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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 do staphlococcus look like under a microscope?

A

Gram positive (purple) cocci in clusters

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2
Q

Are staphlococcus aerobic or anaerobic bacteria?

A

Aerobic and facultatively anaerobic

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3
Q

What are the two important groups of staph. and how can they be differentiated?

A

Staph. aureus and coagulase negative staph (e.g staph. epidermidis). They are differentiated with the coagulase test

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4
Q

In culture what colour does staph aureus appear?

A

Gold

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5
Q

What kind of infections do staph. aureus commonly cause?

A

Wound, skin, bone and joint

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6
Q

Name some toxins which certain strains of s.aureus produce

A

Enterotoxin
PVL
SSSST

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7
Q

What is the drug of choice used to treat staph. aureus skin infections?

A

Flucloxacillin

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8
Q

In which group of people is staphlococcal scalded skin syndrome usually found?

A

Infants

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9
Q

What might a PVL containing strain of s.aureus usually cause?

A

Chronic abscesses (axilla and buttocks)

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10
Q

Coagulase negative staph are usually skin commensals. T/F

A

True

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11
Q

When might coagulase negative staph cause infections?

A

In association with implanted artificial material (e.g heart valves)

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12
Q

What type of infections does staph. saprophyticus usually cause?

A

Urinary tract infection (woman mostly)

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13
Q

What do streptococci look like under a microscope?

A

Gram positive (purple) cocci in chains

strep=strips

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14
Q

Are streptococci aerobic or anaerobic?

A

Aerobic

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15
Q

How are streptococci classified?

A

By type of haemolysis on a blood agar plate (alpha, beta and gamma). Where beta is complete haemolysis, alpha is partial haemolysis and gamma is no haemolysis

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16
Q

Which class of steps are the most pathogenic?

A

Beta haemolytic (group A strep)

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17
Q

When does group B strep cause infection?

A

Usually in neonates

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18
Q

What is the major toxin produced by group B strep which causes destruction of tissues?

A

Haemolysin

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19
Q

How is strep classified within the groupings?

A

Surface antigen structure

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20
Q

What types of infection do group B streps usually cause?

A

Throat and skin infections

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21
Q

What types of infection do group A streps usually cause?

A

Meningitis (neonates)

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22
Q

What are the two most important alpha haemolytic streps? Which infections do they cause respectively?

A

Strep. pneumonia and strep. viridans. Pneumonia and usually commensals of the mouth, throat and vagina

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23
Q

What are the non-haemolytic strep? Which infections do they usually cause?

A

Enterococcus sp. Gut commensals but common cause of urinary tract infection

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24
Q

Which features of the skin protect against infection?

A

Dry surface (doesn’t affect gram positive organisms much), sebum inhibits bacterial growth & competitive resident flora

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25
Q

Name some competitive bacterial flora?

A

Staph. epidermidis, diptheroids, proprionobacterium

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26
Q

What types of skin infections do staph.aureus cause?

A
  • boils and carbuncles (hair follicles)
  • minor skin sepsis (infected cuts)
  • cellulitis
  • infected eczema
  • impetigo
  • wound infection
  • SSSS
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27
Q

Boils and carbuncles must be treated with oral antibiotics (flucloxacillin). T/F

A

False - only carbuncles need treated

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28
Q

Where do carbuncles most commonly occur?

A

At the base of the neck

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29
Q

Which pathogens usually cause impetigo?

A

S.aureus or group A strep

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30
Q

Where does impetigo present?

A

Mucocutaneous junction around the mouth and nose (crusty yellow scabs)

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31
Q

In which age group is impetigo most commonly found?

A

Children

32
Q

How is SSSS treated?

A

Aggressive antibiotics and rehydration therapy

33
Q

What is another name for group A strep?

A

Strep pyrogenes

34
Q

Which types of skin infections do strep pyrogenes cause?

A
  • infected eczema
  • impetigo
  • cellulitis
  • erysipelas
  • necrotising fasciitis (often along with other bacteria)
35
Q

How does cellulitis present?

A

Erythematous skin with no obvious breakage

36
Q

How does erysipelas present?

A

Butterfly pattern of facial swelling and erythema

37
Q

How does necrotising fasciitis present?

A

Purple/erythematous areas with no obvious skin breakage. Patient appears to be suffering pain disproportionate to what can be seen. Rapid spread

38
Q

How are bacterial skin infections diagnoses?

A

Bacterial swab if the skin is broken. If a deeper tissue infection pus/tissue can be used. Blood cultures if pyrexial

39
Q

How are bacterial skin infections treated?

