What Causes Cellulitis
Cellulitis is caused by bacterial invasion of the deep dermis and subcutaneous tissue through breaks in the skin barrier, with beta-hemolytic streptococci (especially group A) being the predominant pathogen, followed by methicillin-sensitive Staphylococcus aureus. 1
Primary Bacterial Pathogens
Most cellulitis infections arise from streptococci, often group A, but also groups B, C, or G, as demonstrated by culture specimens, serologic studies, and immunofluorescent antibody techniques detecting antigens in skin biopsy specimens. 1
Beta-hemolytic streptococci account for approximately 85% of typical cellulitis cases when organisms are identified, making them the single most important causative pathogen. 1, 2
Staphylococcus aureus less frequently causes cellulitis and is typically associated with previous penetrating trauma, including injection sites of illicit drug use. 1
In the 15% of cellulitis cases where organisms are identified, most are due to β-hemolytic Streptococcus and Staphylococcus aureus. 2
Routes of Bacterial Entry
The disrupted cutaneous barrier serves as the portal of entry, which may result from:
- Trauma to the skin surface 1
- Pre-existing skin infections such as impetigo or ecthyma 1
- Ulceration of the skin 1
- Fissured toe webs from maceration or fungal infection (tinea pedis) 1
- Inflammatory dermatoses such as eczema 1
Often the breaks in the skin are small and clinically inapparent, making the exact portal of entry difficult to identify. 1
Bacterial Reservoirs
For lower extremity infections, the responsible streptococci are frequently present in the macerated or fissured interdigital toe spaces, emphasizing the critical importance of detecting and treating tinea pedis and other toe web abnormalities. 1
- Occasionally, the reservoir of streptococci is the anal canal 1
- The vagina may serve as a reservoir, especially for group B streptococci causing cellulitis in patients with previous gynecologic cancer treated with surgery and radiation therapy 1
Special Circumstance Pathogens
Many other infectious agents can produce cellulitis, but usually only in special circumstances:
- With cat or dog bites, Pasteurella species (especially P. multocida) or Capnocytophaga canimorsus are typically responsible 1
- Aeromonas hydrophila may cause cellulitis following immersion in fresh water 1
- Infection after saltwater exposure can arise from Vibrio species, particularly V. vulnificus in warm climates 1
- In rare cases, Streptococcus iniae or Erysipelothrix rhusiopathiae may cause cellulitis 1
Risk Factors That Increase Susceptibility
Surgical procedures that disrupt lymphatic drainage increase the risk for cellulitis, presumably due to impaired local immune surveillance:
- Saphenous venectomy 1
- Axillary node dissection for breast cancer 1
- Operations for gynecologic malignancies involving lymph node dissection, especially when followed by radiation therapy, such as radical vulvectomy and radical hysterectomy 1
Predisposing medical conditions that compromise skin integrity or immune function include:
- Diabetes mellitus 3
- Chronic venous insufficiency 1
- Lymphedema 1
- Alcoholism 3
- Chronic obstructive pulmonary disease 3
Diagnostic Limitations
The bacterial cause of cellulitis cannot be determined by comparing the prevalence and quantity of pathogens from infected and uninfected skin biopsy specimens using current molecular techniques. 4
- Blood culture results are positive in only 5% of cases 1
- Culture of needle aspirations yields variable results, ranging from 5% to approximately 40% in reported series 1
- Culture of punch biopsy specimens yields an organism in 20-30% of cases, but the concentration of bacteria is usually quite low 1
- The majority of cellulitis cases are non-culturable, and therefore the causative bacteria remain unknown 2
Methicillin-resistant S. aureus was not found by culture or PCR, and S. pyogenes was not identified by any technique in one molecular study of cellulitis specimens, highlighting the diagnostic challenges. 4