Causes of Cellulitis
Cellulitis is primarily caused by bacteria entering through breaches in the skin, with beta-hemolytic streptococci and Staphylococcus aureus being the most common causative organisms. 1, 2
Pathophysiology and Entry Points
Cellulitis develops when bacteria penetrate the skin barrier through:
- Trauma (even minor)
- Preexisting skin infections (impetigo, ecthyma)
- Ulcerations
- Fissured toe webs from maceration or fungal infection
- Inflammatory dermatoses (eczema)
- Surgical wounds
- Animal or human bites
Often, the breaks in the skin are small and clinically inapparent 1.
Common Causative Organisms
Primary Pathogens
- Beta-hemolytic streptococci (most common, including Groups A, B, C, and G)
- Staphylococcus aureus (including MSSA and sometimes MRSA)
Special Circumstances
- Pasteurella species (cat or dog bites)
- Capnocytophaga canimorsus (animal bites)
- Aeromonas hydrophila (freshwater exposure)
- Vibrio species (saltwater exposure)
- Proteus mirabilis (rare, more common in compromised tissue) 3
Predisposing Factors
Several conditions increase the risk of developing cellulitis:
Skin barrier disruption:
- Trauma
- Preexisting skin infections
- Inflammatory dermatoses
- Interdigital toe web maceration or fungal infection
Conditions affecting skin integrity:
- Obesity
- Edema from venous insufficiency
- Lymphatic obstruction
- Previous cutaneous damage
Surgical procedures that disrupt lymphatic drainage:
- Saphenous venectomy
- Axillary node dissection
- Gynecologic surgeries with lymph node dissection 1
Reservoir of Pathogens
The source of pathogens is often unclear, but common reservoirs include:
- Macerated or fissured interdigital toe spaces (streptococci)
- Anterior nares (S. aureus)
- Anal canal (occasionally)
- Vagina (particularly for group B streptococci in patients with previous gynecologic cancer) 1
Diagnostic Considerations
- Blood cultures are positive in only 5% of cases
- Cultures of needle aspirations yield variable results (5-40%)
- Punch biopsy specimens yield organisms in 20-30% of cases, but bacterial concentration is usually low
- Most cases of cellulitis (approximately 85%) are non-culturable, making the causative bacteria unknown 4
Prevention of Recurrence
For recurrent cellulitis, addressing predisposing factors is crucial:
- Treatment of tinea pedis and other causes of toe web abnormalities
- Management of edema, obesity, and venous insufficiency
- Prophylactic antibiotics may be considered for patients with 3-4 episodes per year 2
Understanding the causative organisms and entry points of cellulitis is essential for appropriate treatment and prevention of recurrences, ultimately improving patient outcomes and quality of life.