S. aureus - Pathology

1. Nomenclature
a. coagulase positive: primarily Staphylococcus
• Coagulase test
b. coagulase negative: many species of
c. microdase positive: Micrococcus species
• modified oxidase test
2. Epidemiology
a. Normal flora of skin and mucosa
b. Carrier state high in normal population
• easily transmitted
3. Pathogenesis
a. localized skin infections
• follicles, boils, furuncles, carbuncles
3. Pathogenesis
b. toxin mediated diseases
Enterotoxin: food poisoning
Exfoliatin: scalded skin syndrome
TSST-1: toxic shock syndrome
Pantone-Valentine Leukocidin?
3. Pathogenesis
c. systemic infections
• all species: bacteremia
• S. saprophyticus: UTI
• Central Nervous System (CNS): indwelling devices
(catheters, valves, hardware, shunts)
d. antimicrobial resistance
• Methicillin Resistant Staphylococcus aureus (MRSA)
– nosocomial v. community (PVL as marker?)
– screening programs to detect colonization in hospitalized
• Vancomycin Intermediate Staphylococcus aureus (VISA)
– thicker cell wall, grow poorly, vanco less active
• VRSA – vancomycin resistant
– MRSA acquire plasmids containing van genes from Vanco
resistant enterococci (VRE)
1. Nomenclature
a. Hemolytic criteria: alpha, beta, gamma
b. Lancefield group carbohydrates for betahemolytic streptococci: A, B, C, F, G
c. some beta-hemolytic streptococci
produce identical carbohydrate antigens in
unrelated species, and some genetically
related species have heterogeneous
carbohydrate antigens
2. Epidemiology
a. normal flora of skin, mucosa (upper
respiratory tract, GU tract, GI tract)
b. disease can be due to person-person
transmission or from endogenous strains
3. Pathogenesis
a. S. pyogenes
i. Group A, beta-hemolytic
ii. acute pharyngitis, skin infections, bacteremia
with potential for severe systemic effects
– toxic shock syndrome, necrotizing faciitis (“flesh-eating”
iii. many toxins, enzymes, M protein which are all
virulence factors
iv. post infection sequelae: Rheumatic fever, acute
glomerulonephritis (AGN)
• reason for treating strep throat is to prevent these
b. S. agalactiae
i. Group B, beta-hemolytic
ii. neonatal infections following PROM
early onset (day 0 – 5)
late onset (day 7 – 90)
iii. screening of pregnant women at week 3537
c. S. pneumoniae
i. alpha hemolytic
ii. pneumonia, meningitis, and otitis media
iii. capsule has antiphagocytic activity
23 valent polysaccharide vaccine for adults
7 valent conjugated vaccine for kids
c. Enterococcus
i. alpha, beta, or gamma hemolytic
ii. most infections are nosocomial: UTI, bacteremia,
iii. multi-drug resistance contributes to pathogenesis;
penicillins, aminoglycosides, glycopeptides
E. faecalis, vanA, vanB (plasmid, inducible)
E. faecium, vanA, vanB (plasmid)
E. gallinarum, vanC (chromosome)
E. casseliflavus, vanC (chromosome)
d. Viridans streptococci and Abiotrophia sp.
i. normal skin flora; non-pathogens
ii. endocarditis in compromised hosts
iii. alpha hemolytic
iv. Abiotrophia show satelliting
C. Identification algorithms
1. Gram stain: pairs, chains, clusters
2. Direct antigen detection
a. GAS from throat
b. Pneumococcus from CSF or blood
3. Selective media
a. SSA for GAS from throat
b. GBS broth + Grenada agar
4. catalase
a. positive: coagulase
i. positive: CPS (S. aureus)
ii. negative: CNS
Yellow pigment: Micrococcus
high numbers in urine, screen for Staphylococcus
saprophyticus using novobiocin (R = S. saprophyticus)
b. negative: hemolysis
i. beta: typing: A, B, C, D, F
streptolysin O, S
CAMP test for GBS
ii. alpha: optochin
sensitive: S. pneumoniae
resistant: Alpha-hemolytic streptococci
iii. gamma: PYR
positive: Enterococcus
motile: E. casseliflavus or E. gallinarum
non-motile: biochems for E. faecalis or E. faecium
negative: Gamma-hemolytic streptococci
Staphylococcus sp. on Gram stain
S. aureus, beta-hemolytic
S. aureus, gold colonies
Coagulase-negative Staphylococci
Micrococcus sp., yellow colonies
MRSA chromogenic agar
Streptococcus sp. on Gram stain
Alpha-hemolytic S. pneumoniae
CAMP positive GBS
Optichin-sensitive S. pneumoniae
Satelliting of Abiotrophia
S. agalactiae (GBS) on Grenada agar
D. Susceptibility testing
1. Staphylococcus sp.
a. resistance in multiple classes
b. usually test all S. aureus; test Coag Neg Staph if
c. VITEK, disk diffusion or broth dilution
d. inducible clinda resistance
a. not resistant to pen, ceph, vanco
b. resistance to macrolides, so test if considering use
c. disk diffusion or broth dilution
d. inducible clinda resistance in GBS
D. Susceptibility testing
3. S. pneumoniae
a. resistance to pen, ceph, macrolides
b. VITEK, disk diffusion or broth dilution
c. different interpretive criteria for CNS v. non-CNS
4. Enterococcus
a. usually use cell wall active agent plus aminoglycoside
to achieve sufficient bactericidal activity
b. resistance to all clinically useful agents has been
c. test using broth dilution, including high level resistance
to aminoglycosides (VITEK)
5. other strep (not routine)

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