Skin and soft tissue infection

Question 1

What is included in soft tissues?

Answer 1

• Skin commonly infected at various levels
• connective tissues:
  
  • Fascia (rarely infected e.g. necrotising fasciitis) and synovial membranes (infective synovitis or bursitis).
  • also fat, tendons and ligaments (very rarely infected)
• Also muscle (e.g. bacterial or viral myositis)
• nerves (herpes, poliomyelitis)
• blood vessels ( very rarely infected e.g. syphilis, typhus, viral vasculitis, endocarditis and vascular prosthesis infection).

Question 2

Definitions:

Impetigo

Erysipelas

Answer 2

Impetigo = superficial skin infection that only affects the epidermis, usually staphylococcus.

Erysipelas = superficial skin infection that only affects the epidermis byt it tends to be streptococcal

Question 3

Defnitions:

Cellulitis

Answer 3

Cellulitis = skin infection that gets into the dermis and possible into subcutaneous fat (but no further than that).

Most common can be caused by staphylocci or streptococci, or immune supression.

Question 4

Definitions:

Furuncle

Carbuncle

Answer 4

Furuncle = deep infection of the hair follicle = Boil (usually straphyloccoccal)

Carbuncle = connecting collection of furuncles (usually staphylococcal).

Question 5

Definitions:

Necrotising fasciitis

Answer 5

Necrotising fasciitis = deep infection that enters below epidermis/ dermis/ subcutaneous fat and into the fascia, with or without muscle involvement. Necrotising in nature, aggressive and lifethreatening.

Usually streptococcal or mixed bacteria if infection arises from the pelvis

Question 6

Aetiology of skin/ soft tissue infections?

Answer 6

1. Viruses
   
   • viral warts (skin warts, verrucas, genital warts)
   • herpes (labialis HSV 1 and HSV 2 genitalis)
   • viral exanthems (exanthem = rash) (chickenpox, shingles, measles).
   • Molluscum contagiosum (pox virus)
   • small pox
2. Bacteria
   
   • ​​streptococcus and staphylococcus
   • opportunistic bacteria (e.g. immunocompromised, diabetic etc.. )
3. Fungi
   
   • ​Tinea infections (tinea pedis = athletes foot, tinea corporis = of the head)
   • Serborrhoeic dermatitis (dandruff, fungus plays key role in initating the process).
4. Protozoa, Hemlminths
5. Ectoparasites –> scabies and cutaneous myiasis

Question 7

Host microbe interactions

What interactions are there?

Answer 7

• We are not sterile surfaces with only occasional and always pathological interactions with microbes
• We are colonised by microorganisms, various interactions can occur:
  
  • Symbiotic interactions –> interaction between two organisms living in close physical association. Can be commensal, mutualistic or parasitic.
  • Commensal = symbiotic relationship between two different species where one derives some benefit and other is unaffected
  • Colonisation = when microbe grows on/ in another organism without causing disease
  • Pathogen = microbe able to cause disease

Question 8

Define infection

Answer 8

Infection = invasion and multiplication of microbes in an area of the body where they are not normally present, usually leads to disease

Question 9

Describe the host- microbe interactions that can occur on the skin:

What organisms are involved?

Answer 9

• Skin bacteria can be either commensals or pathogens e.g. Staphylococcus species of bacteria
• Staph. epidermidis and other staph species that are not staphylococcus aureus are usually called the coagulase negative staphylococci:
  
  • They lack an enzyme called coagulase and are therefore less aggressive towards humans
  • Colonise the skin of nearly all humans are are usally non pathogens
  • Unless they are able to reach an internal prosthetic surface where they can cause infection
• Staphylococcus aureus = normally considered a pathogen, but can be commensal, has the potential to become a pathogen.
• If colonised, risk of infection with staph. aureus increases. Especially if poor hygiene/ abnormal skin / immunocompromised
• Still needs to invade and multiply to cause infection and so may remain as commensal bacteria
• Hence “de- colonisation” treatment to sterilise skin may be beneficial prior to operation.

Question 10

What is the pathogenesis of bacteria in skin and soft tissue infections?

Answer 10

AH AH Its MERDT

• Access –> most bacteria causing SSTI found on skin as colonisers
• Adherence –> have well developed adhesin molecules
• Invasion -> most need a break in the skin, but some are highly invasive (PVL staph. aureus, aggresive staph aureus, Group A streptococcus more aggresive streptococcus.)
• Multiplication –> colonisation of skin or wounds may precede infection
• Evasion –> e.g. Staph catalase enzyme blocks free radials produced by neutrophils, Strep .M protein blocks complement
• Resistance –> may bacteria causing SSTI have drug resistance (particularly methicillin resistant staph. aureus - MRSA)
• Damage –> affects epidermis, dermis and deeper tissue and may lead to septicaemia
• Transmission –> easily passed from skin by direct or indirect contact

Question 11

What are some of the risk factors of skin and soft tissue infections?

Answer 11

• Direct inoculation e.g. trauma, medical procedures or skin ulcers
• Previous colonisation e.g. poor hygiene and Staph. aureus or MRSA
• Immunosuppression e.g. diabetes mellitus or renal failure.

Question 12

How do we clinically describe a lesion?

Answer 12

• Shape and size
• edge
• colour
• surface -> any breaks, vesicles etc
• distribution

Question 13

How do we diagnose skin and soft tissue infections?

Answer 13

• Full History ->
• Examination –> is this a localised infection, look for sepsis
• Differentials and investigations –> body samples, imaging or physiology and histopathology. Body sample not always needed e.g. skin infection better to give empirical treatment
• Diagnosis and then treatment

Question 14

What investigations can be done in skin and soft tissue infection?

Answer 14

• Swabs with furuncles and pus on the skin, or nore and throat if suspecting staphylococcus aureus colonisation
• Body fluids -> pus, vescile fluid or blood
• body tissue biposy but rare

Question 15

Treatment of SSTIs

viral

bacterial

Answer 15

• Viral skin infections are usually self limiting but ACICLOVIR and related drugs can be used for herpes, chickenpox and shingles
• bacterial skin infections are rarely self limiting but:
  
  • Superificial infections e.g. impetigo can be treated with topical fuscidin
  • Staphyloccocus aureus is usually resistant to antibiotics (MRSA), therfore sampling recommedned and flucoxacillin often used initally
  • Streptococcus rarely resistant to antibiotics therefore penicillin is recommended (benzylpenicillin injected (penicillin g) , phenoxymethylpenicillin oral version (penicillin V) ).
• If undecided if strep or staph, often used combination treatment
• Opportunistic bacterial infections (diabetic ulcer) often polymicrobial , with anaerobic organisms –> sampling recommended
  
  • Co- amoxiclav used initially
• Fungal infections usually superifical and rarely resistant
  
  • Topical terbinafine often used, can be oral if needed

Question 16

What should be considered when choosing antibiotics?

Answer 16

Consider both the organism and the patient

Known organism? –> use sensitivies and resistance profile

Or most likely organism and the expected sensitivity and resistance

Take into account patient factors:

• Allergies
• renal and liver function
• severity of infection or immunocompromisation
• risk of antibiotic associate infection
• route of admin
• interactions with other medication
• age and ethnicity
• pregancy or breast feeding
• oral contraception

Then choose antibiotic, route and duration