Skin & Soft Tissue Infections (SSTI)

Question 1

What tissues soft tissues of the body can become infected?

Answer 1

1. Skin at various levels (most common)
2. Connective tissues
3. Nerves e.g. herpes, leprosy and poliomyelitis
4. Muscle e.g. bacterial/viral myositis (rare)
5. Blood vessels e.g. syphilis, typhus, viral vasculitis, endocarditis and vascular prosthesis infection (rare)

Question 2

What connective tissue structures can become infected?

Answer 2

Widely spread:
- Fibrous tissues

Rarely infected:

• Fascia e.g. necrotising fasciitis
• Fat
• Tendons
• Ligaments
• Synovial membranes e.g. infective synovitis or bursitis (looks like joint infections due to close proximity)

Question 3

What is the difference between impetigo and erysipelas?

Answer 3

Both infections of epidermis but impetigo is usually caused by staphylococcus but erysipelas is normally caused by streptococcus

Question 4

What is cellulitis?

Answer 4

Skin infections of the dermis +/- subcutaneous fat either due to staphylococcus or streptococcus - can surround carbuncles/furuncles

Question 5

What is a furuncle?

Answer 5

Deep infection of a hair follicle i.e. a ‘boil’ usually caused by staphylococcus - can be surrounded by cellulitis

Question 6

What is a carbuncle?

Answer 6

Connecting collection of furuncles or ‘boils’ that is also usually caused by staphylococcus - can be surrounded by cellulitis

Question 7

What is necrotising fasciitis?

Answer 7

Deep infection of the fascia +/- muscle usually caused by streptococcus or mixed bacteria - can be caused by cellulitis getting deeper than the dermis and subcutaneous fat

Question 8

What viruses can cause skin and soft tissue infections?

Answer 8

• Viral warts: skin warts, verruccas, genital warts
• Herpes: herpes simplex 1 (oral) and 2 (genital), herpes labialis, herpes genitalis
• Viral exanthems: chickenpox, shingles, measles
• Molluscum contagiosum
• Small pox

Question 9

What bacteria cause skin and soft tissue infections?

Answer 9

• Streptococcus
• Staphylococcus aureus
• Opportunistic bacteria e.g. in diabetic foot ulcers

Question 10

What fungi cause skin and soft tissue infections?

Answer 10

• Tinea infections: tinea pedis (Athlete’s foot), tinea corporis (ring worm)
• Seborrhoeic dermatitis: dandruff

Question 11

What other organisms cause skin and soft tissue infections?

Answer 11

Protozoa: cutaneous leishmaniasis

Helminths: cutaneous larva migrans e.g. dog parasite crawling through skin (need anti-worm tablets)

Ectoparasites: scabies and cutaneous myiasis (e.g. botfly) that remain on superficial skin

Question 12

How should you clinically describe a skin lesion?

Answer 12

Shape
Size
Edge
Colour
Surface
Distribution

Question 13

What is the characteristic colour of an impetigo lesion?

Answer 13

Golden

Question 14

What are the 3 stages of chicken pox/shingles skin lesions?

Answer 14

1. Papule
2. Blister
3. Ulcer

Question 15

How can you get impetigo?

Answer 15

Poor hygiene

Rugby playing

Question 16

How do you treat carbuncles?

Answer 16

Antibiotics

Incision drainage surgery

Question 17

Where does necrotizing fasciitis tend to occur?

Answer 17

In limbs due to blunt trauma or pelvic area due to operation

Question 18

How do you treat necrotizing fasciitis?

Answer 18

Patient will be septic so surgically removing the skin on top of the fascia to keep patient alive (can sometimes be down to tendons and bones)

Question 19

What will ring worm lesions present like?

Answer 19

Round red mark
Itchy
Flaky

Question 20

What would cutaneous leishmaniasis skin infection present like?

Answer 20

Skin lesion with raised edge with ulcerated area in middle but not as painful as you’d expect (disappears with long-term antibiotics)

Question 21

Who is scabies most likely to effect? How would it present?

Answer 21

In people whose hygiene is compromised presenting with scabby, scaly skin and burrows and scabies can be noticed if you look close enough, intense itching in finger webs that’s worst at night and in hot baths so you need to improve hygiene of the patient along with topical solution

Question 22

What is a symbiotic relationship?

Answer 22

Close and often long-term interaction between 2 different species that can be mutualistic, commensal or parasitic e.g. commensal is where 1 species derives some benefit whilst the other is unaffected

Question 23

What is colonisation?

Answer 23

When a microbe grows on or in another organism w/o causing any disease

Question 24

What is an infection?

Answer 24

Invasion and multiplication of microbes in an area of the body where they are not normally present which usually leads to disease - may cause no symptoms, be sub-clinical or cause varying degrees of symptoms and be clinically apparent

Question 25

What is a pathogen?

Answer 25

Microbe able to cause disease

Question 26

What areas of the body are densely populated by microorganisms? What areas are sparsely populated or sterile?

Answer 26

Densely populated: - Oral cavity - Distal region of urethra - Vagina (mainly lactobacilli) - Colon - URT Sparsely populated: - Skin (gram +ve species) - Stomach Sterile: - Lungs - Internal tissues - Urinary tract

Question 27

How is the population of an area of the body in terms of microorganisms classified?

