Microbiology of Skin Infections

Question 1

What properties of the skin allow it to defend against micro-organisms?

Answer 1

Dry (desiccation of micro-organisms), sebum/fatty acids (inhibit bacterial growth), competitive bacterial flora

Question 2

What are some competitive bacterial flora of the skin?

Answer 2

Staph. epidermidis, Corynebacterium sp. (diphtheroids), Propionibacterium sp.

Question 3

How can skin infections be diagnosed?

Answer 3

Swab lesion if surface broken, pus or tissue if deeper lesion, +/- blood cultures if appropriate

Question 4

What is Staphylococcus?

Answer 4

Gram positive cocci in clusters, aerobic and facultatively anaerobic (grows best in air but can grow anaerobically)

Question 5

What are the two important types of Staphylococcus?

Answer 5

Staph. aureus (coagulase positive), coagulase negative staph. (all other types except staph. aureus, not normally important for skin infections)

Question 6

How is staph. aureus distinguished from all other staph. species?

Answer 6

Produces, enzymes, including coagulase (clots plasma)

Question 7

What kind of infections does Staph. aureus cause?

Answer 7

Wound, skin, bone and joint infection

Question 8

What is the treatment of choice for Staph. aureus infections?

Answer 8

Flucloxacillin

Question 9

What is the resistant strain of Staph aureus called?

Answer 9

Methicillin Resistant Staphylococcus Aureus (MRSA)

Question 10

Why can’t MRSA be treated with flucloxacillin?

Answer 10

Methicillin is very similar to flucloxacillin, its basically a laboratory version of the drug

Question 11

What are some toxins produced by Staph. aureus?

Answer 11

Enterotoxin (food poisoning), Staphylococcal Scalded Skin Syndrome Toxin (SSSST), Panton valentine leucocidin (PVL)

Question 12

What skin infections does staph. aureus cause?

Answer 12

Boils and carbuncles, cellulitis, other minor skin sepsis, infected eczema, impetigo, wound infection, Staph. scalded skin syndrome

Question 13

What are the treatments for skin and soft tissue infections caused by MRSA?

Answer 13

Doxycycline (oral, bacteriostatic), co-trimoxazole (excellent oral bioavailability), Clindamycin and Linezolid (not routinely recommended)

Question 14

What are the bactericidal treatment options for systemic MRSA infections?

Answer 14

Vancomycin, Daptomycin

Question 15

What do coagulase negative staph. tend to be?

Answer 15

Skin commensals (not usually pathogenic)

Question 16

When do coagulase negative staph. cause infection?

Answer 16

May cause infection in association with implanted artificial material (e.g heart valves)

Question 17

What does Staph. saprophticus cause?

Answer 17

UTIs in women of child bearing age

Question 18

What is Streptococci?

Answer 18

Gram positive cocci in chains, aerobic (and facultatively anaerobic)

Question 19

How is Streptococcus classified initially?

Answer 19

By haemolysis on blood agar = beta (complete), alpha (partial), gamma (no haemolysis)

Question 20

What is an example of a toxin produced by beta haemolytic strep?

Answer 20

Haemolysin=damages tissues

Question 21

How can beta haemolytic strep be further classified?

Answer 21

By the antigenic structure on their surface = Group A (throat, severe skin infections), Group B (meningitis in neonates)

Question 22

What are the two important examples of alpha haemolytic strep?

Answer 22

Strep. pneumoniae (commonest cause of pneumonia, pathogen), Strep. viridans group (commensals of mouth, throat and vagina, cause endocarditis)

Question 23

What is an example of a gamma haemolytic strep?

Answer 23

Enterococcus species=commensals of bowel, common cause of UTI

Question 24

What are some examples of infections caused by Strep. pyogenes (Group A strep)?

Answer 24

Infected eczema, impetigo, cellulitis, erysipelas, necrotising fasciitis (also caused by mixed bacterial infection)

Question 25

What is the treatment for Strep. pyogenes infections?

Answer 25

Penicillin (also flucloxacillin)

Question 26

What is necrotising fasciitis?

Answer 26

Bacterial infection that spreads along fascial planes below skin surface causing rapid tissue destruction (little to see on skin surface but severe pain)

Question 27

What causes type 1 necrotising fasciitis?

Answer 27

Mixed anaerobes and coliforms, usually post-abdominal surgery

Question 28

What causes type 2 necrotising fasciitis?

Answer 28

Group A strep infection

Question 29

What is the treatment for necrotising fasciitis?

