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Dermatology > Microbiology of Skin Infections > Flashcards

Microbiology of Skin Infections Flashcards

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Dermatology Board Prep flashcards
1
Q

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

A

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

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2
Q

What are some competitive bacterial flora of the skin?

A

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

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3
Q

How can skin infections be diagnosed?

A

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

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4
Q

What is Staphylococcus?

A

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

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5
Q

What are the two important types of Staphylococcus?

A

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

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6
Q

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

A

Produces, enzymes, including coagulase (clots plasma)

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7
Q

What kind of infections does Staph. aureus cause?

A

Wound, skin, bone and joint infection

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8
Q

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

A

Flucloxacillin

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9
Q

What is the resistant strain of Staph aureus called?

A

Methicillin Resistant Staphylococcus Aureus (MRSA)

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10
Q

Why can’t MRSA be treated with flucloxacillin?

A

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

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11
Q

What are some toxins produced by Staph. aureus?

A

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

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12
Q

What skin infections does staph. aureus cause?

A

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

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13
Q

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

A

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

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14
Q

What are the bactericidal treatment options for systemic MRSA infections?

A

Vancomycin, Daptomycin

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15
Q

What do coagulase negative staph. tend to be?

A

Skin commensals (not usually pathogenic)

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16
Q

When do coagulase negative staph. cause infection?

A

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

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17
Q

What does Staph. saprophticus cause?

A

UTIs in women of child bearing age

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18
Q

What is Streptococci?

A

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

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19
Q

How is Streptococcus classified initially?

A

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

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20
Q

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

A

Haemolysin=damages tissues

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21
Q

How can beta haemolytic strep be further classified?

A

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

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22
Q

What are the two important examples of alpha haemolytic strep?

A

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

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23
Q

What is an example of a gamma haemolytic strep?

A

Enterococcus species=commensals of bowel, common cause of UTI

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24
Q

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

A

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

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25
Q

What is the treatment for Strep. pyogenes infections?

A

Penicillin (also flucloxacillin)

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26
Q

What is necrotising fasciitis?

A

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

27
Q

What causes type 1 necrotising fasciitis?

A

Mixed anaerobes and coliforms, usually post-abdominal surgery

28
Q

What causes type 2 necrotising fasciitis?

A

Group A strep infection

29
Q

What is the treatment for necrotising fasciitis?

A

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

30
Q

What is the underlying problem of leg ulcers?

A

Vascular problems

31
Q

When do you take swabs of leg ulcers?

A

If there are signs of cellulitis or infection

32
Q

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

A

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

33
Q

What are some examples of dermatophyte (fungal infections)?

A

Tinea (ringworm), candida

34
Q

How is tinea named?

A

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

35
Q

What kind of skin does tinea enter?

A

Abraded or soggy skin

36
Q

Where does the hyphae of tinea spread?

A

Stratum corneum

37
Q

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

A

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)

38
Q

What kind of tissue does tinea infect?

A

Only keratinised tissues (hair, nail, skin)

39
Q

Who is more likely to get tinea infection?

A

Males (especially foot and groin ringworm)

40
Q

Who commonly gets scalp ringworm?

A

Mainly children

41
Q

What are some sources of tinea infection?

A

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

42
Q

What is the most common cause of tinea infection?

A

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

43
Q

What is the second most common cause of tinea infection?

A

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

44
Q

What is a rare cause of tinea infection?

A

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

45
Q

How can tinea infection be diagnosed?

A

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

46
Q

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

A

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

47
Q

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

A

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

48
Q

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

A

Oral terbinafine or Itraconazole

49
Q

Where does Candida intertrigo infection occur?

A

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

50
Q

How is candida infection diagnosed?

A

Using a swab for culture

51
Q

How is candida infection treated?

A

Clotrimazole cream, oral Fluconazole

52
Q

What organism causes scabies?

A

Sarcoptes scabies

53
Q

What is the chronic crusted form of scabies called?

A

Norwegian scabies (highly infectious)

54
Q

What is the incubation period of scabies?

A

Up to 6 weeks

55
Q

How does scabies manifest?

A

Intensely itchy rash affecting finger webs, wrists or genital area

56
Q

How is scabies treated?

A

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

57
Q

What organism causes head lice?

A

Pediculus capitus

58
Q

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

A

Pediculus corporis

59
Q

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

A

Intense itch; infection treated with malathion

60
Q

What are some infection control precautions for changing dressings?

A

Wear gloves and plastic aprons

61
Q

What patients need single rooms?

A

Patients with Group A strep, MRSA or scabies

62
Q

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

A

Long sleeved gowns

63
Q

Why are patients with exfoliative skin conditions an infection risk?

A

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

Dermatology (31 decks)

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