Microbiology of skin infections

Question 1

What do staphlococcus look like under a microscope?

Answer 1

Gram positive (purple) cocci in clusters

Question 2

Are staphlococcus aerobic or anaerobic bacteria?

Answer 2

Aerobic and facultatively anaerobic

Question 3

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

Answer 3

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

Question 4

In culture what colour does staph aureus appear?

Answer 4

Gold

Question 5

What kind of infections do staph. aureus commonly cause?

Answer 5

Wound, skin, bone and joint

Question 6

Name some toxins which certain strains of s.aureus produce

Answer 6

Enterotoxin
PVL
SSSST

Question 7

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

Answer 7

Flucloxacillin

Question 8

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

Answer 8

Infants

Question 9

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

Answer 9

Chronic abscesses (axilla and buttocks)

Question 10

Coagulase negative staph are usually skin commensals. T/F

Answer 10

True

Question 11

When might coagulase negative staph cause infections?

Answer 11

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

Question 12

What type of infections does staph. saprophyticus usually cause?

Answer 12

Urinary tract infection (woman mostly)

Question 13

What do streptococci look like under a microscope?

Answer 13

Gram positive (purple) cocci in chains

strep=strips

Question 14

Are streptococci aerobic or anaerobic?

Answer 14

Aerobic

Question 15

How are streptococci classified?

Answer 15

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

Question 16

Which class of steps are the most pathogenic?

Answer 16

Beta haemolytic (group A strep)

Question 17

When does group B strep cause infection?

Answer 17

Usually in neonates

Question 18

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

Answer 18

Haemolysin

Question 19

How is strep classified within the groupings?

Answer 19

Surface antigen structure

Question 20

What types of infection do group B streps usually cause?

Answer 20

Throat and skin infections

Question 21

What types of infection do group A streps usually cause?

Answer 21

Meningitis (neonates)

Question 22

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

Answer 22

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

Question 23

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

Answer 23

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

Question 24

Which features of the skin protect against infection?

Answer 24

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

Question 25

Name some competitive bacterial flora?

Answer 25

Staph. epidermidis, diptheroids, proprionobacterium

Question 26

What types of skin infections do staph.aureus cause?

Answer 26

- boils and carbuncles (hair follicles) - minor skin sepsis (infected cuts) - cellulitis - infected eczema - impetigo - wound infection - SSSS

Question 27

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

Answer 27

False - only carbuncles need treated

Question 28

Where do carbuncles most commonly occur?

Answer 28

At the base of the neck

Question 29

Which pathogens usually cause impetigo?

Answer 29

S.aureus or group A strep

Question 30

Where does impetigo present?

Answer 30

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

Question 31

In which age group is impetigo most commonly found?

Answer 31

Children

Question 32

How is SSSS treated?

Answer 32

Aggressive antibiotics and rehydration therapy

Question 33

What is another name for group A strep?

Answer 33

Strep pyrogenes

Question 34

Which types of skin infections do strep pyrogenes cause?

Answer 34

- infected eczema - impetigo - cellulitis - erysipelas - necrotising fasciitis (often along with other bacteria)

Question 35

How does cellulitis present?

Answer 35

Erythematous skin with no obvious breakage

Question 36

How does erysipelas present?

Answer 36

Butterfly pattern of facial swelling and erythema

Question 37

How does necrotising fasciitis present?

Answer 37

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

Question 38

How are bacterial skin infections diagnoses?

Answer 38

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

Question 39

How are bacterial skin infections treated?

Answer 39

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

Question 40

What is the pathogenesis of necrotising fasciitis?

Answer 40

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

Question 41

What are the two types of necrotising fasciitis?

Answer 41

Type 1 - mixed anaerobes and coliforms | Type 2 - group A strep

Question 42

When does Type 1 necrotising fasciitis usually occur?

Answer 42

Post abdominal surgery

Question 43

Which antibiotics is Type 2 necrotising fasciitis treated with?

Answer 43

Clindomycin and penicillin

Question 44

When should swabs be taken from leg ulcers?

Answer 44

If there are signs of infection/cellulitis

Question 45

Which organisms found in leg ulcers are worth treating?

Answer 45

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

Question 46

Tinea is another word for what?

Answer 46

Ringworm

Question 47

How does tinea capitis present?

Answer 47

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

Question 48

Why doe tinea corporis present as round lesions?

Answer 48

Fungus grows outwards from central infected point

Question 49

Is tinea cruris more common in men or woman?

Answer 49

Men (may grow on scrotum)

Question 50

How does tinea unguum present?

Answer 50

Yellow nails which flake off easily

Question 51

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

Answer 51

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

Question 52

What is the pathogenesis of dermatophyte infection?

Answer 52

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)

Question 53

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

Answer 53

True

Question 54

What is the epidemiology of dermatophyte infection?

Answer 54

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

Question 55

How do dermatophyte infections spread?

Answer 55

Human-human (most common), animals & soil

Question 56

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

Answer 56

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

Question 57

How are dermatophyte infections diagnosed?

Answer 57

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

Question 58

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

Answer 58

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

Question 59

What is the disadvantage of culturing dermatophytes?

Answer 59

It takes forever

Question 60

How does a culture of trichophyton rubrum appear?

Answer 60

White and fluffy on top and brown and orange below

Question 61

How are dermatophyte infections treated?

Answer 61

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

Question 62

Where does candida most commonly infect?

Answer 62

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

Question 63

How are candida infections diagnosed?

Answer 63

Swab for culture

Question 64

How are candida infections treated?

Answer 64

Clotrimazole cream, oral fluconazole

Question 65

What organisms is scabies caused by?

Answer 65

Sarcoptes scabiei

Question 66

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

Answer 66

Norwegian scabies. In immunocompromised patients

Question 67

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

Answer 67

Up to six weeks. Human to human

Question 68

How does scabies present?

Answer 68

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

Question 69

How is scabies treated?

Answer 69

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

Question 70

What are the different types of lice?

Answer 70

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

Question 71

How does lice infection present?

Answer 71

Intense itch

Question 72

When might someone contract pediculus corporus?

Answer 72

Homeless/sleeping on the streets

Question 73

What is the treatment of lice?

Answer 73

Malathion

Question 74

What are the infection control procedures needed in dermatology?

Answer 74

Gloves and aprons required for dressing changes, single room isolation

Question 75

Which patients need single room isolation?

Answer 75

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