C’ chemoattractant)
SpeB (cleaves IgG)
GAS epidemiology?
- Humans are only host
- transient colonization of oropharynx/skin normal
- transmitted person-person via oral secretions or contact w/skin lesions
Non-Suppurative: (primarily pediatric in poor countries)
GAS respiratory tract suppurative disease?
pharyngitis
(complication = scarlet fever thanks to SpeA and SpeC)
- kids 5-15, fever, no cough, purulent exudate, cervical lymphadenopathy
pneumonia (rare)
GAS skin suppurative disease?
impetigo (pyoderma)
erysipelas
necrotizing fasciitis (streptococcal gangrene)
streptococcal toxic shock syndrome
pharyngitis diagnosis?
rapid antigen tests:
- convenient
- good specificity
- negative requires culture
culture:
- gold standard
- takes longer (call outpatient pt if cx is positive and give Abx)
pharyngitis treatment?
- in order to prevent non-suppurative complications
- PCN
- macrolides or clindamycin for pcn allergies (clindamycin does GP and anaerobes so good rx for oropharyngeal stuff)
scarlet fever (scarlatina)
uncommon manifestation of acute infection (usu pharyngitis)
manifestation of SpeA or SpeC
rash begins on trunk
capillary gragility (accentuated in skin folds, petechiae elicited w/blood pressure cuff), subsequent desquamation
impetigo (pyoderma)
superficial skin infection
characteristic “honey colored” crusting from serum exudate
dx clinically or by culture
can treat topically (bacitrain, mupirocin)
systemic for more extensive disease (culture, presumptive therapy w/emoxicillin-clavulanic acid or cephalexin)
erysipelas?
form of cellulitis
highly characteristic of B-hemolytic streptococcus
bright erythema
edema
sharp raised edges
regional lymphadenopathy
systemic sxs
cellulitis w/lymphangitis?
s.pyogenes has peculiar relationship w/lymphatics
cellulitis often complicated by lymphangitis
those w/damaged lymphatics predisposed to recurrent strep cellulitis
necrotizing fasciitis
life threatening deep infection (cuts b/w fascial planes)
clinical clues:
- pain out of proportion for clinical findings
- septic shock assoc w/cellullitis
- hemorrhagic bullae
streptococcal toxic shock syndrome
features:
- occurs in context of S.pyogenes infection (skin)
- fever
- hypotension, shock
- multiple organ syndrome dysfunction
- rash (generalized, macular, may desquamate)
pathogenesis:
- pyogenic exotoxins, esp SpeC (superantigen)
- assoc w/certain HLA alleles
- massive release of pro-inflammatory cytokines
pathogenesis of streptococcus pyogenes nonsuppurative infections?
rheumatic fever & heart disease:
- molecular mimicry (specific M proteins have epitopes shared w/cardiac antigens)
- infiltration of T lymphocytes into heart tissue, inflammatory cytokines, cardiac valve lesions
- assoc w/HLA-DR alleles
- after pharyngitis only
Acute glomerulonephritis:
- deposition of immune complexes in kidney
- activation of enzymes that damage glomerular basement membrane, with protein loss and decreased renal function
clinical syndrome of acute rheumatic fever?
major criteria (2 or 1 plus 2 minor) - polyarthritis, carditis, chorea, erythema marginatum, subQ nodules
minor criteria:
-arthralgia, fever, elevated CRP or ESR, 1st degree heart block
evidence of recent infection: culture, antigen or serology (anti-streptolysin O)
recurrent attacks lead to rheumatic heart disease (irreversible damage to heart valves)
therapy for acute rheumatic fever?
symptomatic w/asa or corticosteroids
primary preventio: tx of pharyngitis
secondary prevention: benzathine PCN G or oral pcn daily
post-streptococcal glomerulonephritis
incubation period 1-2 weeks after pharyngitis, 2-3 weeks after skin infection
result of antigen-Ab complex deposition
manifestations (edema, hypertension, proteinuria, microscopic hematuria, acute renal failure)
differnt strains than those assoc w/acute rheumatic fever
recovery usu complete
Group B strep
S.agalactiae
colonizes GI and female GU tract
significant disease in neonates
- early onset at birth or in utero: sepsis, pna, meningitis
- late onset in 1st month of life: sepsis, meningitis
can cause cellulitis, UTI, sepsis in adults, esp those with underlying disease (Diabetes)
Dx: culture and nucleic acid amplific tests are SENSITIVE and SPECIFIC
tx: pcn (or vancomycin or clindamycin for pcn allergy)
screen all pregnant women and treat colonized women at term
Group C, F, and G
share virulence factors with group A
cause pharyngitis, cellulities, bacteremia without identifiable focus
dx, tx same as other strep
S. anginosis
Group F strep
B-hemolysis, a-hemolysis, or g-hemolysis
abscesses in liver, brain, periodontal
