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Chapter 77: Infections of the Skin & Skin Structures

INTRODUCTION

Infections of the skin and skin structures are some of the most common infectious diagnoses and result in hundreds of thousands of medical office and emergency room visits each year. These infections often occur following a break in normal skin integrity from either trauma or skin disease (e.g., atopic dermatitis). The vast majority of these infections are caused by Staphylococcus aureus and Streptococcus pyogenes (Table 77–1). Rarely, infections in patients with burns, diabetes mellitus, or decubitus ulcers may involve gram-negative rods such as Pseudomonas or anaerobes. Hematogenous seeding of organisms into the skin can occur but is uncommon. Normal histology of the skin can be seen in Figure 77–1.

TABLE 77–1Skin and Skin Structures Infections: Appearance of Lesions, Skin Layers Involved, Common Pathogens, and Treatment Modalities

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TABLE 77–1 Skin and Skin Structures Infections: Appearance of Lesions, Skin Layers Involved, Common Pathogens, and Treatment Modalities

Type of Infection

Appearance of Lesion

Description of Lesion

Layer of Skin Involved

Common Pathogens

Treatment

Impetigo

Vesicles with honey-colored crust, often on the face of a child

Epidermis

Staphylococcus aureus, Streptococcus pyogenes

Few lesions: topical antibiotics (e.g., mupirocin); numerous lesions: systemic therapy (e.g., cephalexin, clindamycin)

Erysipelas

Erythematous, very painful lesion with sharply demarcated, raised, regular border

Superficial dermis

S. pyogenes, Streptococcus agalactiae > S. aureus

Systemic antibiotics (e.g., cephalexin or cefazolin)

Cellulitis

Erythematous, diffuse, flat lesion with irregular border

Deep dermis

S. pyogenes, S. agalactiae > S. aureus

Systemic antibiotics (e.g., cephalexin or cefazolin)

Folliculitis

Localized, inflamed papules containing a small amount of pus

Hair follicle

S. aureus, Pseudomonas aeruginosa (associated with hot tubs)

Antibiotics often not needed; warm, moist compresses are useful

Skin abscess (also known as a boil, furuncle, carbuncle)

Raised, tender, inflamed nodule with central region of purulence; the area of pus initially is firm but then progresses to fluctuance (becomes movable)

Deep dermis

S. aureus

Incision and drainage is mainstay of therapy; antibiotics directed against S. aureus in select cases

Necrotizing fasciitis

Very painful area of inflammation with rapid progression to necrosis, bullae, purpura, anesthesia, and systemic toxicity

Fascia and muscle; local blood vessels and nerves also involved

Monomicrobial form: S. pyogenes, Clostridium perfringens, Vibrio vulnificus

Polymicrobial form: enteric gram-negative rods plus anaerobes

Surgical debridement is critical in addition to broad-spectrum systemic antibiotics

FIGURE 77–1

Layers of the skin and subcutaneous tissue. Note sebaceous glands (purple coils) and hair follicles with protruding hair. (Reproduced with permission from Mescher AL. Junqueira’s Basic Histology: Text & Atlas. 13th ed. New York, NY: McGraw-Hill Education; 2013.)