Skin and Soft Tissue Infections Flashcards
Why is skin intrinsically resistant to infection?
due to low water content, low pH, low temperature, high salt, fats and fatty acids, microbiota
What microbiota live on the skin?
staph epidermidis, staph aureus, diptheroids, streptococci, mostly gram +ve but some gram -ve e.g. pseudomonas, some anaerobes (gram positives and negatives - typically down in follicles) - and some yeasts particularly candida
What are some bacterial causes of skin and soft tissue infections?
- Staphylococcus aureus
- Streptococcus pyogenes
- Clostridium perfringens
- Pseudomonas sp
- Other Gram –ves
What are the 3 types of skin and soft tissue infections?
localised infections (folliculitis, abscess), spreading infections (impetigo, cellulitis) and necrotising infections (fasciitis)
What is folliculitis?
infection of the hair follicle (may progress to an abscess/boil/furuncle)
What is the most common cause of folliculitis?
staph aureus
Is staph aureus coagulase positive or negative?
positive
What is the primary defence against staph?
innate immunity via neutrophils
How does staph bind to damaged tissue?
via adhesins
How does staph inhibit chemotaxis?
via CHIPS - a chemotaxis inhibiting protein
How does staph inhibit phagocytosis?
with its capsule, protein A, staphylokinase, complement inhibitor, haemolysins and leukocidins
What is the mechanism of action of protein A?
binds antibodies at the Fc end to prevent them binding to block opsonisation and phagocytosis
What is the mechanism of action of staphylokinase?
activates plasminogen to dissolve the clot that is trying to keep the infection localised
What is Panton-Valentine leukocidin?
a leukocidin found in methicillin resistant staph aureus
What is the antimicrobial resistance of staph aureus?
90% are resistant to penicillin, and now there is increasing amounts of healthcare acquired methicillin resistant staph and community acquired methicillin resistant staph
What is impetigo?
an infection of the epidermis characterised by bullous, crusted or pustular lesions
What causes impetigo?
staph aureus or strep pyogenes or both
What is bullous impetigo caused by?
staph aureus alone
What is erysipelas?
rapidly spreading superficial erythematous infection (usu. of face, legs or feet) with well-defined border, plus pain and fever
What is cellulitis?
similar to erysipelas but involving subcutaneous fat
What is the most common cause of erysipelas and cellulitis?
strep pyogenes
What is fasciitis?
a rapidly spreading infection along fascial planes that disrupts blood supply and leads to necrosis and may cause gangrene
What are the most common causes of fasciitis?
strep pyogenes and anaerobes
What is the most common cause of gas gangrene?
clostridia
Which lancefield group is strep pyogenes?
group A
Which lancefield group is strep agalactiae?
group B
How is strep pyogenes further categorised?
according to which M proteins it expresses
How does strep pyogenes evade innate immunity?
it has a capsule, it has leukocidal toxins, it has a DNAse to dissolve the NETS created by neutrophils to trap pathogens, and it has a C5a peptidase
What type of bacterium is clostridia?
a gram positive rod that is anaerobic and spore forming
What bacterium is a common component of diabetic foot ulcers?
pseudomonas
What bacterium are burns particularly susceptible to?
pseudomonas
Why is pseudomonas difficult to manage?
because its intrinsically resistant to many antibiotics
What types of fungi can cause skin infections?
dermatophytes (Epidermophyton, Trichophyton, Microsporum) and yeasts (candida and mallasezia)
Which patients are at risk of chronic mucocutaneous candidiasis?
patients with a primary immunodeficiency e.g. a mutation in the AIRE gene
What types of parasites cause skin infection?
arthropods (insects, ticks, mites) and scabies
What types of viruses cause skin and soft tissue infection?
- HSV,
- VZV,
- HPV,
- measles,
- rubella,
- enteroviruses,
- parvo B19,
- Molluscum contagiosum
What is eczema herpeticum?
disseminated herpes virus in patients with eczema
What is the treatment for a wound infection?
remove damaged tissue and foreign material, use a topical disinfectant and wound dressing, consider an oral antibiotic and consider tetanus prophylaxis
What is the treatment for an abscess?
drain, consider oral antibiotics
What is the treatment for impetigo?
soap and water wash with flucloxacillin
What is the treatment for cellulitis?
flucloxacillin
What is the treatment for gas gangrene?
surgery and penicillin G and hyperbaric oxygen
What is the treatment for diabetic foot ulcers?
co amoxyclav and metronidozole
