Skin and soft tissue infections Flashcards
What features of skin make it intrinsically resistant to infections?
Low water content Low pH Low temperature High salt Fats and fatty acids Microbiota
What is the Gram Stain of the vast majority of the skin microbiota?
Gram positive
What is the most common cause of localised skin infections?
Staphylococcus aureus
What is the feature that distinguishes S. aureus from S. epidermidis?
S. aureus - Coagulase +
S. epidermidis - Coagulase -
What is the primary defence against S. aureus?
Neutrophils
What are the virulence factors produced by S. aureus to overcome neutrophils?
Adhesins - bind to damaged tissues
CHIPS - Inhibit chemotaxis
Capsule, protein A, - inhibits phagocytosis
Can resist killing if phagocytosed by PMNs
Can form biofilms on biotic and abiotic surfaces
What percentage of S. aureus is resistant to penicillin?
~90%
What is Erysipelas?
Rapidly spreading superficial erythematous infection (usually. Of face, legs or feet) with well-defined borders plus pain and fever.
How is Cellulitis different to Erysipelas?
Cellulitis usually involves subcutaneous fat, and is caused by a wider range of bacteria.
Which bacterial species most commonly causes gas gangrene?
Clostridia
How are streptococci classified?
Classified according to the type of haemolysis they produce on blood agar.
Which bacterium belongs to Lancefield Group A?
Group A Streptococcus -> S. pyogenes
Which bacterium belongs to Lancefield Group B?
Group B streptococcus -> S. agalactiae
What are the structural virulence determinants of Group A streptococcus?
M-protein, Capsule -> antiphagocytic
Other surface Ags
Cell wall
Carbohydrate
What are some Secreted enzymes that act as virulence determinants for Group A streptococcus?
Enzymes:
- streptokinase (fibrinolysin)
- hyaluronidase (spreading)
- C5a peptidase
- SpeB (a protease)
- DNAases: overcome NETs (Neutrophil Extracellular Traps)
What are some secreted toxins that act as virulence determinants for Group A streptococcus?
Streptolysins (haemolysins)
Leukocidins
Superantigens (e.g. SpeA)
What is the morphology/nutritional requirements of Clostridia?
Gram positive rod
Anaerobes
Spore forming
Outline the pathogenesis of gangrene caused by Clostridium:
Spores or vegetative bacteria from environment, gut or vaginal microbiota enter the wound
Germinate and replicate under anaerobic conditions in devitalised tissue
Secrete many different toxins and enzymes. This leads to gas production, tissue damage and further spread.
What colour pus does Pseudomonas produce?
Blue-green
How are viral skin infections diagnosed?
Nucleic Acid Transferase tests.
How do you treat Gas gangrene?
surgery + penicillin G +/- hyperbaric oxygen
How do you treat Diabetic foot infections?
Co-amoxyclav + metronidazole
What is Co-amoxyclav?
Amoxycillin: Beta-lactam antibiotic
Clavulanic acid: Beta-lactamase inhibitor
What is Flucloxicillin?
Beta-lactam antibiotic
