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LR Dermatology > Microbiology > Flashcards

Microbiology Flashcards

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Dermatology Board Prep flashcards
1
Q

What is meant by virulence?

A

The ability of a microorganism to cause disease in a host

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2
Q

What is meant by a virulence factor?

A

A component of a pathogen that contributes to its ability to cause disease

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3
Q

How do the following virulence factors help a pathogen cause disease?

  • adhesin
  • invasin
  • impedin
  • aggressin
  • modulin
A

Adhesin - enables binding of organism to host tissue

Invasin - enables organism to invade host cell/tissue

Impedin - enables organism to avoid host defence

Aggressin - damages host directly

Modulin - damages host indirectly

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4
Q

Skin infections tend to be gram +ve/-ve

A

Gram +ve staph and strep infections are most common

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5
Q

What are the 2 most common staphylococcus skin infections?

A
  • S. aureus

- S. epidermis

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6
Q

S. epidermis has a ?% colonisation

A

100%

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7
Q

In what individuals is S. epidermis infection most commonly seen?

A

In immunocompromised/ hospital patients, particularly in association with foreign devices e.g., catheters

Everyone carries S. epidermis on their skin (100% colonisation), it doesn’t usually cause infection

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8
Q

S. aureus has a ?% colonisation

A

20% (-60%)

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9
Q

In what individuals is S. aureus infection seen in?

A

In the community and in the hospital (nosocomial) setting

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10
Q

Which surfaces are most commonly colonised by…
- S. aureus
- S. epidermis
…?

A

S. aureus -> anterior nostrils and perineum

S. epidermis -> skin and mucous membranes

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11
Q

How can you distinguish between staph aureus and staph epidermis?

A

Coagulase testing

Staph aureus is coagulase +ve and staph epidermis is coagulase -ve

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12
Q

What is MRSA?

Where is it seen?

A

Methicillin resistant staph aureus -> a strain of staph aureus which is resistant to flucloxacillin

It is mainly seen in the hospital (nosocomial) setting in elderly and immunocompromised patients

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13
Q

What are the various presentations of staph aureus skin infection?

A
  • Rash
  • Abscess
  • Folliculitis
  • Carbuncle
  • Impetigo
  • Scalded skin syndrome
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14
Q

What are the important virulence factors that make staph aureus a highly effective pathogen?

A

Fibrinogen binding protein ->
allows staph aureus to create a fibrinogen shield around itself to avoid the immune response

Superantigens -> bacterial proteins that massively overstimulate the T cell immune response

Protein A ->
binds antibodies the wrong way round so they are not activated and the pathogen is not cleared

Panton-Valentine Leukocidin (PVL) ->
a toxin that destroys leukocytes, associated with severe infections

Toxic shock syndrome toxin (TSST-1) ->
a toxin that acts as a superantigen to cause toxic shock syndrome

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15
Q

What are toxinoses? Where do they affect an individual?

A

Syndromes caused by a bacterial toxin

They affect the host away from the site of colonisation

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16
Q

Give 2 examples of pathogenesis caused by staph aureus toxins

A

Toxic shock syndrome

Scalded skin syndrome

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17
Q

What is the diagnostic criteria for toxic shock syndrome?

A
  • Fever
  • Diffuse macular rash
  • Hypotension
  • > = 3 organ systems involved
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18
Q

Why is toxic shock syndrome associated with the use of tampons?

A

Staph aureus often colonises the perineum which is in close proximity with a tampon

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19
Q

PVL is a toxin produced by some strains of staph aureus.

What severe skin infection is this strain associated with?

A

Necrotising pneumonia

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20
Q

Describe the progression of necrotising pneumonia

A
  • Flu like syndrome
  • Necrotising haemorrhagic pneumonia
  • Rapid progression
  • Acute respiratory distress
  • Deterioration of pulmonary function
  • Refractory hypoxaemia
  • Multi-organ failure despite antibiotic therapy
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21
Q

What is the most common streptococcus skin infection?

A

Streptococcus pyogenes

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22
Q

What are the most common presentations of strep pyogenes?