A

+ minor skin sepsis - no treatment
+ s.aureus - flucloxacillin
+ strep. pyrogenes - penicillin (but also flucloxacillin)
+ necrotising fasciitis - immediate surgical debridement & antibiotics

40
Q

What is the pathogenesis of necrotising fasciitis?

A

Bacterial infection which spreads along the fascial planes below the skin surface causing rapid tissue destruction

41
Q

What are the two types of necrotising fasciitis?

A

Type 1 - mixed anaerobes and coliforms

Type 2 - group A strep

42
Q

When does Type 1 necrotising fasciitis usually occur?

A

Post abdominal surgery

43
Q

Which antibiotics is Type 2 necrotising fasciitis treated with?

A

Clindomycin and penicillin

44
Q

When should swabs be taken from leg ulcers?

A

If there are signs of infection/cellulitis

45
Q

Which organisms found in leg ulcers are worth treating?

A

Strep. pyrogenes, s.aureus & anaerobes (diabetic patients)

46
Q

Tinea is another word for what?

A

Ringworm

47
Q

How does tinea capitis present?

A

An area of hair loss (hair itself is infected and breaks off) with scale

48
Q

Why doe tinea corporis present as round lesions?

A

Fungus grows outwards from central infected point

49
Q

Is tinea cruris more common in men or woman?

A

Men (may grow on scrotum)

50
Q

How does tinea unguum present?

A

Yellow nails which flake off easily

51
Q

How does tinea pedis present? What is another name for tinea pedis?

A

Itch & soggy area of broken skin between toes (usually between fifth and fourth toes). Athlete’s foot

52
Q

What is the pathogenesis of dermatophyte infection?

A

Fungus enters broken/soggy skin > hyphae spread in keratin layer > increased epidermal turnover causing scaling > inflammatory response > hair follicles invaded > lesion grows outward and heals in centre (target lesion)

53
Q

Dermatophytes only infect keratinised tissues (skin, nails, hair). T/F

A

True

54
Q

What is the epidemiology of dermatophyte infection?

A

Men more commonly affected (pedis and cruris most commonly). Children more commonly affected with tinea capitis.

55
Q

How do dermatophyte infections spread?

A

Human-human (most common), animals & soil

56
Q

What are the three most common dermatophytes to cause infection? How are the spread?

A

Trichophyton rubrum, trichophyton mentagraphytes & microsporum canis. Human, human and animal

57
Q

How are dermatophyte infections diagnosed?

A

Clinically, woods light, microscopy (most accurate) and culture of skin scrapings, nail clippings & hair

58
Q

How must skin scrapings, nail clippings and hair be sent to the laboratory?

A

Within a dermapak (black so samples are more easily seen)

59
Q

What is the disadvantage of culturing dermatophytes?

A

It takes forever

60
Q

How does a culture of trichophyton rubrum appear?

A

White and fluffy on top and brown and orange below

61
Q

How are dermatophyte infections treated?

A

+ small areas of infection - clotrimazole cream, topical nail paint (amorolfine)
+ extensive infection - oral terbinafine and oral itraconazole

62
Q

Where does candida most commonly infect?

A

Within skin folds (under breasts, groin area, abdominal folds)

63
Q

How are candida infections diagnosed?

A

Swab for culture

64
Q

How are candida infections treated?

A

Clotrimazole cream, oral fluconazole

65
Q

What organisms is scabies caused by?

A

Sarcoptes scabiei

66
Q

What is the chronic, crusted form of scabies called? How does this occur?

A

Norwegian scabies. In immunocompromised patients

67
Q

What is the incubation period of scabies? How is it transferred?

A

Up to six weeks. Human to human

68
Q

How does scabies present?

A

Itchy rash often forming burrows and affecting the genital area, wrists and finger webs

69
Q

How is scabies treated?

A

Malathion lotion (applied overnight to whole body and washed off next day). Benzyl benzoate (NOT IN CHILDREN)

70
Q

What are the different types of lice?

A

Pediculus capitis (head lice), pediculus corporus (body louse), phthirus pubis (pubic lice)

71
Q

How does lice infection present?

A

Intense itch

72
Q

When might someone contract pediculus corporus?

A

Homeless/sleeping on the streets

73
Q

What is the treatment of lice?

A

Malathion

74
Q

What are the infection control procedures needed in dermatology?

A

Gloves and aprons required for dressing changes, single room isolation

75
Q

Which patients need single room isolation?

A

Group A strep infection, MRSA, scabies (long sleeved gowns required

Dermatology (26 decks)

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