Answer 29

Urgent surgical opinion and debridement needed, antibiotic treatment depends on the organisms isolated from the tissue taken at operation

Question 30

What is the underlying problem of leg ulcers?

Answer 30

Vascular problems

Question 31

When do you take swabs of leg ulcers?

Answer 31

If there are signs of cellulitis or infection

Question 32

What are some organisms worth treating if found in a leg ulcer?

Answer 32

Strep. pyogenes, Staph. aureus, other beta haemolytic strep (B, C, G), anaerobes (especially in diabetic patients)

Question 33

What are some examples of dermatophyte (fungal infections)?

Answer 33

Tinea (ringworm), candida

Question 34

How is tinea named?

Answer 34

According to where it is found (e.g tinea pedis = foot)

Question 35

What kind of skin does tinea enter?

Answer 35

Abraded or soggy skin

Question 36

Where does the hyphae of tinea spread?

Answer 36

Stratum corneum

Question 37

What is the pathogenesis of tinea once it has entered the stratum corneum?

Answer 37

Increases epidermal turnover causing scaling, inflammatory response is provoked in dermis, hair follicles and shafts are invaded, lesions grow outwards and heal in centre (causing a ring appearance)

Question 38

What kind of tissue does tinea infect?

Answer 38

Only keratinised tissues (hair, nail, skin)

Question 39

Who is more likely to get tinea infection?

Answer 39

Males (especially foot and groin ringworm)

Question 40

Who commonly gets scalp ringworm?

Answer 40

Mainly children

Question 41

What are some sources of tinea infection?

Answer 41

Other infected humans (anthropophilic), animals (cats, dogs, cattle-zoophilic), soil (less common in UK-geophilic)

Question 42

What is the most common cause of tinea infection?

Answer 42

Trichophyton rubrum = >70% of isolates, human-human transmission

Question 43

What is the second most common cause of tinea infection?

Answer 43

Trichophyton mentgraphytes = >20% of isolates, human-human transmission

Question 44

What is a rare cause of tinea infection?

Answer 44

Microsporium canis = occasional isolate, cat/dog-human transmission

Question 45

How can tinea infection be diagnosed?

Answer 45

Clinical appearance, Woods light (fluorescence), skin scrapings/nail clippings sent to lab in Dermapak for microscopy and culture

Question 46

How long does a tinea culture take, and where should skin scrapings be taken from?

Answer 46

2 weeks +; scrapings should be taken from scaly edge of lesion

Question 47

How are small areas of skin/nails infected with tinea treated?

Answer 47

Clotrimazole (Canestan) cream, topical nail paint (Amorolfine)

Question 48

How are extensive tinea infections, or nail/scalp infections treated?

Answer 48

Oral terbinafine or Itraconazole

Question 49

Where does Candida intertrigo infection occur?

Answer 49

In skin folds where area is warm and moist = under breast in females, groin areas, abdominal skin folds, nappy area in babies

Question 50

How is candida infection diagnosed?

Answer 50

Using a swab for culture

Question 51

How is candida infection treated?

Answer 51

Clotrimazole cream, oral Fluconazole

Question 52

What organism causes scabies?

Answer 52

Sarcoptes scabies

Question 53

What is the chronic crusted form of scabies called?

Answer 53

Norwegian scabies (highly infectious)

Question 54

What is the incubation period of scabies?

Answer 54

Up to 6 weeks

Question 55

How does scabies manifest?

Answer 55

Intensely itchy rash affecting finger webs, wrists or genital area

Question 56

How is scabies treated?

Answer 56

Malathion lotion (applied overnight to whole body and washed off next morning), Benzyl benzoate (avoid in children)

Question 57

What organism causes head lice?

Answer 57

Pediculus capitus

Question 58

What organism causes body lice (Vagabond's disease)?

Answer 58

Pediculus corporis

Question 59

What is associated with a lice infection and how is the infection treated?

Answer 59

Intense itch; infection treated with malathion

Question 60

What are some infection control precautions for changing dressings?

Answer 60

Wear gloves and plastic aprons

Question 61

What patients need single rooms?

Answer 61

Patients with Group A strep, MRSA or scabies

Question 62

What kind of gown needs to be worn when seeing to a patient with Norwegian scabies?

Answer 62

Long sleeved gowns

Question 63

Why are patients with exfoliative skin conditions an infection risk?

Answer 63

They shed huge numbers of skin scales and associated bacteria (gram positive bacteria can survive in environment)