Lecture 16: Skin and soft tissue infection

Question 1

What are the signs and symptoms of Skin and Soft Tissue Infections (SSTIs)?

Answer 1

• Erythema (redness)
• Warmth
• Tenderness
• Pain (dep. on type as to degree)
• Fever (deeper -> systemic inf.)
• Purulence (pussy)
• Fever
• Increased HR, WBC

Question 2

Relate depth of infection to skin layers:

Answer 2

Inf of epidermis = Erysipelas impetigo

Inf of dermis = Cellulitis

Inf of epidermis -> Fascia = Necrotizing soft tissue infection

Infection of fascia = Necrotizing fascitis

Question 3

What organisms cause SSTI?

Answer 3

Mainly:
Streptococcus pyogenes
Staphylococcus aureus

Some other bacteria i.e vibrio vulnificus, clostridium perfringens

Fungi: Tinea

Viruses i.e chicken pox

Question 4

What are some examples of non-purulent SSTI?

Answer 4

Cellulitis
Erysipelas
Necrotising fasciitis

Question 5

What are purulent SSTI?

Answer 5

Carbuncle
Impetigo

Question 6

What are some notes on cellulitis presentation:

Answer 6

Cellulitis
- Edema, pain
- Poorly demarcated erythema (no clear boundary b/w healthy and unhealthy)
- Often around wound/abscess i.e cellulitis around abscess

Question 7

Write some notes on erysipelas:

Answer 7

Swelling/redness

Well demarcated erythema

Fever

Question 8

Write some notes on necrotising fasciitis

Answer 8

Evolves rapidly, excruciating pain, fever, systemic erythematous rash

Question 9

Write notes on impetigo:

Answer 9

Could be staph or strep pyogenes or polymicrobial

Painless, erythematous base with honey crusted exudate

Question 10

Whats a carbuncle?

Answer 10

Big abscess following folliculitis

Staph aureus only

Question 11

Whats the function of innate immune response?

Answer 11

• Non-specific, generic response to pathogens
• Immediate response

Question 12

What recognize PAMPS?

Answer 12

Pattern recognition receptors (on macrophages, secrete pro-inflam cytokines upon binding)

PAMPS - highly conserved structures

Question 13

How does infection relate to inflammation?

Answer 13

• Infection leads to tissue damage and activation of mast cells
• Release of heparin and histamine from mast cells
• Innate immune mechanisms (PAMPS, complement) lead to activation of resident macrophages
• Vasodilation, increased permeability of blood vessels. (vascular leakage, swelling)
• Leukocyte extravasation
• Migration via chemokine (IL8) and C5a

Question 14

Write some notes on streptococcus:

Answer 14

• Gram positive spherical or oval cocci
• Catalase negative (compared to staphylococci)

Causes:
- SSTI
- Severe systemic diseases (streptococcal toxic shock syndrome)
- Pharyngitis/tonsilitis (SPECIFIC for STREPT)
- Acute rheumatic fever

Question 15

What is the lancefield classification?

Answer 15

Uses sero typing i.e specific antibody against group A antigen isolate S pyogenes

i.e Further classification

Question 16

Is GAS commensual to humans?

Answer 16

Group A streptococcus

• Exclusively found in humans
• Asymptomatic in many
• Transient colonization of skin
• Transmission by human contact
• High infection rates in overcrowding

Question 17

How does S pyogenes infect skin?

Answer 17

MSCRAMMS (microbial surface components recognizing adhesive matrix molecules)

• Cell wall attatched adhesions
• Specific binding to host ECM proteins
  i.e M protein

MSCRAMMS are found in ALL pathogenic GRAM POSITIVE bacteria

Question 18

How can S pyogenes evade the immune system?

Answer 18

• Hyaluronic acid capsule (prevents opsonization and phagocytosis)
• M protein = Binds factor H, which prevents opsonisation with C3b.
• Secretes toxins i.e
  -> Streptolysins (lyse immune cells)
  -> C5a peptidase (preventing neutrophil chemotaxis)
  -> SNases (Degrade neutrophil EC traps (nets))
  -> SpyCEP (Destroys IL8 to prevent neutrophil chemotaxis)

Question 19

What virulence factors allow S pyogenes to invade deeper tissues?

Answer 19

Spreading factors:
- Proteases
- Lipases
- Hyaluronidase
- Streptokinase (anticoagulant that activates plasminogen to plasmin, degrading fibrin)

Question 20

How can S pyogenes be diagnosed?

Answer 20

• Swab of purulent material
• Identified in microbiology lab
• If in hospital then blood culture should be taken. (bacteremia could lead to sepsis or streptococcal toxic shock syndrome)

Question 21

How can S pyogenes be identified diagnostically?

Answer 21

Gram +ive cocci
-> Catalase test
-> Grow on blood agar (hemolysis)
-> Bacitracin susceptible

= S pyogenes

Question 22

How can S-pyogenes be treated?

Answer 22

• Supportive care
• Rest and elevation
• Analgesia
• Antimicrobial drugs (penicillin)

Question 23

What do you give for staph aureus?

Answer 23

• Beta lactamase resistance penicillin i.e flucoxacillin (except for MRSA strains)

Question 24

How does penicillin work?

Answer 24

Penicillin binds to the transpeptidase enzyme (Aka penicillin binding protein)

Prevents formation of peptide cross links in bacterial cell wall

Results in weak cell wall and cell lysis

Question 25

What are the three main classes of beta lactam antibiotics?

Answer 25

- Penicillins -> Pen g, Pen V and amoxycillin - Cephalosporins - Carbapanems

Question 26

How effective is penicillin?

Answer 26

All s. pyogenes are susceptible to penicillin but only 10% of staph aureus are