Skin and Soft Tissue Infections

Question 1

Why is skin intrinsically resistant to infection?

Answer 1

due to low water content, low pH, low temperature, high salt, fats and fatty acids, microbiota

Question 2

What microbiota live on the skin?

Answer 2

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

Question 3

What are some bacterial causes of skin and soft tissue infections?

Answer 3

• Staphylococcus aureus
• Streptococcus pyogenes
• Clostridium perfringens
• Pseudomonas sp
• Other Gram –ves

Question 4

What are the 3 types of skin and soft tissue infections?

Answer 4

localised infections (folliculitis, abscess), spreading infections (impetigo, cellulitis) and necrotising infections (fasciitis)

Question 5

What is folliculitis?

Answer 5

infection of the hair follicle (may progress to an abscess/boil/furuncle)

Question 6

What is the most common cause of folliculitis?

Answer 6

staph aureus

Question 7

Is staph aureus coagulase positive or negative?

Answer 7

positive

Question 8

What is the primary defence against staph?

Answer 8

innate immunity via neutrophils

Question 9

How does staph bind to damaged tissue?

Answer 9

via adhesins

Question 10

How does staph inhibit chemotaxis?

Answer 10

via CHIPS - a chemotaxis inhibiting protein

Question 11

How does staph inhibit phagocytosis?

Answer 11

with its capsule, protein A, staphylokinase, complement inhibitor, haemolysins and leukocidins

Question 12

What is the mechanism of action of protein A?

Answer 12

binds antibodies at the Fc end to prevent them binding to block opsonisation and phagocytosis

Question 13

What is the mechanism of action of staphylokinase?

Answer 13

activates plasminogen to dissolve the clot that is trying to keep the infection localised

Question 14

What is Panton-Valentine leukocidin?

Answer 14

a leukocidin found in methicillin resistant staph aureus

Question 15

What is the antimicrobial resistance of staph aureus?

Answer 15

90% are resistant to penicillin, and now there is increasing amounts of healthcare acquired methicillin resistant staph and community acquired methicillin resistant staph

Question 16

What is impetigo?

Answer 16

an infection of the epidermis characterised by bullous, crusted or pustular lesions

Question 17

What causes impetigo?

Answer 17

staph aureus or strep pyogenes or both

Question 18

What is bullous impetigo caused by?

Answer 18

staph aureus alone

Question 19

What is erysipelas?

Answer 19

rapidly spreading superficial erythematous infection (usu. of face, legs or feet) with well-defined border, plus pain and fever

Question 20

What is cellulitis?

Answer 20

similar to erysipelas but involving subcutaneous fat

Question 21

What is the most common cause of erysipelas and cellulitis?

Answer 21

strep pyogenes

Question 22

What is fasciitis?

Answer 22

a rapidly spreading infection along fascial planes that disrupts blood supply and leads to necrosis and may cause gangrene

Question 23

What are the most common causes of fasciitis?

Answer 23

strep pyogenes and anaerobes

Question 24

What is the most common cause of gas gangrene?

Answer 24

clostridia

Question 25

Which lancefield group is strep pyogenes?

Answer 25

group A

Question 26

Which lancefield group is strep agalactiae?

Answer 26

group B

Question 27

How is strep pyogenes further categorised?

Answer 27

according to which M proteins it expresses

Question 28

How does strep pyogenes evade innate immunity?

Answer 28

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

Question 29

What type of bacterium is clostridia?

Answer 29

a gram positive rod that is anaerobic and spore forming

Question 30

What bacterium is a common component of diabetic foot ulcers?

Answer 30

pseudomonas

Question 31

What bacterium are burns particularly susceptible to?

Answer 31

pseudomonas

Question 32

Why is pseudomonas difficult to manage?

Answer 32

because its intrinsically resistant to many antibiotics

Question 33

What types of fungi can cause skin infections?

Answer 33

dermatophytes (Epidermophyton, Trichophyton, Microsporum) and yeasts (candida and mallasezia)

Question 34

Which patients are at risk of chronic mucocutaneous candidiasis?

Answer 34

patients with a primary immunodeficiency e.g. a mutation in the AIRE gene

Question 35

What types of parasites cause skin infection?

Answer 35

arthropods (insects, ticks, mites) and scabies

Question 36

What types of viruses cause skin and soft tissue infection?

Answer 36

• HSV,
• VZV,
• HPV,
• measles,
• rubella,
• enteroviruses,
• parvo B19,
• Molluscum contagiosum

Question 37

What is eczema herpeticum?

Answer 37

disseminated herpes virus in patients with eczema

Question 38

What is the treatment for a wound infection?

Answer 38

remove damaged tissue and foreign material, use a topical disinfectant and wound dressing, consider an oral antibiotic and consider tetanus prophylaxis

Question 39

What is the treatment for an abscess?

Answer 39

drain, consider oral antibiotics

Question 40

What is the treatment for impetigo?

Answer 40

soap and water wash with flucloxacillin

Question 41

What is the treatment for cellulitis?

Answer 41

flucloxacillin

Question 42

What is the treatment for gas gangrene?

Answer 42

surgery and penicillin G and hyperbaric oxygen

Question 43

What is the treatment for diabetic foot ulcers?

Answer 43

co amoxyclav and metronidozole