What are the causes of cellulitis?

Causes of Cellulitis

Cellulitis is predominantly caused by β-hemolytic streptococci (particularly group A Streptococcus pyogenes) and Staphylococcus aureus, with streptococci being the most common pathogen in typical cases without penetrating trauma or abscess formation. 1, 2

Primary Causative Organisms

Most Common Pathogens

• β-hemolytic Streptococcus species are the leading cause of typical cellulitis, including groups A, B, C, and G streptococci, with group A Streptococcus (S. pyogenes) being most frequent 1, 2
• Staphylococcus aureus (both methicillin-sensitive and methicillin-resistant strains) causes cellulitis less frequently than streptococci, but is more common when associated with penetrating trauma, injection drug use sites, or underlying abscesses 1, 3, 2
• In the 15% of cellulitis cases where organisms are successfully cultured and identified, most are due to these two bacterial groups 2

Anatomical Reservoirs for Streptococci

• The anal canal can serve as a reservoir for streptococci 1
• The vagina, particularly for group B streptococci, especially in patients with previous gynecologic cancer treated with surgery and radiation therapy 1

Special Circumstance Pathogens

Animal Bite-Associated

• Pasteurella species (especially P. multocida) and Capnocytophaga canimorsus cause cellulitis following cat or dog bites 1

Water Exposure-Associated

• Aeromonas hydrophila may cause cellulitis after fresh water immersion 1
• Vibrio species (particularly V. vulnificus in warm climates) cause infection after saltwater exposure 1
• Streptococcus iniae or Erysipelothrix rhusiopathiae may rarely cause cellulitis in these settings 1

Uncommon Gram-Negative Organisms

• Proteus mirabilis, typically a urinary pathogen, can occasionally cause cellulitis, particularly in patients with vascular disease or compromised tissue 4

Critical Diagnostic Challenge

A major limitation in understanding cellulitis etiology is that in more than 80% of nonpurulent cellulitis cases, no pathogen can be isolated from cultures, leaving the specific causative organism unconfirmed. 1 This diagnostic gap means treatment must be empirically directed against the most likely pathogens (streptococci and staphylococci) based on clinical presentation rather than confirmed microbiology 1.

Predisposing Factors That Facilitate Infection

Skin Barrier Disruption

• Tinea pedis (athlete's foot) and interdigital maceration create entry points for bacteria 1
• Direct trauma to skin, including minor injuries 3
• Insect bites or other breaks in the skin barrier 3
• Preexisting skin infections such as impetigo 3

Underlying Medical Conditions

• Diabetes impairs immune function and increases risk of more severe infections 3, 5
• Chronic venous insufficiency and venous eczema ("stasis dermatitis") compromise local tissue defenses 1, 5
• Lymphedema creates an environment conducive to bacterial growth 1, 5
• Obesity is associated with defects in skin barrier, immune system, and circulatory function 5
• Old age correlates with multiple physiologic impairments that predispose to infection 5

Lymphatic Damage

• Each episode of cellulitis causes lymphatic inflammation and potentially permanent damage 1
• Severe or repeated episodes may lead to substantial lymphedema, sometimes progressing to elephantiasis 1

Emerging Resistance Patterns

The increasing prevalence of community-acquired MRSA has complicated empiric treatment decisions, as traditional agents like semisynthetic penicillins, cephalosporins, erythromycin, and clindamycin are no longer universally effective against S. aureus. 1, 3 Additionally, streptococcal resistance to erythromycin and clindamycin has been reported regionally, with macrolide resistance among group A streptococci increasing in parts of the United States 1.