What are the most common organisms responsible for cellulitis?

Common Organisms Responsible for Cellulitis

Streptococci, particularly group A streptococci, are the most common causative organisms of cellulitis, followed by Staphylococcus aureus which typically causes cellulitis only when associated with abscess formation or penetrating trauma. 1

Primary Causative Organisms

Most Common Pathogens

• Streptococci - The predominant cause of typical cellulitis
  • Group A streptococci (Streptococcus pyogenes)
  • Other β-hemolytic streptococci (Groups B, C, G) 1, 2
• Staphylococcus aureus - Less frequently causes cellulitis unless associated with:
  • Underlying abscess
  • Penetrating trauma
  • Injection sites (particularly in illicit drug users) 1

Microbiological Diagnosis Challenges

• Blood cultures positive in only 5% of cases
• Needle aspiration culture yields vary widely (5-40%)
• Punch biopsy specimens yield organisms in 20-30% of cases
• Bacterial concentration in tissue is typically low 1

Specific Clinical Scenarios and Associated Organisms

Purulent vs. Non-purulent Cellulitis

• Non-purulent cellulitis: Primarily caused by β-hemolytic streptococci 2, 3
• Purulent cellulitis/abscess: More commonly caused by S. aureus, including MRSA 2, 4

Environmental and Exposure-Related Organisms

• Animal bites:
  • Cat/dog bites: Pasteurella species (especially P. multocida), Capnocytophaga canimorsus 1
• Water exposure:
  • Fresh water: Aeromonas hydrophila
  • Salt water: Vibrio species (particularly V. vulnificus in warm climates) 1

Anatomical Considerations

• Lower extremity infections: Often associated with streptococci present in macerated or fissured interdigital toe spaces 1
• Orbital cellulitis: Usually a complication of sinusitis caused by:
  • Streptococcus pneumoniae
  • Nontypeable Haemophilus influenzae
  • Streptococcus pyogenes
  • Moraxella species
  • Anaerobic bacteria
  • Aspergillus species
  • Zygomycetes 1
• Periorbital cellulitis: Usually results from localized trauma or bacteremia caused by:
  • Staphylococcus aureus
  • Streptococcus pyogenes
  • Streptococcus pneumoniae 1

Special Populations

Immunocompromised Patients

• Broader spectrum of potential pathogens including:
  • Gram-negative bacteria
  • Enterobacteriaceae (Serratia spp., Proteus spp., Citrobacter) 5

Treatment Implications

Antibiotic Selection Based on Likely Pathogens

• For typical non-purulent cellulitis:

  • First-line: Antibiotics active against streptococci (penicillin, amoxicillin, cephalexin) 1, 2, 6
  • Many clinicians also cover S. aureus empirically, though this may not be necessary in uncomplicated cases 6

• For purulent cellulitis or associated abscess:

  • Coverage for MRSA should be considered (clindamycin, trimethoprim-sulfamethoxazole, tetracyclines) 2, 4

Clinical Pearls

• Despite rising rates of community-acquired MRSA, empiric coverage for non-purulent cellulitis is generally not recommended unless there are specific risk factors 3
• The source of pathogens is frequently unclear, but in lower extremity infections, streptococci are often present in toe web spaces, highlighting the importance of treating tinea pedis 1
• The diagnosis of cellulitis is primarily clinical, as microbiological diagnosis is often challenging due to poor culture yield 3

Common Pitfalls

• Failing to distinguish between purulent and non-purulent cellulitis, which have different predominant pathogens and treatment approaches
• Over-treating typical cellulitis with broad-spectrum antibiotics when narrower coverage for streptococci would be sufficient
• Missing special circumstances where unusual organisms may be involved (water exposure, animal bites, immunocompromised state)