Skin Infections Flashcards
What is the structure of skin?
The human skin can be generally divided into
three sections:
❖ Epidermis,
❖ Outermost layer,
❖ Dermis in the middle,
❖ Hypodermis (or subcutaneous tissue)
Functions of skin
Epidermis
Immune defences:
o Filters out UV damage
o Produces pigmentation
o Defence against
pathogens
o Moderates temperature
Functions of skin
Dermis
o Pliability
o Tensile strength
o Elasticity
Functions of skin
Hypodermis
Insulation
o Calorie reserve
o Cushioning and shock
absorption
What is normal skin flora?
Normal flora, mostly commensal microorganisms:
❖ Staphylococcus aureus
❖ Staphylococcus epidermidis
❖ Propionibacterium
❖ Streptococci spp.
❖ Candida albicans (yeast)
➢ Can become pathogenic if skin barrier is broken by injury, burn, insect bite or surgery,
or if the immune system becomes suppressed
Normal skin defence?
Colonisation is limited by:
❖ Limited moisture availability.
❖ Acid pH.
❖ Surface temperature.
❖ Salty sweat.
❖ Excreted chemicals – sebum/fatty acids.
❖ Competition
What is the pathogenesis of skin infections?
Skin infections can be caused by viruses, bacteria, fungi, or parasites.
▪ Bacteria cause cellulitis, impetigo.
▪ Viruses cause herpes simplex.
▪ Fungi cause athlete’s foot and yeast infections.
▪ Parasites cause body lice, head lice.
Infections may arise from pathogens that gain entry through damaged skin or via haematogenous spread (meningococcal skin rash).
Bacterial toxins produced by infections at other sites may also result in skin changes
What are Enzymatic Bacterial Defences?
Hyaluronidase
Staphylokinase
➢ Lipase
Β-Lactamase (penicillinase)
What are Hyaluronidase?
breaks down hyaluronic acid, major component of
extracellular matrix, enables bacteria to spread between cells in the body
What are Staphylokinase
digests blood clots once nutrients and space become
limiting to spread to new areas of the body, only produced by S. aureus.
What are Lipase
digests lipids, allows growth on skin surface and cutaneous oil glands
What are Β-Lactamase (penicillinase)
breaks down beta-lactam antimicrobial drugs,
such as penicillin and cephalosporin. Present in over 90% of S. aureus strains.
What is cellulitis?
Spreading skin infection of the subcutaneous layer.
❖ Results in swelling (oedema), redness (erythema),
tenderness & pain.
❖ Common causes are S. aureus (50%) and S.
pyogenes.
❖ Originate from skin lesions such as boils and ulcers.
❖ Can infect the bloodstream in extreme cases.
❖ Initial treatment should therefore be focused on
treating S. aureus and S. pyogenes.
What are the risk factors for cellulitis?
Immunocompromised patients
Oedema caused by venous insufficiency
Lymphoedema: Lymphatic obstruction resulting in chronic swelling of the arms or legs
History of cellulitis: Having had cellulitis before increases the risk of getting it again
Diabetes and obesity
How is cellulitis diagnosed?
Medical History:
▪ Symptoms experienced, including duration and severity
▪ Recent trauma to the skin. For example, a bite, burn, or laceration
▪ Underlying comorbidities
physical exam
Blood sample: levels of creatinine, bicarbonate, creatine phosphokinase, and C-reactive
protein (CRP).
Skin swab/aspirate for bacterial culture
If cellulitis is diagnosed, categorise the severity using the Eron classification system (1-4)
Treatment of cellulitis?
Oral Antibiotics: 10 to 21 days
Painkillers
IV Antibiotics
Surgery
When choosing antibiotics: take account of the severity of symptoms, the site of infection,
the risk of uncommon pathogens and any microbiological results and MRSA status if known
Managing underlying conditions such as diabetes, venous insufficiency, eczema and
oedema
What is impetigo?
Infection of the epidermis.
Mainly affects children.
Transmitted via skin contact.
Secondary spread as a result of scratching.
Main cause is S. pyogenes but also S. aureus.
Mostly no treatment required but antibiotic creams administered to help prevent spread of infection.
What is folliculitis?
inflammation of hair follicle
What is boil/furuncle?
If pimples spread and abscess is visible on
the outside of skin, then it is a
boil/furuncle
What is furnuncle?
If furuncle spreads deeper, then it
becomes a carbuncle
Features of folliculitis, furuncles and carbuncles
Commonly caused by Staphylococcus aureus
Perianal lesions are often caused by a mixture of faecal microorganisms (including
Escherichia coli , Pseudomonas aeruginosa , anaerobes and the Streptococcus
anginosus group)
Pain and tenderness increase with pressure
Most will mature and emerge into a lesion that discharges yellowish pus
May produce fever and elevation of WBC count
