L4- skin and soft tissue infection

Question 1

what is Cellulitis

Answer 1

infection of the dermis and subcutaneous tissue

- often seen arounf injury site or deep abscess

Question 2

eryipelas definitions

Answer 2

impetigo - common acute superficial s.pyogenes skin infection of dermis and the lymphatics

Question 3

how does the innate immune system recognise the pathogens?

Answer 3

recognises pathogen associated molecular patterns (PAMPS) by pattern recognition receptors.

Question 4

binding of PAMPS to the pattern recognising receptors causes..

Answer 4

propagation of action potential –> causes transcription for cytokine reelase (IL8)

Question 5

what effect does cytokine release have on the endothelial cells?

Answer 5

causes expression of E-selectin on endothelial cell

- binds to neutrohils (slows down)

Question 6

what is diapedesis?

Answer 6

neutrophils squeeze through the endothelial barriers (leukocyte extravasation)

Question 7

what are the main organisms that cause SSTIs?

Answer 7

s. pyogenes
s. aureus
other bacteria/ fungi
viruses

Question 8

characterstics of streptococcus?

shape, what can it cause?

Answer 8

gram positive spherical/cocci
- SSTI
- Toxic shock
- pharyngitis
acute rheumatic fever

Question 9

how to test for streptococcus?

Answer 9

known antibody is added to the agar plate sample, should bind to antigen –> cuause agglutination

Question 10

what is the route of infection of GAS?

- mode of transmission

Answer 10

asymptomatic colonisation of the oropharynx (relatively common)

• they are opportunistic
• transmitted by human contact

Question 11

how does the s. pyogenes recognise teh host cell?

Answer 11

has MSCRAMMS on their cell surface (microbial surface components recognising adhesive matrix molecules)
- helps them bind to components of the host cell (fibronectin, elastin and collagen etc)

Question 12

How does s. Pyogenes evade immune responses?

what are the 3 structures that prevent it from being detected by the immune system?

Answer 12

hyaluronic acid
M protein
secretion of toxins

Question 13

what is the function of hyaluronic acid in avoiding the immune response

Answer 13

prevents opsonisation and phagocytosis

Question 14

role of M protein for S. pyogenes

Answer 14

binds to factor H (regulates the complement system)- prevents with C3b opsonisation

Question 15

3 examples of toxins released by s.pyogenes to prevent immune responses

Answer 15

streptolysins (lyse immune cells)

C5a pepetidase (prevents neutrophil chemotaxis)

DNases (destroys neutrophils)

Question 16

what is necrotising fasciculitis?
what is the prognosis?
treatment?

Answer 16

deep infection of the skin causing destruction of the tissue and fascia.

• often develops into severe systemic disease (high mortality)
• remove necrosed layers, pump with IV antibiotics and amputations.

Question 17

how are skin infections diagnosed?

Answer 17

swab the pus –> microbiology

Question 18

explain the order of testing process to differentiate gram positive bacteria the microbial cause of infection?

Answer 18

catalase test

haemolysis test

bacitracin susceptibility

Question 19

how does the catalase test work?

- negative/positive test results differentiate what?

Answer 19

if the bacteria has the catalase enzyme it will convert H2O2 into O2 (see bubbles)

• negative (strep)
• positive (staphylococci)

Question 20

haemolysis testing- procedue and result inrterpretation

Answer 20

not haemolytic (gamma)
partial haemolytic (alpha)
complete haemolysis (beta)

Question 21

bacitracin susceptibility

- sensitive/insensitive indications

Answer 21

sensitive - pyogenes

resistant - other forms of strep

Question 22

what is the treatment for SSTI?

how does this differ between s.pyogenes and s aureus

Answer 22

supportive care
analgesia
antimicrobial drugs
- pyogenes - penicillin or amoxiillin

staph- b-lactamase resistant penicillin

Question 23

what is the mechanism of penicilllin?

Answer 23

bacterial cell wall consists of 2 types of sugar complexes (NAM and NAG)
- bound by transpeptidase enzyme (this is inhibited by penicillin) = prevented by penicillin so weaker cell wall