Skin and Soft tissue Infiltration

Question 1

Impetigo

typically streptococcal but can be staph

Answer 1

Aka pyoderma, infection of superficial layer of skin
Found in crowded areas, low hygiene, humid.
Associated with rheumatic fever

Question 2

Erysepelas

Answer 2

typically streptococcus pyogenes

Clear line between infected an normal skin

Question 3

Cellulitis

Answer 3

Infection of the dermis and subcutaneous tissue
often seen around injury site or deep wound.
Staph or streptococcus

Question 4

General innate immune response to bacteria

Answer 4

Platelets form blood clots
Mast cells cause vessel vasodilation (heparin, histamine)
Macrophages secrete cytokines such as IL-8, a chemokine due to activation by PAMPs
More vasodilation (leakage, sweeling)
Neutrophils pass through leaky vessels (diapedesis) and phagocytose bacteria

Question 5

Gram -ve and gramp +ve PAMPS

Answer 5

Gram -ve: LPS (recognised by TLR4 on macrophages)

Gram +ve:LTA

Question 6

How do neutrophils enter tissue

Answer 6

Process of diapedesis
Selectin mediated ahesion, a weak connection
Movement enhanced by action of chemokines such as IL-8, C5a

Question 7

Description of streptococcus pyogenes

Answer 7

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)

Question 8

What is unique about streptococcus pyogenes surface antigens?

Answer 8

Has the group A antigen

Synonomous with group A streptococcus

Question 9

How does a Strep pyogenes bacterium colonise on skin?

Answer 9

Presence of MSCRAMMS, that recognise host ECM molecules.

• Are adhesins
• Specific to elastin, fibronectin, laminin, vitronectin, collagen
• E.g M protein (rheumatoc fever??)

Question 10

How can strep pyogenes avoid our immune response

Answer 10

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

Question 11

How can strep pyogenes invade deeper tissues?

result?

Answer 11

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

Can cause necrotising fascitis

Question 12

How can a strep pyogenes infection be diagnosed?

Answer 12

Swab of purulent material
Cultivate and identify
Should check blood culture. Positive cultures in blood can lead to sepsis, toxic shock syndrome

Question 13

Diagnostic approach

Answer 13

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 14

Treatment of strep

antibiotics

Answer 14

Well in leg example, supportive care, rest and elevation.
Analgesia
Antimicrobials: If strep pyogenes, penicillin or derivative (amoxycillin oral)
If staph aureus, B-lactamase resistant penicillin-flucloxacillin (not MRSA)

Question 15

How does penicillin work

Answer 15

Binds to transpeptidase enzyme which prevents bacterial cell walls creating peptide cross links. (between NAM and NAG parallelly =)
Weak cell wall and lysis