Lecture 14: Skin and soft tissue infection

Question 1

What is impertigo?

Risk factors?

Answer 1

• Infection of upper layer of the skin
• Vesicles form
• Risk factors:
  
  • Crowding
  • Poor hygiene
  • High temperatures

Question 2

How you differentiate between erysepelas and cellulitis?

Answer 2

-Clear demarcation line in erysipelas

Question 3

Cellulitis is an infection in which layer of skin?

Answer 3

Lower dermis and subcutaneous tissue

*note that often seen around injury site or deep abcess

Question 4

Erysepelas is an infection of what layers of the skin?

Answer 4

-Upper dermis and also superficial lymphatics

Question 5

What is the predominant causative organism for erysipelas?

Answer 5

Group A beta haemolytic streptococci e.g. S Pyrogenes

Question 6

How do macrophages recognise bacteria?

Answer 6

• PAMPs on bacterial surface

- Macrophages have pattern recognition receptors

Question 7

General innate immune response to bacteria?

Answer 7

• Platelets form blood clots
• Mast cells cause vessel vasodilation (heparin, histamine)
• Macrophages secrete cytokines
• More vasodilation (leakage, sweeling)
• Neutrophils pass through leaky vessels (diapedesis) and phagocytose bacteria

Question 8

Which two substances guide leucocyte migration?

• Macrophages
• Complement

Answer 8

• IL8 (macrophages)

- C5a (activated complement)

Question 9

What are the 3 complement pathways involved in innate immunity?

Answer 9

• Classical
• Lectin
• Alternative

Question 10

At which step do the 3 complement pathways merge?

Answer 10

Conversion of C3 to C3a and C3b

Question 11

What organisms causes SSTI?

Answer 11

• Strep pyogenes
• Staph. aureus
• some other bacteria e.g Vibrio vulnificus
• Fungi e.g dermatophytic moulds, tinea
• Viruses e.g chicken pox

Question 12

General features of streptococcus pyogenes

Answer 12

-GRAM POSITIVE
-Spherical or ovoid cocci (0.5-1 micron)
-Often an asymptomatic colonisation in 15-20% of people
-Are Catalase negative (in contrast to staphylococci)
-Has the group A antigen
Synonomous with group A streptococcus

Question 13

What are four complications of streptococcus infection?

Answer 13

• Skin/soft tissue infection
• Severe systemic disease (TSS)
• Pharyngitis/tonsillitis
• Acute rheumatic fever

Question 14

Through what mechanism does group A streptococcus colonise the skin?

Answer 14

MSCRAMMS on bacteria cell wall bind to host ECM proteins

Question 15

What are three methods of immune evasion by group A strep?

Answer 15

• Hylauronic acid capsule prevents opsonisation and phagocytosis
• M protein binds with factor H, prevents opsonisation with C3b
• Toxins Streptolysins, C5a peptidase (no chemotaxis), DNAases (degrdae neutrophil extracellular traps) and SpyCEP (destroys IL-8)

Question 16

How can strep pyogenes invade deeper tissues?

result?

Answer 16

Spreading factors: proteases, lipases, hyaluronidase, streptokinase (activates plasmin, fibrinogen to fibrin)

Can cause necrotising fascitis

Question 17

What is streptokinase?

Answer 17

• Anticoagulant

- Activates plasminogen to plasmin which degrades fibrin

Question 18

Diagnostic approach

Answer 18

Gram positive, then conduct catalase test
if negative, like streptococcus, pos staph
The haemolysis on blood agar: is beta haemolytic
Bacitracin will kill GAS, but not GBS and GCS

Question 19

What are the 3 classes of beta lactam antibiotics?

Answer 19

• Penicillins
• Cephalosporin
• Carbapenems

Question 20

Penicillin is most effective against gram ___ bacteria

Answer 20

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