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)
