Bacterial skin infections are common presentations to both general practice and the emergency department.

The optimal treatment for purulent infections such as boils and carbuncles is incision and drainage. Antibiotic therapy is not usually required.

Most uncomplicated bacterial skin infections that require antibiotics need 5–10 days of treatment.

There is a high prevalence of purulent skin infections caused by community-acquired (non multiresistant) methicillin-resistant Staphylococcus aureus. It is therefore important to provide adequate antimicrobial coverage for these infections in empiric antibiotic regimens.

MRSA methicillin-resistant Staphylococcus aureus

Impetigo is a superficial bacterial infection that can develop either through direct invasion of normal skin (primary) or infection at sites of damaged skin (secondary) (Fig. 1). It is common in children and is highly contagious. There are two forms:

• non-bullous or crusted impetigo – distinct yellow, crusting lesions that may be itchy. Typically involves face or extremities
• bullous impetigo – usually caused by Staphylococcus aureus. Presents as bullae that rupture to form a brown crust.

Both cellulitis and erysipelas manifest as spreading areas of skin erythema and warmth. Localised infections are often accompanied by lymphangitis and lymphadenopathy. Not infrequently, groin pain and tenderness due to inguinal lymphadenitis will precede the cellulitis. Some patients can be quite unwell with fevers and features of systemic toxicity. Bacteraemia, although uncommon (less than 5%), still occurs.

Erysipelas involves the upper dermis and superficial lymphatics. Skin lesions are usually raised with a clear demarcation of infected skin. Classically, erysipelas affects the face (Fig. 2), but it can also involve other areas such as the lower limb. It is most commonly caused by Streptococcus pyogenes (group A streptococcus).

Cellulitis is usually caused by either S. aureus or beta-haemolytic streptococci (groups A, B, C or G). Differentiating between these two organisms can help guide therapy. Streptococcal infection is usually characterised by acute onset of rapidly spreading erythema, lymphangitis and lymphadenopathy. Staphylococcal cellulitis is usually associated with purulent lesions with erythema. Cultures from wounds or blood can further help delineate the causative organism. In the absence of positive cultures however, it can be difficult to discriminate between the two and antibiotic therapy to cover both organisms (for example flucloxacillin, dicloxacillin, cephalexin, clindamycin) is often used.

Diagnostic approach to cellulitis

When evaluating a patient with cellulitis, review systemic features. Potential portals of entry for infection should also be looked for. These include:

• disruption to the skin barrier, insect bites, wounds, abrasions 
• pre-existing skin infection, tinea pedis, impetigo
• underlying skin disease, eczema, psoriasis
• lymphoedema or surgical disruption of the lymphatic or venous system
• peripheral vascular disease with impaired arterial supply
• chronic venous insufficiency.

It is important to consider less common causes of skin infection associated with specific clinical circumstances or exposures (Table 2). In these cases, specimens should be collected for culture and sensitivity testing and treatment regimens broadened to cover likely pathogens. In difficult-to-treat or atypical infections, specialist opinion is recommended.

Many conditions may masquerade as cellulitis (see Box 1). These conditions should always be considered in atypical cases to avoid the unnecessary use of antibiotics.

Necrotising skin infections, the best known of which is necrotising fasciitis, are a medical and surgical emergency that require prompt debridement and appropriate intravenous antibiotics. Infections can be caused by single or multiple pathogens (e.g. S. pyogenes, Gram negatives, Clostridium).

Infection usually involves the necrosis of underlying soft tissues or muscle. Typical early clinical features are induration and erythema of the affected area with pain out of proportion to overlying skin changes. As infection progresses, the skin can change colour to purple or blue and eventually breaks down to form bullae and gangrene (Fig. 4). The patient is usually quite unwell with systemic toxicity, haemodynamic instability and multi-organ failure. 

Urgent hospital referral is essential in all cases. Surgical exploration is the only way to establish the diagnosis of necrotising fasciitis and is also the definitive management in all cases. Exploration also allows material to be obtained for appropriate cultures to guide antibiotic therapy.

Patients with severe disease who are systemically unwell will require assessment in hospital for monitoring and intravenous antibiotics. Parenteral antibiotics can either be administered as an inpatient or through an Outpatient Parenteral Antibiotic Treatment or Hospital in the Home program. Factors that would favour hospital management of cellulitis include:⁵

• comorbid conditions (renal impairment, diabetes, congestive cardiac failure, splenectomy) or immunosuppression
• rapidly progressive infection
• concern for deep space infection (presence of bullae, necrosis or muscle involvement)
• high fevers and rigors
• haemodynamic instability
• suppurative wound or bite (especially on face or hand) requiring surgical drainage
• lack of systemic or local response to oral antibiotics, or rising or unchanging C-reactive protein concentrations despite adequate therapy
• positive blood cultures
• inability to tolerate or absorb oral antibiotics.

Recurrent cellulitis is extremely challenging. Each repeated episode of cellulitis can cause inflammation and disruption of the lymphatic system and subsequent lymphoedema. The affected limb is subsequently more prone to infection and a vicious cycle of cellulitis and limb swelling is established.

Treating the underlying cause of infection is the most important step in management. In cases of chronic lymphoedema and venous stasis, compression of the affected limb by bandaging or stockings helps to increase venous return and contractility of the lymphatic ducts, therefore decreasing swelling and cellulitis. Further supportive measures such as elevation of the limb may also confer symptomatic relief. For example in cellulitis of the leg, raising the foot higher than the hip with supportive cushions helps to reduce swelling and pain. Prophylactic long-term suppressive antibiotics offer symptomatic benefit and cost–benefit in cases of recurrent streptococcal cellulitis.6,7 Options include twice-daily oral penicillin or cephalexin.

For recurrent staphylococcal infections, decolonisation measures should be considered (Box 2).8 In difficult cases of recurrent infections despite prophylactic antibiotics, expert consultation with an infectious disease specialist is recommended.

Bacterial skin infections have a variety of presentations from localised, trivial infection to rapidly progressive infection with systemic toxicity and considerable mortality. It is important to be able to recognise and treat these infections in the community, and in cases of severe infection to refer the patient promptly for specialist care.

Conflict of interest: none declared