Lecture 4: Skin and Soft Tissue Infection

Question 1

What are the 2 types of Superficial skin infections

Answer 1

Superficial skin infections include:

• Impetigo
  
  • bacterial infection that involves vesicles and _yellowish crust_s of superficial skin on face, arms, or legs; often in low hygiene and tropic areas; highly infectious;
  • caused by staph. aureus or strep. pyogenes,
• Erysipelas
  
  • acute infection of upper dermis and superficial lymphatics; erythematous skin lesion enlarges rapidly and has a sharply demarcated, raised edge;
  • caused by strep. bacteria.

Question 2

A 55-year old NZ European man
(Bob), who is overweight, attends his
GP’s practice with a painful red rash
across his left foot. His foot is warm,
swollen and he has an antalgic gait.

Diffuse, red, absesses

What is this?

Answer 2

Probably Cellulitis

infection of the dermis and subcutaneous tissue

Often seen around in_jury site or deep abscess_

Question 3

What is Cellulitis?

Answer 3

Cellulitis is infection of the dermis and subcutaneous tissue (more diffused than erysipelas).

Often seen aroun_d injury site_ or d_eep absces_s.

It is caused by staphylococcus or streptococcus bacteria.

Question 4

Describe the Inflammation in response to bacterial infection.

Answer 4

1. Bacteria and other pathogens enter wound (injured site)
2. Platelets from blood release blood-clotting proteins (fibrin) at wound site to contain bacteria
3. Mast cells secrete factors that mediate vasodilation and vascular constriction. D_elivery of blood, plasma and cells_ to injured area increases.
4. Neutrophils secrete factors that kill and degrade pathogens
5. Neutrophils and macrophages remove pathogens by phagocytosis
6. Macrophages secrete pro-inflammatory cytokines and chemokines (IL-8) that attract immune system cells to the site and activate cells involved in tissue repair.
7. Inflammatory response continues until the foreign materials is eliminated, and w_ound is repaired._

Question 5

What are Innate Immune Responses?

Answer 5

Innate immunity is non-specific, generic response to pathogens.

It is an _immediate respons_e, but does not confer long lasting protective immunity.

Question 6

Describe the Innate Immune Response

Answer 6

There are specific patterns on the pathogens (Pathogen associated molecular patterns)

1. Pathogen-associated molecular patterns (PAMPs) are recognised by pattern recognition receptors of macrophages, e.g. toll-like receptors (TLR).
   
   • PAMPs are highly conserved, including PSA, CPS, LPS for Gram negative; PGN, CPS, LTA, CPS for Gram positive.
2. Macrophages attaches to PAMPs.
   
   • Macrophages then initiates signal cascade, which leads to secretion of pro-inflammatory cytokines and chemokines (IL-1b, TNF-a, IL-8) to attract neutrophils.
3. Leukocyte Extravasation (Diapedesis)
   • Selectin mediated adhesion to leukocyte is weak, and allows leukocytes to roll along the vascular endothelial surface (slowly). Then tight binding is initiated, so neutrophils can no longer roll along with blood flow.
   • Cytokines then allows diapedesis (open gaps between endothelial cells) so l_eukocyte to squeeze through these blood vessels._
   • Then, l_eukocytes follow chemotactic gradient_ (IL-8, C5a7) to site of infection.

Question 7

Describe the process of Leukocyte extravasation

Answer 7

Leukocyte Extravasation (Diapedesis)

• Selectin mediated adhesion to leukocyte is weak, and allows leukocytes to roll along the vascular endothelial surface (slowly). Then tight binding is initiated, so neutrophils can no longer roll along with blood flow.
• Cytokines then allows diapedesis (open gaps between endothelial cells) so leukocyte to squeeze through these blood vessels.
• Then, leukocytes follow _chemotactic gradient (_IL-8, C5a7) to site of infection.

Question 8

Briefly describe the complement system

Answer 8

The complement system has three main pathways, including:

1. Classical pathway
2. Lectin pathway
3. Alternative pathway

In all 3 pathways…..

• These pathways all converge to produce C3b.
  
