Cellulitis is an acute bacterial infection of the skin.

Cellulitis refers to an acute bacterial skin infection that affects both the dermis and subcutaneous tissue. It may occur anywhere on the body and ranges form a self-limiting infection to severe necrotising infection.

Cellulitis is a very common condition. It can occur at any age, but is mostly seen in middle-aged to older adults and usually affects the lower limbs.

Aetiology & pathophysiology

Cellulitis is commonly caused by both Streptococcus and Staphylococcus species.

Cellulitis occurs due to disruption of the skin barrier (e.g. cut, abrasion), which allows entry of microorganisms. Both Streptococcus and Staphylococcus are common skin commensal organisms that can enter the skin and cause infection. 


  • Beta-haemolytic streptococcus (Groups A, B, C, G, F)
  • Staphylococcus aureus 
  • Less common: gram negative bacilli (e.g. Pseudomonas aeruginosa), Clostridium species

Microorganisms in specific cases

  • Pasteurella multocida: due to animal bites (e.g. dog or cat)
  • Streptococcus pneumoniae: typical in orbital cellulitis
  • Aeromonas hydrophila: following fresh water exposure
  • Mycobacterium marinum: in aquarium keepers

Risk factors

Anything that may affect the integrity of the skin barrier or impair venous/lymph drainage can lead to cellulitis.

  • Trauma
  • Leg ulceration
  • Fungal infection
  • Lymphoedema
  • Venous insufficiency
  • Obesity
  • Pregnancy

Clinical features

Cellulitis is characterised by the hallmarks of inflammation including pain, swelling, warmth and erythema.


  • Pain
  • Redness
  • Swelling
  • Fever
  • Malaise


  • Pain on palpation
  • Erythema
  • Skin warmth
  • Superficial bullae or blisters: may be present
  • Abscess: may complicate cellulitis. Collection of pus within dermis or subcutaneous space. Painful and fluctuant. 
  • Lymphadenopathy


Cellulitis is a clinical diagnosis based on the classic appearance of erythematous, warm and oedematous skin.

Laboratory investigations are often unnecessary if the patient is otherwise clinically well. They are more likely to be completed in patients presenting to secondary care (e.g. hospital)


This describes a form of cellulitis that involves the more superficial dermal structures. It is characterised by a raised, well demarcated border and usually occurs secondary to beta-haemolytic streptococcal infection. Treatment is also with antibiotics. 

Eron classification

The severity of cellulitis may be assessed using the Eron classification:

  • Class I: no signs of systemic toxicity. No uncontrolled co-morbidity (e.g. diabetes mellitus)
  • Class II: systemically unwell or systemically well but with a comorbidity 
  • Class III: significant systemic upset (e.g. tachycardia, tachypnoea), unstable co-morbidities or limb-threatening infection due to vascular compromise.
  • Class IV: sepsis or severe life-threatening infection (e.g. development of necrotizing fasciitis)

Differential diagnosis

Several infective and non-infective aetiologies may be confused with cellulitis.


  • Necrotising fasciitis: a severe skin infection that causes progressive destruction of the muscle fascia and overlying subcutaneous fat. It is a surgical emergency requiring urgent debridement.
  • Toxic shock syndrome: invasive group A streptococcal infection that may cause necrotising soft tissue infection. Due to the release of exotoxins that act as ‘superantigens’ and large release inflammatory cytokines.
  • Septic arthritis: joint infection. May be difficult to distinguish if cellulitis overlies a joint
  • Bursitis: inflammation of a bursa (fluid-filled sac). May present alongside cellulitis
  • Osteomyelitis: may be present with overlying cellulitis. 


  • Deep vein thrombosis: can also cause a painful, swollen, erythematous leg
  • Contact dermatitis: characteristically pruritic and no systemic signs of infection
  • Drug reaction: usually distinguished by typical location (trunk, proximal extremities) and erythematous maculopapular rash
  • Insect bite: may trigger local inflammatory reaction at punctured site. Alternatively, may be delayed skin reaction with features of local swelling, itching and erythema.
  • Lymphoedema: abnormal accumulation of interstitial fluid
  • Stasis dermatitis: inflammatory reaction due to chronic venous insufficiency


Often completed as part of the work-up in patients systemically unwell or where an alternative diagnosis is suspected.

If patients are clinically well then further investigations may not be needed. However, if patients are systemically unwell, at risk of deterioration, or suspected of having complications then investigations are warranted. 


  • Skin swabs: debrided material, open wound, obvious portal for bacterial entry
  • Blood glucose: patients with diabetes at increased risk


  • FBC
  • U&E
  • LFT
  • CRP
  • Blood cultures: < 10% of blood cultures are positive in cellulitis
  • HbA1c


  • Ultrasound: may be used to look for underlying abscess
  • X-ray: may be needed to assess for osteomyelitis


  • Dependent on suspected alternative diagnosis (e.g. d-dimer if suspected deep vein thrombosis)


The principle management of cellulitis is with antibiotics.

Patients with mild cellulitis (e.g. Eron I and some Eron II) may be treated in the community with oral antibiotics. Typical antibiotic choices are penicillin (e.g. phenoxymethylpenicillin) and/or flucloxacillin due to the high rate of streptococcal and staphylococcal infections. Erythromycin/clarithromycin are good alternatives in patients with penicillin allergy (local antibiotics guidelines should always be sought). 

Some patients may require hospital admission for assessment and intravenous antibiotics. These include:

  • Eron class III or IV
  • Severe, rapidly developing cellulitis
  • High-risk patient: very young (e.g. < 12 months old), frail, immunocompromised
  • Orbital/facial cellulitis: high risk of complications
  • Significant lymphedema (unlikely to resolve with oral antibiotics)
  • Suspected complications: e.g. septic arthritis or osteomyelitis

Intravenous flucloxacillin, clindamycin and/or vancomycin may be used as intravenous antibiotic choices. Clindamycin is particularly important in patients with suspected group A streptococcal infection as it suppresses bacterial exotoxin production (plus other beneficial effects). Clindamycin may be needed in combination with a beta-lactam antibiotic (e.g. penicillin, cephalosporin, carbapenem). 

In patients with severe cellulitis or systemically unwell, advice from microbiology is usually required. It is always recommended to follow local antibiotic guidelines.


Cellulitis may cause life-threatening sepsis or necrotising infections (e.g. necrotising fasciitis).

Acute complications

  • Necrotising infections (e.g. necrotising fasciitis)
  • Sepsis
  • Abscess: may require incision and drainage
  • Myositis: infection of muscle

Chronic complications

  • Persistent leg ulceration
  • Recurrent cellulitis
  • Lymphoedema: damage to lymphatic system

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