A
  • Streptococcal sore throat

- Scarlet fever

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23
Q

According to the Lancefield system, strep pyogenes is a group A/B streptococcus

A

Group A strep (GAS)

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24
Q

Strep pyogenes is…

  • catalase +ve/-ve
  • alpha/beta/gamma haemolytic
A
  • Catalase -ve

- Beta haemolytic

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25
Q

The ? system groups gram +ve streptococci depending on their cell wall antigens

A

Lancefield

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26
Q

How are Group A Strep (GAS) further subdivided?

A

According to M protein antigens on their surface

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27
Q

Name 3 skin infections caused by strep pyogenes/GAS

A
  • Impetigo
  • Cellulitis
  • Necrotising fasciitis

(progressively deeper infections)

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28
Q

What is impetigo?

A

A red rash, usually on the face, with the infection immediately below the skins surface

It is highly contagious through contact with discharge on the face

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29
Q

What is the most common presentation of GAS in young children?

A

Impetigo

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30
Q

What is cellulitis?

What other symptoms may accompany it?

A

A deeper skin infection in the dermis but that is not associated with necrosis

Can be accompanied by fever, rigours and nausea

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31
Q

What is necrotising fasciitis?

A

An invasive GAS disease (iGAS) known as ‘the flesh eating bug’ that rapidly destroys connective tissue of the fascia below the skin

32
Q

Describe the rapid progression of necrotising fasciitis from day 0 (insult) to day 4

A

Day 0 - discomfort around area of trauma, pain in area of infection

Day 1 - flu-like symptoms, vomiting, fever, diarrhoea

Day 2 - swelling/sunburn in the area of trauma

Day 3 - boil-like blisters and smelly pus

Day 4 - gangrene

33
Q

How can bacteria pass on the ability to express virulence factors and toxins?

A

Via horizontal gene transfer

34
Q

What is the treatment of choice for staph aureus?

A

Flucloxacillin

35
Q

What is the treatment of choice for strep pyogenes?

A

Penicillin

36
Q

What defences does the skin have against infection?

A
  • Intact keratin layer
  • Dry surface dries out microorganisms
  • Sebum inhibits bacterial growth
  • Commensal bacteria
  • Competition between bacterial flora
37
Q

In what ways can a bacterial infection be diagnosed?

A
  • Clinical diagnosis (no test required)
  • Swap of lesion if the surface is broken
  • Pus or tissue culture if deeper lesion
  • Blood cultures
38
Q

What is the most common skin infector?

A

Staph aureus

39
Q

Coagulase negative staph (e.g., s epidermis) are usually pathogenic/commensal bacteria

A

Commensal

40
Q

Name 3 bacteria that may be commensal skin bacteria

A

Staph epidermis
Corynebacterium sp (aka diphtheroids)
Propionibacterium sp

41
Q

List 8 skin presentations of a staph aureus infection

A
  • Boils
  • Carbuncles
  • Minor skin infection e.g., infected cut
  • Cellulitis
  • Infected eczema
  • Impetigo
  • Wound infection
  • Staphylococcal scalded skin syndrome
42
Q

What are the treatment options for MRSA infection?

A
  • Oral doxycycline
  • Vancomycin
  • Co-trimoxazole
  • Clindamycin (beware of C. diff)
43
Q

Staph and strep are both aerobic and facultatively anaerobic. True/false

A

True

44
Q

Name a toxin produced by beta-haemolytic strep sp that causes damage to tissues

A

Haemolysin

=blood lysis

45
Q

List 5 skin presentations of strep pyogenes (GAS) skin infection

A 
Infected eczema
Impetigo
Cellulitis 
Erysipelas
Necrotising fasciitis
46
Q

Alpha-haemolytic strep don’t cause skin infections as commonly as beta-haemolytic. Which 2 alpha-haemolytic strep are most common if they do infect the skin?

A

Strep pneumoniae (pathogen, most common cause of pneumonia)

Strep viridans (commensals of mouth, throat & vagina, cause infective endocarditis)

47
Q

What are erysipelas?

What do they look like?

A

Bacterial infection of the superficial layers of the skin (most often caused by GAS/strep pyogenes)

They appear as a fiery red rash with raised edges

48
Q

What is the treatment of choice for necrotising fasciitis?

