Microbiology of Skin Infections (Bacterial/Fungal/Parasitic)

Question 1

What is a furuncle?

Answer 1

AKA boil; a deep foliculitis (infection of the hair follicle) most commonly caused by Staph. aureus; this results in a painful swelling on the skin, of pus and necrotic tissue

Question 2

Appearance and environment of Staphylococcus sp. ?

Answer 2

Gram +ve cocci in CLUSTERS that are aerobic and facultatively anaerobic (they grow best aerobically but may grow anaerobically)

Question 3

2 important types of Staph. aureus?

Answer 3

Staph. aureus (coagulase +ve) - produces coagulase enzyme that clots plasma

Coagulase -ve Staph, e.g: Staph. epidermidis

Question 4

Colours of Staph colonies?

Answer 4

Staph. aurues (golden)

Coagulase -ve Staph (white)

Question 5

Which Staph. is resistant to Novobiocin?

Answer 5

S. saprophyticus; all other Staph. are sensitive

Question 6

Results of latex agglutination with different Staph. species?

Answer 6

+ve for Staph. aureus and -ve for a others

Question 7

Infections caused by Staph. aureus?

Answer 7

Wound, skin, bone and joint infections

Question 8

Toxins produced by certain strains of Staph. aureus?

Answer 8

Enterotoxin - food poisoning

SSSST - Staph. Scalded Skin Syndrome Toxin

PVL - Panton Valentine Leukocidin

Question 9

Normal role of coagulase -ve Staph. and infections caused by them?

Answer 9

Skin commensals (not usually pathogenic)

May cause infection in assoc. with prosthetic material (produce a slime), e.g: heart valves, catheters

Staph. saprophyticus causes UTIs in women of child-bearing age

Question 10

Appearance of Streptococcus species?

Answer 10

Gram +ve cocci in CHAINS that are aerobic and facultatively anaerobic

Question 11

Classification of Strep. species?

Answer 11

α-haemolysis (partial)

β-haemolysis (complete) - pathogenic organisms; produce many toxins that damage tissues, one being haemolysin

γ-haemolysis (non-haemolytic)

Question 12

Further classification of β-haemolytic Strep. ?

Answer 12

By antigenic structure on the surface:
Group A (throat and severe skin infections)
Group B (meningitis in neonates)

Question 13

2 important categories of α-haemolytic Strep. ?

Answer 13

Strep. pneumoniae (commonest bacterial cause of pneumonia)

Strep. “viridans” (commensals of the mouth, throat and vagina that rarely cause infection)

Question 14

Important γ-haemolytic Strep. (normal role and infections caused)?

Answer 14

Enterococcus species (E. faecalis, E. faecium)

Commensals of the bowel; commonest cause of UTI

Question 15

How does the skin defend against infection?

Answer 15

Intact skin is less likely to become infected

Dry surface allows dessication of microorganisms

Sebum (fatty acids) inhibits bacterial growth

Competitive bacterial flora

Concept of resident and transient flora

Question 16

Examples of competitive bacterial flora?

Answer 16

Staph. epidermidis

Corynebacterium sp. (diphtheroids)

Proprionobacterium sp.

Question 17

Bacterial skin infections caused by Staph. aureus?

Answer 17

Cellulitis
Infected eczema
Impetigo
Wound infection
Staph. scalded skin syndrome

Question 18

Bacterial skin infections caused by Strep. pyogenes (Group A Strep.)?

Answer 18

Infected eczema

Impetigo

Cellulitis

Erysipelas (acute infection of the upper dermis that is more superficial than cellulitis)

Necrotising fasciitis (infection resulting in necrosis of the soft tissues)

Question 19

How to diagnose the bacteria of a skin lesion?

Answer 19

Swab of the lesion if the surface is broken; if deeper lesion, pus/tissue

+/- blood cultures, e.g: fever, if appropriate

Question 20

Treatment of bacterial infections?

Answer 20

Minor skin sepsis may not require antibiotics

Staph. aureus (FLUCLOXACILLIN is the antibiotic of choice for sensitive strains)

Strep. pyogenes (penicillin or flucloxacillin)

Necrotising fasciitis (life-threatening and require immediate surgical debridement, as well as antibiotics)

Question 21

When should leg ulcer swabs be taken?

Answer 21

If signs of cellulitis or infection are present; organisms worth treating:
Strep. pyogenes (Group A Strep.) and other -haemolytic Strep.
Staph. aureus
Anaerobes (part. in diabetic patients)

Question 22

Different names of Dermatophyte infection (ringworm)?

Answer 22

Tinea capitis - scalp
Tinea barbae - beard
Tinea corporis - body
Tinea manuum - hand
Tinea unguium - nails
Tinea cruris - groin
Tinea pedis - foot
(athlete’s foot)

Question 23

Pathogenes of Dermatophyte?

Answer 23

Infects keratinised tissues only (skin, hair, nails)

Fungus enters abraded or soggy skin and hyphae spread in stratum corneum

Increased epidermal turnover causes scaling

Inflammatory response provoked (dermis) and the hair follicles and shafts are invaded

Lesion grows outward and heals in centre, giving a “ring” appearance

Question 24

Groups mainly affected by Dermatophyte?

Answer 24

Males more commonly affected

Scalp ringworm mainly affects children

Foot and groin ringworm mainly affects men

Question 25

Sources of Dermatophyte infections?

Answer 25

Other infected humans (most likely source)

Animals

Soil

Question 26

Dermatophyte causal organisms?

Answer 26

Trichophyton rubrum (human-human transmission)

Tricophyton mentagraphytes (human-human)

Microsporum canis (cats, dogs-humans)

Question 27

Diagnosis of dermatophyte infections?

Answer 27

Clinical appearance

Woods light (fluorescence)

Skin scrapings (from the SCALY EDGE of the lesion), nail clippings, hair can be sent to the lab for microscopy and culture

Question 28

Treatment of Dermatophyte infection?

Answer 28

For small areas of infection skin/nails - Clotrimazole (Canestan) cream or topical nail paint

Extensive skin infection , nail infection, scalp infections:
Terbinafine orally
Itraconazole orally

Question 29

Where does Candida cause skin infection?

Answer 29

In skin folds where there is warmth and moisture; this is seen under the breasts in females, groin areas, abdominal skin folds, nappy area in babies

Question 30

Diagnosis and treatment of Candida?

Answer 30

Swab for culture

Clotrimazole cream and oral fluconazole

Question 31

Describe scabies

Answer 31

Caused by Sarcoptes scabei; the chronic, crusted form is called “Norwegian Scabies” and is highly infectious

There is an incubation period of a up to 6 weeks; rash is intensely itchy and affects the finger webs, wrists, genital area

Question 32

Treatment of scabies?

Answer 32

Malathion lotion is applied overnight to the whole body and washed off the next day

Benzyl benzoate (avoid in children)

Question 33

Different types of lice?

Answer 33

Pediculus capitis (head louse)

Pediculus corporis (body louse) - Vagabond’s disease

Phthirus pubis (pubic louse)

Assoc. with intense itch

Question 34

Treatment of lice?

Answer 34

Malathion

Question 35

Why is infection control important?

Answer 35

Patients with exfoliative skin conditions shed huge numbers of skin scales & associated bacteria into the environment

Gram positive bacteria can survive in the environment because of their cell wall structure

These may then become a source of infection for other patients

Question 36

Which patients need single room isolation and contact precautions?

Answer 36

Patients with Group A Strep. infection

Patients with MRSA infection

Patients with Scabies (long sleeved gowns also required for Norwegian scabies)