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Micro - Staphylococci (i) S Aureus (produces extracellular substances that…
Micro - Staphylococci (i) S Aureus
Intro
causes common but serious infections
2 portals of entry: infection only occurs if theres a break in the skin or mucous membranes (surgery) - penetration, or via enterotoxin food poisoning - ingestion (esp in dairy)
colonises NP mucosa + moist skin areas
no clinical significance
not indicative of hygiene
in 20-40% of healthy individuals
higher in DM, IV drug users, foreign bodies
high virulence
lots of virulence factors
can survive in phagocytes
coag +ve (differentiates it from other staph)
Cell attachment
surface proteins
attach to ECM (laminin/fibronectin), foreign bodies (e.g. catheter, slime formation - biofilm)
capsule
inhibits chemotaxis + phagocytosis
facilitates adherence to foreign bodies
fibrin/fibronogen
clumping factor
attaches to blood clots + traumatise tissue
matrix BPs
fibronectin
fibrinogen
collagen
= adhesin found in strains causing osteomyelitis + septic arthritis
produces extracellular substances that promote invasion (invasions + enzymes)
Coagulase
binds to prothrombin, activates thrombin, converts fibrinogen to fibrin, clotting (protection against phagocytosis)
Protein A
cell wall surface protein
binds to IgG (via FC region) + prevents phagocytosis
coded by spa gene
varies in strains, used for genotyping
alpha toxin (alpha-haemolysin)
binds to platelets + monocytes
pore formation - osmotic lysis
leukocidin
acts on neutrophils
expressed by 2%
causes necrotising fasciitis
staphylokinase
activates plasminogen
fibrin (clot) lysis - aids bacterial spread
hyaluronidase
degrades hyaluronic acid in connective tissue
aids spread
proteases
alpha lipase
deoxyribonuclease (DNase)
FAME (fatty acid modifying enzyme)
catalase
damage to host cells
direct damage
peptidoglycan wall stims ck release
enzymes
exotoxins
superantigens
TSST1
enterotoxins
6 antigenic types (ABCDEF)
exaggerate immune response - massive ck release
exfoliative toxins
e.g. scalded skin syndrome
split intracellular bridges in skin layers
cytotoxins
epidermolytic toxin
spreading
person-person (direct contact, skin carriage-eps hands)
environment (skin sheds onto surfaces + bacterium can withstand drying)
Skin + soft tissue infections
folliculitis
mastitis (esp in breastfeeding)
styes (often resolve on their own)
boils (often resolve on their own)
impetigo
also caused by GAS
no scarring
superficial skin
esp face, arms, legs
cellulitis (subcut tissue)
wound/SSI
carbuncle (cluster of boils/infected hair follicles)
can be invasive...
scalded skin syndrome
spectrum of superficial blistering skin disorders
toxin-mediated
extremely painful
localised blisters + general exfoliation of entire body surface
mainly in kids < 6y/o (eco neonates), immunosuppression, renal failure
contagious
mucous membranes usually spared
Nikolsky's sign
large patches of necrotic epidermis slide off underlying layers @ slightest pressure
prognosis in kids usually good unless sepsis, underlying condition, superinfection or dehydration (complications)
Tx: dehydration, wound care, antibiotics
TSS
necrotising fasciitis
deep muscle involvement
potentially fatal