Skin and Soft Tissue Infections: Definitions and Pathophysiology

Life-Threatening Infections

• Necrotizing Fasciitis: A rapidly progressive, limb- and life-threatening infection involving extensive destruction of the skin, subcutaneous tissue, fascia, and muscle. It is often caused by a combination of aerobic and anaerobic bacteria and requires urgent surgical debridement.

Pathophysiology: The Body's Defense vs. Pathogens

Infections occur when the skin's protective defense mechanisms are compromised, allowing foreign bacteria to outcompete the normal skin flora.

Skin Protective Defenses

The skin maintains a robust barrier through several key mechanisms:

• Stratum Corneum: Maintains an acidic pH to inhibit the growth of many pathogens.
• Epidermal Renewal: The continuous shedding of skin facilitates the disposal of contaminants.
• Sebaceous Secretions: These produce free fatty acids that help inhibit bacterial reproduction.
• Microcirculation: Ensures the delivery of essential nutrients and immune system response factors to the site of potential injury.

Normal Skin Flora

Under healthy conditions, the skin is colonized by a variety of Gram-positive bacteria that compete with foreign pathogens for space and nutrients. This normal flora includes:

• Staphylococcus epidermidis
• Staphylococcus aureus
• Streptococcus spp.
• Corynebacterium spp.
• Propionibacterium spp.
• Candida spp. (Yeast)

Common Pathogens in SSTIs

The specific pathogen involved often dictates the severity and the necessary antibiotic therapy.

Community-Acquired Pathogens

In typical community-acquired SSTIs, the most common culprits are:

• Staphylococcus aureus (including MRSA)
• $\beta$-hemolytic streptococci (specifically Groups A, B, C, and G)

Exam Tip: In normal hosts, $\beta$-hemolytic streptococci are significantly more common than S. aureus in cases of erysipelas and cellulitis.

Less Common and Specialized Pathogens

Other pathogens may be encountered depending on the patient's environment or medical history:

• Enterobacteriaceae (such as E. coli) and Enterococcus spp.
• Pseudomonas aeruginosa (often associated with hot tubs or freshwater exposure)
• Zoonotic or environmental pathogens: Such as Vibrio vulnificus (saltwater), Aeromonas spp. (freshwater), or Pasteurella spp. (animal bites)

Worked Example: Clinical Differentiation

Scenario: A 67-year-old female presents with a 3-day history of worsening redness, pain, and swelling of her left lower leg following an insect bite. The area is warm, tender, and shows diffuse erythema extending from the ankle to below the knee. There is no purulent drainage or fluctuance.

Analysis:

1. Identify the depth: The infection is diffuse and involves the dermis and subcutaneous tissue, but lacks a localized collection of pus (abscess) or involvement of the hair follicle (folliculitis).
2. Differentiate: Because the erythema is diffuse and poorly defined, rather than sharply demarcated (which would suggest erysipelas), the most likely diagnosis is cellulitis.
3. Assess Severity: Since the patient's vital signs are stable and there are no systemic signs (like fever or tachycardia), this would be classified as a mild infection.