L4- skin and soft tissue infection Flashcards
what is Cellulitis
infection of the dermis and subcutaneous tissue
- often seen arounf injury site or deep abscess
eryipelas definitions
impetigo - common acute superficial s.pyogenes skin infection of dermis and the lymphatics
how does the innate immune system recognise the pathogens?
recognises pathogen associated molecular patterns (PAMPS) by pattern recognition receptors.
binding of PAMPS to the pattern recognising receptors causes..
propagation of action potential –> causes transcription for cytokine reelase (IL8)
what effect does cytokine release have on the endothelial cells?
causes expression of E-selectin on endothelial cell
- binds to neutrohils (slows down)
what is diapedesis?
neutrophils squeeze through the endothelial barriers (leukocyte extravasation)
what are the main organisms that cause SSTIs?
s. pyogenes
s. aureus
other bacteria/ fungi
viruses
characterstics of streptococcus?
shape, what can it cause?
gram positive spherical/cocci - SSTI - Toxic shock - pharyngitis acute rheumatic fever
how to test for streptococcus?
known antibody is added to the agar plate sample, should bind to antigen –> cuause agglutination
what is the route of infection of GAS?
- mode of transmission
asymptomatic colonisation of the oropharynx (relatively common)
- they are opportunistic
- transmitted by human contact
how does the s. pyogenes recognise teh host cell?
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)
How does s. Pyogenes evade immune responses?
what are the 3 structures that prevent it from being detected by the immune system?
hyaluronic acid
M protein
secretion of toxins
what is the function of hyaluronic acid in avoiding the immune response
prevents opsonisation and phagocytosis
role of M protein for S. pyogenes
binds to factor H (regulates the complement system)- prevents with C3b opsonisation
3 examples of toxins released by s.pyogenes to prevent immune responses
streptolysins (lyse immune cells)
C5a pepetidase (prevents neutrophil chemotaxis)
DNases (destroys neutrophils)
what is necrotising fasciculitis?
what is the prognosis?
treatment?
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.
how are skin infections diagnosed?
swab the pus –> microbiology
explain the order of testing process to differentiate gram positive bacteria the microbial cause of infection?
catalase test
haemolysis test
bacitracin susceptibility
how does the catalase test work?
- negative/positive test results differentiate what?
if the bacteria has the catalase enzyme it will convert H2O2 into O2 (see bubbles)
- negative (strep)
- positive (staphylococci)
haemolysis testing- procedue and result inrterpretation
not haemolytic (gamma) partial haemolytic (alpha) complete haemolysis (beta)
bacitracin susceptibility
- sensitive/insensitive indications
sensitive - pyogenes
resistant - other forms of strep
what is the treatment for SSTI?
how does this differ between s.pyogenes and s aureus
supportive care
analgesia
antimicrobial drugs
- pyogenes - penicillin or amoxiillin
staph- b-lactamase resistant penicillin
what is the mechanism of penicilllin?
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
