soft tissue and skin infections

Question 1

describe cellulitis

Answer 1

• infection of dermis and subcutaneous tissue

- often seen around injury site or deep abscesses

Question 2

describe impetigo

Answer 2

• infection of epidermis by group A step or S. aureus
• characterised by vesicles, crusted erosions
• risk factors include crowded places, lower hygiene, high temps
• highly contagious

Question 3

how does infection cause inflammation

Answer 3

1. bacteria and other pathogens enter wound
2. bacteria produces toxins, damaging tissue
3. this activates mast cells which secrete factors that mediate vasodilation and vascular constriction
4. delivery of blood, plasma and cells to site of injury increases
5. PAMPs & complement activate macrophages which produce pro-inflammatory cytokines
6. this attracts neutrophils to the site which destroys bacteria via phagocytosis
7. inflammatory responses continue until foreign material eliminated and wound repaired

Question 4

describe PAMPs

Answer 4

pattern-associated molecular patterns

• macrophages recognise these with receptors
• activates macrophages which then secrete pro-inflammatory cytokines (e.g. IL-1, TNF-a)

Question 5

describe leukocyte extravasation (diapedesis)

Answer 5

• pro-inflammatory cytokines up-regulate molecules on endothelial membranes
• facilitating a stronger interaction with neutrophils and therefore movement into tissues

Question 6

organisms causing skin and soft tissue infections

Answer 6

• streptococcus pyogenes
• staphylococcus aureus
• other bacteria, fungi, viruses

Question 7

infectivity of S. pyogenes

Answer 7

• exclusively human pathogen
• asymptomatic colonisation of oropharynx
• transient colonisation of skin
• transmission by human contact, promoted by overcrowding, kindergarten etc

Question 8

colonisation of skin by S pyogenes

Answer 8

MSCRAMMs
microbial surface recognising adhesive matrix molecules - on bacterial surface

• they interact with ECM proteins e.g. collagen, elastin

Question 9

evasion of immune system by S. pyogenes

Answer 9

• hyaluronic acid capsule = prevents opsonisation and phagocytosis
• M protein = binds factor H preventing C3b opsonisation
• toxin secretion
  
  • C5a peptidase = prevents chemotaxis
  • streptolysins = lyses immune cell

Question 10

spreading factors of S pyogenes

Answer 10

• proteases
• lipases
• hyaluronidase
• streptokinase

Question 11

diagnosis of skin and soft tissue infections

Answer 11

• swab for purulent material and maybe blood culture

- cultivate and identify causative organsism

Question 12

how is S pyogenes diagnosed

Answer 12

gram stain = positive
- chain

catalase test = negative

blood agar testing for haemolysis

• a-hemolytic gives green (partial)
• B-hemolytic gives yellow = S. pyogenes
• Y-hemolytic is non haemolytic

Question 13

treatment of skin and soft tissue infection

Answer 13

• supportive care
• rest, elevation
• analgesia
• antibiotics
  
  • S pyogenes = penicillin e.g. amoxycillin
  • S aureus = B-lactamase resistance penicillin
    e. g. flucloxacillin

Question 14

mechanism of penicillin

Answer 14

• binds to transpeptidase enzyme
• prevents peptide cross-links in bacterial cell wall
• weak cell wall and lysis

Question 15

what guides migration of leukocytes to a site of infection

Answer 15

chemokine IL-8 (macrophages)

C5a (activated complement)