16th July 2026

Bacterial Infections

Impetigo

Impetigo is a superficial bacterial skin infection that primarily affects young children, though it can occur at any age. It is highly contagious and spreads through direct skin-to-skin contact or contaminated surfaces. The most common causative organisms are Staphylococcus aureus and Streptococcus pyogenes (Group A Streptococcus). 

There are two main forms:

• Non-bullous impetigo (more common): Starts as erythematous macules that evolve into vesicles and pustules, which rupture to form honey-coloured crusts. Lesions typically affect the face.
• Bullous impetigo: Caused by exfoliative toxin-producing S. aureus. It presents with flaccid bullae which rupture, leaving shallow erosions.

Risk factors include poor hygiene, minor skin trauma (e.g. insect bites), eczema or other dermatoses, hot and humid climates, and nasal carriage of S. aureus.

Diagnosis

Diagnosis is clinical. Swabs for culture and sensitivities are reserved for:

• Recurrent or resistant cases
• Widespread infection or suspected MRSA
• Outbreak settings

Management

• Localised, non-bullous impetigo:
  • Hydrogen peroxide 1% cream (first-line)
  • Topical fusidic acid or mupirocin if needed
• Extensive or bullous impetigo:
  • Oral flucloxacillin or clarithromycin

Children should also be excluded from school until lesions are crusted or 48h after antibiotics.

Erysipelas

Erysipelas is a superficial bacterial skin infection affecting the upper dermis and lymphatics. It is caused almost exclusively by Streptococcus pyogenes (Group A streptococcus)

It presents with sudden-onset fever, chills, and a well-demarcated, raised, bright red rash. The face and lower limbs are most commonly affected. Risk factors include breaks in the skin barrier, lymphoedema, obesity, and immunosuppression.

Diagnosis

Diagnosis is clinical. Blood cultures may be taken in systemic illness. Differential diagnoses include cellulitis, DVT, contact dermatitis, and drug reactions.

Management

• Oral phenoxymethylpenicillin (or flucloxacillin) for mild cases
• IV benzylpenicillin or flucloxacillin for systemic features
• Clarithromycin if penicillin-allergic

Cellulitis

Cellulitis is an acute infection involving the deep dermis and subcutaneous tissues. It typically presents with localised pain, erythema, warmth, and swelling, often with systemic signs like fever.

The most common pathogens are Streptococcus pyogenes and Staphylococcus aureus. Risk factors include diabetes, peripheral vascular disease, venous insufficiency, tinea pedis, and obesity.

Diagnosis

Diagnosis is clinical. Imaging may be considered if an abscess or necrotising fasciitis is suspected. Differentials include DVT, contact dermatitis, erysipelas, and stasis dermatitis.

Management

• Mild: oral flucloxacillin
• Severe or systemically unwell: IV flucloxacillin, consider MRSA cover if risk factors present
• Treat underlying risk factors

Necrotising Fasciitis

Necrotising fasciitis is a rapidly progressing infection of the fascia and subcutaneous tissues, causing tissue necrosis. It is a surgical emergency with high mortality.

There are two main types:

• Type I (polymicrobial)
• Type II (usually Group A Streptococcus)

Presentation includes extreme pain out of proportion to findings, systemic toxicity, skin discolouration, crepitus, and hypotension.

Diagnosis

CT or MRI may show fascial thickening and gas in soft tissues. Bloods show raised inflammatory markers. Surgical exploration is often diagnostic.

Management

• Urgent surgical debridement
• Broad-spectrum IV antibiotics: meropenem + clindamycin + vancomycin
• Supportive care in ICU if required