  • This binds to bacteria and targets for destruction either via macrophages (opsonisation of phagocytosis), or via lysis.
• In addition, C5a attracts neutrophils to site of infection.

Question 9

Summarise the process of Inflammation after an Infection

Answer 9

1. Infection leads to t_issue damage_ and activation of mast cell
2. Release of heparin and histamine from mast cells.
3. Innate immune mechanisms (PAMPs, complement) lead to activation of residential macrophages.
4. Release of pro-inflammatory cytokines and chemokines (IL-8)
5. Vasodilation and increased permeability of blood vessels (vascular leaking, swelling)
6. Leukocyte extravasate from the blood and migrate to the site of infection
7. Migration is guided by the chemokine interleukin-8 (macrophages) and C5a (activated complement)

Question 10

What organisms cause SSTI?

Answer 10

What Organisms Cause SSTI?

• Streptococcus pyogenes *
• Staphylococcus aureus *
• Some other bacteria (e.g. vibrio vulnificus)
• Fungi (e.g. dermatophytic moulds, tinea)
• Viruses (e.g. chick pox) * (if you itch it)

Question 11

Describe the features of Streptococcus

Answer 11

Streptococcus

Streptos means easily bent or twisted, like a chain (Greek).

Streptococcus is gram-positive spherical or ovoid cocci (0.5-1um).

It is catalase negative (in contrast to staphylococci).

It can cause:

• Skin and soft tissue infection
• Severe systemic disease (streptococcal toxic shock syndrome)
• Pharyngitis, tonsillitis
• Acute rheumatic fever

Question 12

Describe Streptococcus Pyogenes

How can we determine if someone is infected by it?

Answer 12

Streptococcus pyogenes has specific group A antigen, thus also called group A streptococcus.

• It is exclusively found in humans. Transmission by human contact.
• Transient colonisation of skin. Asymptomatic colonisation of oropharynx in 15-20% population (opportunistic infection).
• High infection rates in overcrowded houses, kindergarten, etc.

Lancefield Classification by Sero-typing

• Specific antibody against g_roup A antigen_ is used in latex agglutination test

​

• If you add the antibody that recognises the group A antigen, if group A antigen is present, you’d get agglutination (positive when agglutinate)
• If the antibody doesn’t recognise anything on the surface, you won’t get agglutination.

Question 13

Where can GAS be found?

Answer 13

GAS is exclusively found in humans

• Asymptomatic colonisation of oropharynx in 15-20% of population
• Transient colonisation of skin (not long)
• Transmission by human contact
• High infectionr ates in overcrowded houses, kindergarden etc.

Question 14

How do S.pyogenes infect our cells?

Answer 14

Streptococcus pyogenes has _microbial surface components recognising adhesive matrix molecule_s (MSCRAMMS), which are a large protein family.

Example include M protein.

• MSCRAMMS have c_ell wall-attached adhesins_
• MSCRAMMS have specific binding to host extracellular matrix (ECM) proteins (fibronectin, elastin, laminin, collagen), thus infecting host cells.
  
  • ​It therefore allows them to adhere and colonise the tissue

Question 15

How do S.Pyogenes evade our immune response?

Answer 15

Streptococcus pyogenes has:

1. Hyaluronic acid capsule, which prevents opsonisation and phagocytosis (it’s immunologically inate- so we ignore it)
2. M protein binds factor H (regulate our complement system- we use it so we don’t attact our own cells), which prevents opsonisation with C3b
3. Secretion of toxins, which include:

• Streptolysins (lyse immune cells)
• C5a peptidase (destroys C5a to prevent neutrophil chemotaxis)
• DNases (degrade neutrophil extracellular traps NETs)
• SpyCEP (_destroy IL-8 t_o prevent neutrophil chemotaxis)

Question 16

How do S.pyogenes spread into deeper tissue?

Answer 16

Spreading factors helps bacteria to destroy deeper tissue, and eventually reach bloodstream. They include:

• Protease;
• Lipases;
• Hyaluronidase;
• Streptokinase (anticoagulant that activates plasminogen to plasmin which degrades fibrin, - which prevent coagulation and l_ocalisation of injury_)

When bacteria are not contained, patient develops necrotising fasciitis (flesh-eating disease):

• Deep infection of the skin, d_estruction of tissue and fascia._
• Often development into s_evere systemic disease_ with high mortality.

