What is cellulitis, how does it occur, and what are the typical causes in an elderly patient with potential comorbidities such as poor circulation or diabetes?

What is Cellulitis and Why Does It Occur?

Cellulitis is an acute bacterial infection of the deeper dermis and subcutaneous tissue that occurs when organisms—predominantly β-hemolytic streptococci (especially group A Streptococcus pyogenes) and Staphylococcus aureus—enter through breaches in the skin barrier. 1

Pathophysiology: How Cellulitis Develops

Cellulitis arises when bacteria penetrate disrupted skin and invade the deeper dermal and subcutaneous layers, triggering an acute inflammatory response. 1 The infection involves deeper tissue than erysipelas, which affects only the upper dermis and superficial lymphatics. 1

The Entry Points

The organisms gain access through various breaks in the skin, which may be:

• Clinically apparent trauma: cuts, abrasions, surgical wounds, or penetrating injuries 1
• Preexisting skin infections: impetigo, ecthyma, or other superficial infections that extend deeper 1
• Fungal infections: particularly tinea pedis with fissured, macerated toe webs—a critical and often overlooked reservoir of streptococci in lower extremity cellulitis 1
• Inflammatory dermatoses: eczema, venous stasis dermatitis, or other conditions that compromise skin integrity 1
• Insect bites or animal bites: creating portals for bacterial entry 1, 2
• Microscopic breaks: often the disruption is so small it remains clinically inapparent 1

Primary Causative Organisms

Streptococci (Most Common)

β-hemolytic streptococci—particularly group A, but also groups B, C, and G—cause the majority of typical cellulitis cases. 1 Evidence from culture studies, serologic investigations, and immunofluorescent antibody techniques consistently identifies streptococci as the predominant pathogens. 1 In lower extremity infections, these organisms frequently colonize the macerated interdigital toe spaces, emphasizing why treating tinea pedis is crucial for preventing recurrence. 1

Staphylococcus aureus (Less Common in Typical Cases)

S. aureus causes cellulitis less frequently than streptococci in typical presentations, but becomes more likely with specific risk factors such as penetrating trauma or injection drug use. 1 Despite rising community prevalence of methicillin-resistant S. aureus (MRSA), this organism remains an uncommon cause of typical nonpurulent cellulitis—even in high-prevalence settings—with beta-lactam monotherapy succeeding in 96% of cases. 3

Special Circumstance Pathogens

Certain exposures predict unusual organisms:

• Cat or dog bites: Pasteurella multocida or Capnocytophaga canimorsus 1
• Freshwater exposure: Aeromonas hydrophila 1
• Saltwater exposure: Vibrio species 1
• Aquaculture or meatpacking employment: various occupational pathogens 1

Predisposing Factors: Why Certain Patients Develop Cellulitis

Conditions That Weaken Skin Integrity

• Obesity: makes skin more fragile and local defenses less effective 1
• Venous insufficiency: causes chronic edema and skin changes that predispose to infection 1
• Lymphatic obstruction or lymphedema: impairs local immune surveillance and fluid drainage 1
• Previous cutaneous damage: from any cause 1

Surgical Disruption of Lymphatic Drainage

Specific procedures dramatically increase cellulitis risk by disrupting lymphatic flow:

• Saphenous venectomy for vascular procedures 1
• Axillary node dissection for breast cancer 1
• Radical vulvectomy or radical hysterectomy with lymph node dissection, especially when followed by radiation therapy 1

Comorbidities in Elderly Patients

In elderly patients with multiple comorbidities, several factors converge:

• Poor circulation from peripheral vascular disease or venous insufficiency creates chronic edema and skin breakdown 1
• Diabetes mellitus impairs immune function, causes neuropathy (leading to unrecognized trauma), and promotes microvascular disease 1
• Chronic edema from heart failure or renal disease provides a favorable environment for bacterial invasion 1
• Immobility in long-term care facilities increases pressure ulcer risk, which can become infected 1

Clinical Manifestations

Cellulitis presents with rapidly spreading areas of:

• Erythema (redness) that expands over hours to days 1
• Edema (swelling) that may create a "peau d'orange" (orange peel) appearance as superficial edema surrounds hair follicles 1
• Heat over the affected area 1
• Tenderness and pain 4

Systemic manifestations may include fever, tachycardia, confusion, hypotension, and leukocytosis—sometimes appearing hours before visible skin changes. 1 Vesicles, bullae, petechiae, or ecchymoses may develop on inflamed skin; widespread hemorrhagic changes with systemic toxicity should raise concern for necrotizing fasciitis. 1

Diagnostic Challenges

Cellulitis remains a clinical diagnosis without a gold standard test. 4, 5 Blood cultures yield positive results in only 5% of cases. 1 Needle aspiration of inflamed skin produces variable culture results (5-40% positive), likely depending on patient population and whether isolates represent true pathogens versus contaminants. 1 Punch biopsy cultures yield organisms in 20-30% of cases, but bacterial concentrations are typically quite low. 1

Common Pitfall: The Streptococcal Reservoir

A critical and frequently overlooked factor is the role of interdigital toe web abnormalities as a reservoir for streptococci in lower extremity cellulitis. 1 Macerated, fissured toe spaces from tinea pedis or other causes harbor the responsible streptococci in many infections. 1 Failure to identify and treat these predisposing conditions leads to recurrent episodes, with annual recurrence rates of 8-20% in patients with previous leg cellulitis. 3