A

Immediate surgical debridement and antibiotics

49
Q

What is a common presentation of necrotising fasciitis? Why is this the case?

A

Excruciating pain out of proportion to the redness of the skin

This is because the infection is spreading along the fascia below the skin

50
Q

Swabs should only be taken from leg ulcers (caused by vascular problems) if there are signs of…

A

Cellulitis or infection

51
Q

What are the main fungal skin pathogens?

A
  • Candidiasis
  • Tinea (dermatophytoses)
  • Pityriasis versicolor
  • Emboli
52
Q

Candida is an intertrigo. What does this mean?

A

It causes infection in skin folds where the area is warm and moist

53
Q

How is candida infection diagnosed and treated?

A

Diagnosis: swab for culture

Treatment: clotrimazole cream, oral fluconazole

54
Q

Tinea infections are dermatophytoses. What does this mean?

A

Ringworms

55
Q

Describe how dermatophytes infect a person

A
  • Fungus enters abraded/soggy skin
  • Hyphae spread in stratum corneum, infecting keratinised tissues only (skin, hair, nails)
  • Increased epidermal turnover causes scaling
  • Inflammation occurs
  • Hair follicles are invaded
  • Lesion grows outwards and heals in the centre, giving it a ringed appearance
56
Q

State which part of the body is infected in the following fungal infections:

Tinea capitis 
Tinea barbae
Tinea corporis
Tinea manuum
Tinea unguium
Tinea cruris
Tinea pedis
A

Tinea capitis = scalp

Tinea barbae = beard

Tinea corporis = body

Tinea manuum = hand

Tinea unguium = nails

Tinea cruris = groin

Tinea pedis = foot (athletes foot)

57
Q

The most likely source of dermatophyte infection is…

A

Other infected humans e.g., swimming pool

58
Q

What is the most common dermatophyte causal organism?

A

Trichophyton rubrum

59
Q

How can a dermatophyte infection be diagnosed?

A
  • clinical appearance
  • Wood’s light
  • microscopy and culture of skin scrapings, nail clippings and hair
60
Q

How can dermatophyte infections be treated?

A

Skin and nails:

  • Clotrimazole cream
  • Topical nail paint

Scalp:

  • Oral terbinafine
  • Oral itraconazole
61
Q

Name 2 common skin parasites

A
  • Scabies

- Lice

62
Q

What is Scabies?

A

An intensely itchy rash caused by the parasite Sarcoptes scabiei

It affects the finger webs, wrists and the genital area where the scabies burrow

63
Q

What is the incubation period for scabies?

A

Up to 6 weeks

64
Q

What is the first-line treatment for Scabies?

A

Malathion lotion

Applied overnight to the whole body and washed off the next day

65
Q

Malathion lotion is used to treat Scabies and…

A

Lice

66
Q

The main symptom of lice is usually…

A

Intense itching in the head/body/pubic area

67
Q

Patients with the following infections need single room isolation and contact precautions…?

A
  • Group A Strep
  • MRSA
  • Scabies
68
Q

List the most common viral skin infections

A
  • Herpes simplex virus
  • Human papillomavirus
  • Poxvirus
  • Varicella-zoster virus
  • Viral exanthems
69
Q

Why are viral infections spread so rapidly?

A

They are associated with an incubation period so affected individuals are infectious before they are symptomatic

70
Q

List 3 erythematous exanthems that tend to start on the face

A

Measles
Rubella
Erythema infectiosum

71
Q

Why should you isolate a patient who reports a rash that started on the face?

A

It could be measles

The patient can come out of isolation if their results are negative upon testing

72
Q

List 2 erythematous exanthems that tend to start on the trunk

A

Roseola

Scarlet fever

73
Q

List 2 papulo-vesicular exanthems

A

Chickenpox (varicella)

Gianotti-Crosti syndrome

74
Q

Name an exanthem of the extremities

A

Hand, foot and mouth disease

75
Q

List 6 possible skin manifestations of COVID-19

A
  • Morbilliform (measle-like, starts in the face then spreads)
  • “COVID toe” (chillblain-like)
  • Lace-like purpura
  • Urticaria
  • Vesicular blisters
  • Multisystem inflammation in children

LR Dermatology (11 decks)

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