Question 17

What is streptokinase

Answer 17

Secreted by S.pyogenes- help it spread into deeper tissue

Anticoagulant that activates plasminogen to plasmin which degrades fibrin, - which prevent coagulation and localisation of injury

Question 18

Describe necrotising fasciitis

Answer 18

Flesh-eating disease

When bacteria are not contained, patient develops necrotising fasciitis (flesh-eating disease):

• Deep infection of the skin, destruction of tissue and fascia.
• Often development into severe systemic disease with high mortality.

Question 19

What is another name for Flesh-eating disease?

Answer 19

necrotising fasciitis

Question 20

How do you diagnose S.Pyogenes?

Answer 20

Investigations

• Swab of purulent material (e.g. pus)
  
  • Cultivate and identify causative organism in microbiology lab
  • Swab of intact skin (absence of pus) is not useful (commensal skin bacteria)
• Hospitalised patients (signs the bacteria is in the blood) should also have blood culture taken
  
  • Positive cultures (bacteraemia) could lead to further complications (sepsis, STSS)

Confirmation

• 1) Histology
  
  • Strep. pyogenes (GAS) is g_ram-positive_
• 2) Catalyse test
  
  • Take bacteria, put them on a slide, put a drop of Hydrogen Peroxide in there. If the enzyme catalase is present (convert H2O2 to H2O and O2), it will produce bubbles.
  • Differentiate between streptococci (don’t have catalase) and staphlococci that do have calatase
  • S.Pyogenes = catalase negative,
• 3) Hemolysis on blood sugar
  
  • Swab on horse blood agar plates.
  • Different Streptococci have different abilities to lyse blood cells.
    
    • Some don’t lyse at all - non-haemolytic
    • Some partially lyse RBC - Virudence (green) streptococci
    • Completely lyse RBC- BHaemolysis (S.Pyogenes does this)
  • beta-haemolysis (complete RBC lysis),
• 4) Bacitracin susceptibiltiy
  
  • Antibacteria that is toxic- so you don’t prescribe it.
  • Swab the bacteria all over the agar plate. Put a disc of the antibacteria on it.
  • Susceptible to bacitracin (diagnostic only, toxic, not for treatment).
    
    • If S. pyogenes, lots of bacteria should be found on the agar plate, but not anywhere around the disc

Question 21

How do you treat Skin and Soft Tissue infection by Streptococcus Pyogenes?

Answer 21

Treatment Regime

• Supportive care
• Rest and elevation
• Analgesia
• Antimicrobial drugs
• If causative agent identified as streptococcus pyogenes, use penicillin or derivative such as amoxicillin (very effective against gram-positive bacteria)
• If staphylococcus aureus (or polymicrobial infection) is a possible cause, use beta-lactamase resistant penicillin such as flucloxacillin (except for MRSA strains)

Question 22

How does Penicillin work?

Answer 22

Penicillin binds to transpeptidase enzyme (i.e. p_enicillin-binding protein_).

• It p_revents formation of peptide cross-lin_ks in bacterial cell wall between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) (don’t need to know names)
• This results in weak cell wall and cell lysis.

Question 23

What are the different classes of B-lactam antibiotics?

Answer 23

Beta-lactam antibiotics have three main class of antibiotics, which include

• penicillin (from Penicillium - mould),
• cepgalosporins (from Acremonium- mould),
• carbapenems (from Streptomyces cattleya).

Each class has many derivatives (+ side chains), but all have beta-lactam ring.

• Penicillin derivatives include:
  
  • Penicillin G (benzylpenicillin)
    
    • Intramuscular injections
  • _Penicillin V (_phenoxymethylpenicillin)
  • Amoxicillin
• All streptococcus pyogenes is susceptible to penicillin. But only 10% of staphylococcus aureus are.

Question 24

All ________________ is susceptible to penicillin. But only 10% of _______________ are.

Answer 24

All streptococcus pyogenes is susceptible to penicillin. But only 10% of staphylococcus aureus are.