Skin Infections (Cellulitis & Impetigo)
Bacterial skin infections — recognition, management and red flags
Bacterial skin infections are a common reason for GP and emergency department visits. Cellulitis and impetigo are the two most frequent types, and while most cases respond well to treatment, it is important to recognise the warning signs that indicate a more serious infection requiring urgent attention.
What are Bacterial Skin Infections?
The skin is the body's first line of defence against infection. When the skin barrier is broken — through cuts, insect bites, eczema, athlete's foot, or other conditions — bacteria can enter and cause infection. Cellulitis is a bacterial infection of the deep layers of the skin and the tissue beneath it, causing redness, warmth, swelling, and tenderness. It most commonly affects the lower legs but can occur anywhere on the body. Impetigo is a highly contagious superficial skin infection that is particularly common in children, presenting as honey-coloured crusted sores or fluid-filled blisters. Erysipelas is a form of cellulitis affecting the upper layers of the skin, characterised by a sharply demarcated, bright red, raised area — often on the face or legs.
Causes & Risk Factors
Cellulitis is most commonly caused by Streptococcus pyogenes (Group A Strep) and Staphylococcus aureus. Impetigo is caused by the same bacteria, usually Staphylococcus aureus or Group A Strep. Tinea pedis (athlete's foot) is the most common entry point for cellulitis of the lower leg — treating the fungal infection is an important part of preventing recurrence. Other risk factors include skin conditions such as eczema and psoriasis (which disrupt the skin barrier), lymphoedema (impaired lymphatic drainage), obesity, diabetes (impaired immunity and circulation), venous insufficiency, previous cellulitis, and any wound or skin break. MRSA (meticillin-resistant Staphylococcus aureus) should be considered in recurrent or treatment-resistant cases.
Signs & Diagnosis
Cellulitis presents as a warm, red, swollen, and tender area of skin that spreads over hours to days. There may be a clear border between infected and normal skin, or the margin may be indistinct. Systemic features such as fever, chills, and feeling unwell indicate more severe infection. Impetigo presents as small red sores that quickly rupture, weep, and develop into characteristic honey-coloured or golden crusts, most commonly around the nose and mouth or on exposed skin. Diagnosis is clinical — based on the appearance, spread, and associated symptoms. Clinical evaluation includes assessing the extent of infection using validated severity tools, marking the border of redness with a pen to monitor progression, checking for systemic features (fever, tachycardia, hypotension), and taking a wound swab or blood cultures when infection is severe.
Management & Red Flags
Mild cellulitis can often be treated at home with oral antibiotics prescribed by a GP, elevation of the affected limb, and regular review. Marking the border of redness with a skin pen and photographing it allows you to monitor whether the infection is spreading. Impetigo is treated with topical antiseptic cream or topical antibiotics for mild cases, with oral antibiotics reserved for more extensive infections. Children with impetigo should stay away from school until 48 hours after starting treatment or until the sores have dried and crusted. Seek urgent medical help if: the redness is spreading rapidly beyond the marked borders, you develop a high fever or feel very unwell, the skin becomes painful out of proportion to its appearance (this can indicate a rare but life-threatening deep tissue infection called necrotising fasciitis), or there is blistering, skin breakdown, or purple discolouration over the affected area.