Answer 27

In aerobic organisms per cm squared: 10^2 = dry 10^7 = moist

Question 28

Cutaneous flora may be ___ or ___ (e.g. ___).

Answer 28

Resident Transient MRSA

Question 29

How is staphylococcus epidermidis normally classified along with other staphylococcus species (not S. Aureus)?

Answer 29

Commensal coagulase -ve bacteria that colonise the skin of nearly all humans and are usually non-pathogenic causing infections only if they reach internal prosthetic material

Question 30

How is staphylococcus aureus normally classified?

Answer 30

As a pathogen that may colonise skin esp. if a patient has poor hygiene or abnormal skin but still need to invade and multiply to cause infection so remain commensal until that happens - 'de-colonisation' treatment to sterilise skin may be beneficial

Question 31

What do cultures of staphylococcus aureus typically look like?

Answer 31

Golden in colour

Question 32

What is the pathogenesis of skin and soft tissue infections (SSTIs)?

Answer 32

1. Access: most bacteria already found on skin as colonisers 2. Adherence: well-developed adhesin molecules allow colonisation 3. Invasion: most need a skin break but PVLSA and GAS are highly invasive 4. Multiplication: colonisation of skin/wounds may precede infection 5. Evasion: staph catalase blocks free radicals whilst the strep M protein blocks complement 6. Resistance: many bacteria causing SSTIs have drug resistance e.g. MRSA 7. Damage: affects epidermis, dermis, deeper tissue and can lead to septicaemia 8. Transmission: easily passed from skin by direct or indirect contact

Question 33

What are the risk factors for skin and soft tissue infections?

Answer 33

``` Direct inoculation (e.g. trauma, medical procedures and skin ulcers) Previous colonisation (e.g. poor hygiene, S. Aureus or MRSA) Immunosuppression (e.g. DM and renal failure) ```

Question 34

What is PVLSA?

Answer 34

Panton Valentin Leukocidin Staphylococcus Aureus - particularly aggressive as it has a special chemical that kills WBCs so spreads through blood and organs rapidly with high mortality

Question 35

How should you diagnose a skin and soft tissue infection?

Answer 35

1. History: PC, HPC, PMH, ADT, FH, SH and ROS - check for risk factors 2. Exam: general, CVS, RS, abdo, NS, LMS, skin, etc. - check if its localised 3. Differential diagnosis 4. Investigations: bed side, body samples, imaging, physiology and histopathology 5. Diagnosis 6. Treatment

Question 36

What microbiology investigations can be carried out for a skin or soft tissue infection?

Answer 36

1. Swabs: pus (e.g. from boil but clean surface 1st then push more pus out) or skin - look for colonisation in nose and throat e.g. S. Aureus 2. Body fluids: pus, vesicle fluid and blood 3. Body tissues: biopsies rarely

Question 37

What information can you get from a swab?

Answer 37

Resistance/sensitivities to antibiotics (Meticillin used in lab as marker for multi-resistance) Genetic testing can tell you whether its VPL +ve

Question 38

How can you look for more deeply seated infections?

Answer 38

Radiology e.g. in necrotizing fasciitis you may see gas in soft tissues from GAS necrotic process and also in gas gangrene - DO NOT use to diagnose as clinical suspicion followed by antibiotics should occur straight away as patient will be contagious (put in side room) with the potential to colonize others and potentially infect if IS is dampened

Question 39

How should you treat viral skin and soft tissue infections?

Answer 39

Viruses: self-limiting but give Aciclovir if not for herpes, chickenpox and shingles

Question 40

How should you treat bacterial skin and soft tissue infections?

Answer 40

Bacteria: - Topical fuscidin for superficial infections e.g. impetigo - S. Aureus is usually resistant e.g. MRSA so sampling and Flucoxacillin initially - Streptococcus not normally resistant so penicillin - Opportunistic e.g. diabetic ulcers often polymicrobial so sampling and Co-Amoxiclav initially

Question 41

How should you treat fungal skin and soft tissue infections?

Answer 41

Usually superficial but rarely resistant to antimicrobials so topical Terbinafine often used which can be given orally if necessary

Question 42

If you are unsure of the cause of a bacterial skin and soft tissue infection, what would you give as treatment?

Answer 42

Put the patient on IV and oral antibiotics e.g. Benzylpenicillin (IV) and Phenoxymethylpenicillin (oral)

Question 43

When treating skin and soft tissue infections, what must you consider about the organism?

Answer 43

Is it a known organism +/- sensitivities and resistance profile? OR What is the most likely organism and their expected sensitivities/resistances?/

Question 44

When treating skin and soft tissue infections, what must you consider about the patient?

Answer 44

``` Allergy/intolerance Renal/liver function Severity of infection or immunosuppression Risk of antibiotic-associated infection Route of administration Interactions w/ other medicines Age or ethnic group Pregnant/breast-feeding or taking oral contraception ```

Question 45

When you have took into account the organism and patient, what can you then do?

Answer 45

Choose antibiotic, dose, route and duration (or consult guidelines)