Purulent skin infections
Skin infections sorted by the layer they reach: impetigo in the epidermis, boils down the hair follicle, erysipelas and cellulitis in the dermis, necrotising fasciitis along the fascia, and the staphylococcal toxin that splits the epidermis.
By the end, you can
- Relate each purulent skin infection to the layer of skin it involves, from the stratum corneum to the fascia.
- Compare how Staphylococcus aureus and Streptococcus pyogenes use toxins and enzymes to stay localised or to spread.
- Explain how exfoliative toxin splits the epidermis in bullous impetigo and in scalded skin syndrome, and why the two differ.
- Explain why necrotising fasciitis causes pain out of proportion to the skin and spreads so fast.
- Recognise the classic appearances, classifications and tests of skin infections, and explain how drainage, antibiotics and debridement act on the mechanism.
What it is
Purulent (pyogenic) skin infections are bacterial infections of the skin and the soft tissue beneath it, caused mostly by Staphylococcus aureus and Streptococcus pyogenes (group A streptococcus). They are best understood by depth: impetigo lies in the epidermis; folliculitis, furuncles and carbuncles follow the hair follicle; abscesses collect in the dermis and subcutis; erysipelas and cellulitis spread through the dermis and fat; and necrotising fasciitis runs along the fascia. Some of their toxins act far from the infection itself.
4 types to tell apart
- Superficial (epidermal)
- Impetigo, non-bullous or bullous, confined to the epidermis and healing without a scar; ecthyma is its deeper, ulcerating form.
- Purulent (localised)
- Folliculitis, furuncle, carbuncle and skin abscess: pus collects and is walled off. Mostly S. aureus, including MRSA.
- Non-purulent (spreading)
- Erysipelas, most cellulitis and necrotising fasciitis: infection spreads through tissue with little pus. Mostly streptococci; necrotising fasciitis can also be a mixed infection.
- Toxin-mediated
- Staphylococcal scalded skin syndrome and toxic shock syndrome: toxins from a small or hidden focus act on the whole skin or the whole body.
Normal function
Skin layers as barriers, and the gaps between them
Each layer of the skin resists bacteria in its own way. The is a dry, slightly acidic wall of dead, keratin-filled cells that is constantly shed. Beneath it the living keratinocytes of the and layers are riveted together by desmosomes, whose adhesion proteins include desmoglein 1, most abundant in the upper epidermis, and desmoglein 3, mainly in the lower layers.
The and hold blood vessels, lymphatics and resident immune cells. Below lies the , crossed by that carry vessels and nerves up from the deep fascia over the muscles. Each layer is a plane along which infection can spread once it gets in.
The barrier has natural gaps. Every is a warm, moist, oily pocket of epidermis reaching deep into the dermis, where bacteria can settle. Many healthy people carry S. aureus in the nose and on the skin, and streptococci live on the skin and in the throat. Infection usually needs a break in the barrier: a scratch, an insect bite, eczema, a fungal crack between the toes or a leg ulcer.
| Feature | Layer involved | Usual organism | Hallmark |
|---|---|---|---|
| Non-bullous impetigo | Beneath the stratum corneum | S. aureus, S. pyogenes or both | Honey-coloured crusts |
| Bullous impetigo | Split at the granular layer | S. aureus making exfoliative toxin | Flaccid blisters containing bacteria |
| Ecthyma | Through the epidermis into the dermis | Streptococci, staphylococci | Punched-out ulcers that scar |
| Folliculitis | Upper hair follicle | S. aureus; Pseudomonas after hot tubs | Pustule pierced by a hair |
| Furuncle and carbuncle | Dermis round one follicle; several linked through the fat | S. aureus | A pointing nodule; several draining openings |
| Erysipelas | Upper dermis and its lymphatics | S. pyogenes (group A) | Raised, shiny plaque with a sharp edge |
| Cellulitis | Deeper dermis and subcutaneous fat | Streptococci; S. aureus if purulent | Flat redness with an ill-defined edge |
| Necrotising fasciitis | Subcutaneous fat and the fascia beneath | Mixed (type I) or group A streptococcus (type II) | Pain out of proportion; dusky skin |
| Staphylococcal scalded skin syndrome | Granular layer everywhere, via the blood | S. aureus exfoliative toxin | Tender skin that shears off; sterile blisters |
Schematic; organisms overlap, and deeper infections tend to cause more systemic illness.
Underlying change · Step 1 of 6
Two cocci, two strategies: stay put or spread
The full lesson is part of Student Pro
It takes you through all 6 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 2 of 6
In the epidermis: impetigo and the toxin that splits it
The full lesson is part of Student Pro
It takes you through all 6 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 3 of 6
Down the hair follicle: folliculitis, boils and carbuncles
The full lesson is part of Student Pro
It takes you through all 6 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 4 of 6
Through the dermis and fat: erysipelas and cellulitis
The full lesson is part of Student Pro
It takes you through all 6 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 5 of 6
Along the fascia: necrotising fasciitis
The full lesson is part of Student Pro
It takes you through all 6 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Consequences · Step 6 of 6
Toxins at a distance: scalded skin and toxic shock
The full lesson is part of Student Pro
It takes you through all 6 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Symptoms and signs
Symptoms, signs and findings
The full lesson is part of Student Pro
It takes you through all 6 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Exam detail
Exam detail
The full lesson is part of Student Pro
It takes you through all 6 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Treatment
How treatment works
The full lesson is part of Student Pro
It takes you through all 6 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Recap
Recap
The full lesson is part of Student Pro
It takes you through all 6 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Self-check
Check your understanding
The full lesson is part of Student Pro
It takes you through all 6 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .