Strep etc Flashcards
Strep pneumoniae pathogenesis
Oropharynx - small population, no capsular Ig produced
Aspiration - rare, must have compromise (cilia, alveolar macrophage)
Capsule - antiphagocytic, anti-capsule Ig is key for immunity
IgA protease
Pneumolysin - lytic enzyme
Strep pneumoniae bacteriology
Gram + diplococci (short end apposed/"lancet" vs Neisseria) Alpha hemolytic Optochin sensitive (vs Strep viridans) Bile soluble (vs Strep viridans) Large capsule - multiple serotypes, ID via Quellung reaction (visualize capsule)
Strep pneumoniae clinical
Most common cause of bacterial pneumonia -> rusty sputum
-> 10-20% bacteremia, 5-20% fatality of hospitalized
- requires humoral immunity -> anti-capsule Ig
- requires spleen (higher risk if sickle cell)
Rare meningitis (children, elderly), otitis media (children), sinus
Dx: sputum Gram and culture, blood, CSF
- do not usually determine specific type
Tx: frequent multi antibiotic resistance (inc penicillin)
- must get sensitivity
Strep pneumoniae epidemiology
Colonization - 10% adult, higher children, no animal
Pneumovax = 23 serotype capsular polysaccharide
-> T-cell indep (poor booster response)
- recommended >65 or >2 and at risk (sickle cell, spleen, immune)
Prevnar = 13 serotype capsule + diphtheria conjugate
-> T-cell dep (more effective infants, etc)
- recommended for peds, prevents meningitis
Overview of Streptococcus
Gram + cocci in chains
Facultative anaerobes
Catalase (-)
Different carb antigens (Lancefield) -> groups A, B, C, D, non-type
- precipitation tests with anti-serum (rabbit)
- can also distinguish with hemolysis (ex A-O -> beta except D)
Important
- beta hemolytic -> A (pyogenes, oral), B (vagina), D (enterococcus, colon)
- alpha hemolytic -> pneumococcus, viridans (both oral)
Not important - Groups C, D (beta, bacitracin resistant)
Types of hemolysis
Gamma = no hemolysis
Alpha -> green zone (RBCs intact with altered heme pigment)
Beta -> clear zone (RBCs lysed)
- may be cloudy if doesn’t extend through full thickness
Strep pyogenes bacteriology
aka Group A beta-hemolytic Strep
Gram + streptococci, aerotolerant
Subdivide by M protein - extends through hyaluronic capsule from cell wall
-> 90 specific serotypes
Also have pilus, F protein, etc
5-10% carry in nasopharynx - nasal more infectious than oral, skin
Spread respiratory droplet - large inoculum
Strep pyogenes pathogenesis
Extracellular proteins -> invasive disease
- streptolysins -> beta hemolysis
- O = O2 labile, antigenic -> ASO titer (anti-streptolysin O)
- S = O2 stable, not antigenic
- streptokinase - triggers proteolysis of fibrin clots
- DNAse
- Spe A, B, C -> rash of scarlet fever, toxic shock
(both syndrome prevented by specific antibody)
Antiphagocytic
- hyaluronic capsule
- M protein - sticks through capsule -> adherence
- virulence proportional to M protein
- anti-M Ig -> immunity
Strep pyogenes clinical syndromes
Toxin-mediated - pyogenic exotoxins (Spe A, B, C)
- superantigens -> overproduction of cytokines
- Scarlet fever = pharyg-tonsil -> rash
- toxic shock = hypotension + renal, liver, resp, DIC, soft tissue, rash
Invasive disease
- puerperal fever - childbirth -> uterus (mostly developing)
- acute pharyng-tonsil (aka strep throat)
- impetigo - superficial skin lesion -> minor infection
- erysipelas - severe cellulitis of dermis
- necrotizing fasciitis - highly invasive strains -> surgery, life threat
- other: sinus, mastoid, otitis media, meningitis, pneumonia (2t flu)
Strep pyogenes sequelae
Treat all Strep pyogenes (within 8 days) to prevent sequelae!
Do not involve Strep bacteria at site -> immunological
Can test ASO titer, ESR (inflammation)
Rheumatic fever: throat -> 2-3 wks
- > rash (erythema marginatum), nodules, heart lesions, joint pain
- cross-reactive antibodies - some strains/M proteins more likely
- short term except for heart (esp mitral, aortic valves)
- prior rheumatic -> penicillin prophylaxis
- still common in developing world
Glomerulonephritis: skin -> 1 week -> edema, hematuria
- immune complex deposition (with Strep antigen)
- only few M-protein serotypes (usu skin, sometimes throat)
Strep pyogenes clinical
Dx:
- do not routinely subtype
- Gram stain -> Gram + streptococci
- beta-hemolysis
- bacitracin sensitive -> Group A (other betas are resistant)
- rapid - beads with anti-A antibody -> agglutination
- “reasonably specific, not as sensitive as culture”
- ASO titer (vs streptolysin O) - >160 or 4x increase
TREAT ALL WITH PENICILLIN to prevent sequelae
(no resistance!)
- erythromycin second line
- ampicillin prophylaxis for prior rheumatic
Group B strep overview
Beta hemolytic (smaller zone than group A)
Bacitracin resistant
Hippurate hydrolysis
cAMP test - more beta hemolysis next to Staph
Most imp species - Strep agalactiae
Can be part of normal colonic/vaginal flora -> neonatal, etc
Sensitive to penicillin
Group B strep clinical
Up to 1/3 neonatal infections -> pneu, meningitis, septicemia, death
Early onset (within 1 week) - from vaginal flora
- prevent with vaginal + colonic cultures -> intrapartum penicillin, ampicillin
- highest risk if no maternal Ig - screen 36 weeks for new infection
- higher risk if premature, long rupture of membranes
Later onset - infant-infant, nosocomial
- less fulminant, lower mortality
Also endometriosis (postpartum), infections in older (>65), etc
Enterococci
aka Group D strep
- Enterococcus faecalis, faecium
Variable hemolysis
Can grow in high salt, bile (hydrolyzes esculin)
Common in GI -> UTI, abdominal, wound, sepsis (esp elderly)
Lots of antibiotic resistance -> vanco + aminoglycoside (some resistance)
- prophylaxis for heart patients (GI,GU procedures) - amp + gent
Strep viridans
S mutans, S sanguis, S mitis
Alpha or gamma hemolysis
Not lysed by bile, resists optochin (vs Strep pneumo)
Normal oral/pharyngeal -> minor oral trauma -> transient bacteremia -> adhere to pre-existing endocardial lesion (valve)
- > infective endocarditis -> subacute with intermittent bacteremia -> death
- detect vegetation on echo
Tx: penicillin (+ aminoglycoside if endocarditis)
