What is the number one cause of cellulitis?

Number One Cause of Cellulitis

The number one cause of cellulitis is streptococci, particularly Group A Streptococcus (Streptococcus pyogenes), followed by other beta-hemolytic streptococci groups such as B, C, and G. 1 This is supported by multiple lines of evidence including serologic studies, immunofluorescent antibody techniques, and culture data from skin biopsy specimens.

Microbiology of Cellulitis

Primary Causative Organisms

• Streptococci: Most common cause of typical non-purulent cellulitis

  • Group A Streptococcus (S. pyogenes) is predominant
  • Groups B, C, and G streptococci also frequently implicated
  • Often present in macerated or fissured interdigital toe spaces 1

• Staphylococcus aureus: Less frequently causes cellulitis, except in specific circumstances:

  • When associated with underlying abscess
  • Following penetrating trauma
  • At injection sites (particularly in drug users)
  • MRSA is more common in purulent infections but rarely causes typical cellulitis 2, 3

Special Circumstances with Different Pathogens

• Animal bites:
  • Dog/cat bites: Pasteurella species (especially P. multocida) or Capnocytophaga canimorsus 1
• Water exposure:
  • Freshwater: Aeromonas hydrophila
  • Saltwater: Vibrio species (particularly V. vulnificus in warm climates) 1

Diagnostic Challenges

Understanding the causative organism is complicated by several factors:

• Blood cultures are positive in only 5% of cases 1
• Needle aspirations of inflamed skin yield variable results (5-40%) 1
• Punch biopsy specimens yield organisms in only 20-30% of cases 1
• Modern molecular techniques (PCR, pyrosequencing) have failed to definitively identify causative organisms by comparing infected and uninfected sites 4

This diagnostic uncertainty explains why empiric therapy targeting streptococci remains the standard of care despite limited microbiological confirmation.

Risk Factors and Predisposing Conditions

Cellulitis typically occurs when bacteria enter through disruptions in the skin barrier:

• Trauma (even minor)
• Preexisting skin infections (impetigo, ecthyma)
• Skin ulceration
• Fissured toe webs from maceration or fungal infection
• Inflammatory dermatoses (eczema)
• Lymphatic disruption (following surgeries like saphenous venectomy, axillary node dissection) 1, 5

Treatment Implications

The predominance of streptococci as the causative organism has important treatment implications:

• First-line therapy should target streptococci with beta-lactam antibiotics:

  • Cephalexin (500mg four times daily)
  • Dicloxacillin (500mg four times daily)
  • Amoxicillin-clavulanate (875/125mg twice daily) 5

• MRSA coverage is generally not needed for typical non-purulent cellulitis, despite rising community MRSA rates 3

• Treatment duration: 5-7 days is sufficient for uncomplicated cases with good response 5

Common Pitfalls

• Misdiagnosis: Cellulitis is often confused with non-infectious conditions like venous stasis dermatitis, contact dermatitis, and lymphedema 3, 6
• Overtreatment: Unnecessary MRSA coverage for typical non-purulent cellulitis 5
• Neglecting predisposing factors: Failure to identify and treat underlying conditions like tinea pedis, which often harbors streptococci in lower extremity infections 1, 5

In summary, while multiple organisms can cause cellulitis in specific circumstances, streptococci remain the predominant causative pathogens in typical cases of non-purulent cellulitis, informing both diagnostic approaches and empiric antibiotic selection.