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LR Dermatology > Microbiology > Flashcards

Microbiology Flashcards

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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
The ? system groups gram +ve streptococci depending on their cell wall antigens
Lancefield
26
How are Group A Strep (GAS) further subdivided?
According to M protein antigens on their surface
27
Name 3 skin infections caused by strep pyogenes/GAS
- Impetigo - Cellulitis - Necrotising fasciitis (progressively deeper infections)
28
What is impetigo?
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
29
What is the most common presentation of GAS in young children?
Impetigo
30
What is cellulitis? | What other symptoms may accompany it?
A deeper skin infection in the dermis but that is not associated with necrosis Can be accompanied by fever, rigours and nausea
31
What is necrotising fasciitis?
An invasive GAS disease (iGAS) known as 'the flesh eating bug' that rapidly destroys connective tissue of the fascia below the skin
32
Describe the rapid progression of necrotising fasciitis from day 0 (insult) to day 4
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
How can bacteria pass on the ability to express virulence factors and toxins?
Via horizontal gene transfer
34
What is the treatment of choice for staph aureus?
Flucloxacillin
35
What is the treatment of choice for strep pyogenes?
Penicillin
36
What defences does the skin have against infection?
- Intact keratin layer - Dry surface dries out microorganisms - Sebum inhibits bacterial growth - Commensal bacteria - Competition between bacterial flora
37
In what ways can a bacterial infection be diagnosed?
- Clinical diagnosis (no test required) - Swap of lesion if the surface is broken - Pus or tissue culture if deeper lesion - Blood cultures
38
What is the most common skin infector?
Staph aureus
39
Coagulase negative staph (e.g., s epidermis) are usually pathogenic/commensal bacteria
Commensal
40
Name 3 bacteria that may be commensal skin bacteria
Staph epidermis Corynebacterium sp (aka diphtheroids) Propionibacterium sp
41
List 8 skin presentations of a staph aureus infection
- Boils - Carbuncles - Minor skin infection e.g., infected cut - Cellulitis - Infected eczema - Impetigo - Wound infection - Staphylococcal scalded skin syndrome
42
What are the treatment options for MRSA infection?
- Oral doxycycline - Vancomycin - Co-trimoxazole - Clindamycin (beware of C. diff)
43
Staph and strep are both aerobic and facultatively anaerobic. True/false
True
44
Name a toxin produced by beta-haemolytic strep sp that causes damage to tissues
Haemolysin | =blood lysis
45
List 5 skin presentations of strep pyogenes (GAS) skin infection
``` Infected eczema Impetigo Cellulitis Erysipelas Necrotising fasciitis ```
46
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?
Strep pneumoniae (pathogen, most common cause of pneumonia) Strep viridans (commensals of mouth, throat & vagina, cause infective endocarditis)
47
What are erysipelas? | What do they look like?
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
What is the treatment of choice for necrotising fasciitis?
Immediate surgical debridement and antibiotics
49
What is a common presentation of necrotising fasciitis? Why is this the case?
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
Swabs should only be taken from leg ulcers (caused by vascular problems) if there are signs of...
Cellulitis or infection
51
What are the main fungal skin pathogens?
- Candidiasis - Tinea (dermatophytoses) - Pityriasis versicolor - Emboli
52
Candida is an intertrigo. What does this mean?
It causes infection in skin folds where the area is warm and moist
53
How is candida infection diagnosed and treated?
Diagnosis: swab for culture Treatment: clotrimazole cream, oral fluconazole
54
Tinea infections are dermatophytoses. What does this mean?
Ringworms
55
Describe how dermatophytes infect a person
- 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
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 ```
Tinea capitis = scalp Tinea barbae = beard Tinea corporis = body Tinea manuum = hand Tinea unguium = nails Tinea cruris = groin Tinea pedis = foot (athletes foot)
57
The most likely source of dermatophyte infection is...
Other infected humans e.g., swimming pool
58
What is the most common dermatophyte causal organism?
Trichophyton rubrum
59
How can a dermatophyte infection be diagnosed?
- clinical appearance - Wood's light - microscopy and culture of skin scrapings, nail clippings and hair
60
How can dermatophyte infections be treated?
Skin and nails: - Clotrimazole cream - Topical nail paint Scalp: - Oral terbinafine - Oral itraconazole
61
Name 2 common skin parasites
- Scabies | - Lice
62
What is Scabies?
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
What is the incubation period for scabies?
Up to 6 weeks
64
What is the first-line treatment for Scabies?
Malathion lotion Applied overnight to the whole body and washed off the next day
65
Malathion lotion is used to treat Scabies and...
Lice
66
The main symptom of lice is usually...
Intense itching in the head/body/pubic area
67
Patients with the following infections need single room isolation and contact precautions...?
- Group A Strep - MRSA - Scabies
68
List the most common viral skin infections
- Herpes simplex virus - Human papillomavirus - Poxvirus - Varicella-zoster virus - Viral exanthems
69
Why are viral infections spread so rapidly?
They are associated with an incubation period so affected individuals are infectious before they are symptomatic
70
List 3 erythematous exanthems that tend to start on the face
Measles Rubella Erythema infectiosum
71
Why should you isolate a patient who reports a rash that started on the face?
It could be measles The patient can come out of isolation if their results are negative upon testing
72
List 2 erythematous exanthems that tend to start on the trunk
Roseola | Scarlet fever
73
List 2 papulo-vesicular exanthems
Chickenpox (varicella) | Gianotti-Crosti syndrome
74
Name an exanthem of the extremities
Hand, foot and mouth disease
75
List 6 possible skin manifestations of COVID-19
- 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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