Anatomy of the Skin

The skin is divided into three distinct layers: the epidermis (the outermost layer), the dermis, and the subcutaneous tissue (Figure 76-1). The epidermis is made up of layered squamous epithelium. Hair follicles, sebaceous glands (oil-producing), and sweat glands open to the skin surface through the epidermis. The dermis is composed of dense connective tissue rich in blood and nerve endings, and this is where some hair follicles and sebaceous glands originate. The subcutaneous tissue contains loose connective tissue and is rich in fat. Deeper hair follicles and sweat gland originate in this layer. Below the subcutaneous layer are thin fascial membranes (sheets or bands of fibrous tissue) covering muscles, ligaments, and other connective tissues.

Function of the Skin

The skin is the body’s largest and thinnest organ. It forms a self-repairing and protective boundary between the body’s internal environment and the external environment. Skin plays a crucial role in the control of body temperature, excretion of water and salts, synthesis of important chemicals and hormones, and as a sensory organ. The skin has an important protective function because of the composition of the outermost layer of the epidermis, which is composed of cells containing keratin, a water-repellent protein. The skin’s normal microbial flora, pH, and chemical defenses (high salt and acidic environment) also help prevent colonization by many pathogens. The resident microbial flora is listed in Box 76-1.

Prevalence, Etiology, and Pathogenesis

Approximately 15% of all patients who seek medical attention have either some skin disease or skin lesion, many of which are infectious. Various bacteria, fungi, and viruses may be involved. These infections can include one or several causative agents. Because of the diversity of etiologic agents and the potential complexity of these infections, only the most common infections involving the skin and subcutaneous tissues will be addressed.

Skin infections can arise from the invasion of certain organisms from the external environment through breaks in the skin or from organisms that reach the skin through the blood as part of a systemic disease. In some infections, such as staphylococcal scalded-skin syndrome, toxins produced by the bacteria cause skin lesions. In others, lesions are a result of the host’s immune response to microbial antigens.

Because of the diversity of etiologic agents, clinicians will often rely on the appearance of skin lesions for diagnostic clues in order to determine the required laboratory testing. The physical characteristics of the lesions can indicate the need for smear, culture, biopsy, or surgical procedures. Some of the terms most frequently used to describe manifestations of skin infections are provided in Table 76-1. Figure 76-2 shows examples of some skin lesions.

Infections in or around Hair Follicles

Folliculitis, furuncles, and carbuncles are localized abscesses either in or around hair follicles. These infections are distinguished from one another based on size and the extent of involvement in subcutaneous tissues. Table 76-2 summarizes each infection’s respective clinical features. For the most part, these infections are precipitated by blockage of the hair follicle with skin oils (sebum) or because of minor trauma resulting from friction such as that caused by clothes rubbing the skin. Staphylococcus aureus is the most common etiologic agent for all three infections. Members of the Enterobacteriaceae family may also cause folliculitis. Outbreaks of folliculitis caused by Pseudomonas aeruginosa have been reported to be associated with the use of whirlpools, swimming pools, and hot tubs.

Infections in the Keratinized Layer of the Epidermis

Because of their ability to utilize the keratin in the epidermal cells, the dermatophyte fungi are significant and well-suited pathogens for infection. Unlike the previously discussed infections, dermatophytes do not invade the deeper layers of skin. Because keratin is also present in hair and nails, these fungi may also cause superficial infections at these sites (see Chapter 61 for more information).

Infections in the Deeper Layers of the Epidermis and Dermis

Most infections in the deeper layers of the epidermis and dermis result from the inoculation of microorganisms by traumatic breaks in the skin. These superficial skin infections usually do not require surgical intervention. Table 76-3 summarizes these infections. In most instances, these infections resolve with local care and only occasionally require antimicrobial therapy.

Cutaneous ulcers usually involve a loss of epidermal and part of the dermal tissues. In contrast, nodules are inflammatory foci in which the epidermal and dermal layers remain largely intact. Various bacteria and fungi can cause ulcerative or nodular skin lesions following direct traumatic inoculation. Examples of these causative agents include Bacillus anthracis, Corynebacterium diphtheriae, Mycobacterium marinum, Nocardia spp., and Sporothrix schenckii.

Infections of the Subcutaneous Tissues

Infections of the subcutaneous tissues may manifest as abscesses, ulcers, or boils. The most common etiologic agent of subcutaneous abscesses in healthy individuals is S. aureus. Many subcutaneous abscesses contain mixed bacteria. To a large degree, the organisms isolated from these abscesses depend on the site of infection. For example, anaerobes are commonly isolated from abscesses of the perineal, inguinal, and buttock area, whereas nonperineal infection is commonly caused by a mixed infection containing facultative organisms.

Progressive synergistic gangrene, or Meleney’s ulcer, is a slowly progressive infection of the subcutaneous tissue that usually begins as an ulcer following trauma or surgery. The infection leads to subcutaneous necrosis and enlargement of a visible ulcer. This is a true polymicrobial infection in which microaerophilic streptococci grow synergistically with S. aureus. The infection may also include other facultative or anaerobic organisms.

In many instances, infections of the epidermis and dermis extend deeper and become subcutaneous infections and may even reach the fascia or muscle. For example, erysipelas (Figure 76-3) can develop into subcutaneous cellulitis and eventually necrotizing fasciitis. Similarly, folliculitis can readily develop into a subcutaneous abscess or carbuncle that can extend to the fascia. Cellulitis can also extend to the subcutaneous tissues (Figure 76-4). Anaerobic cellulitis is associated with the production of large amounts of gas by organisms that may be present in the subcutaneous tissue. This type of infection is most often located in the extremities and is particularly common among patients with diabetes. The infection may involve the neck, abdominal wall, perineum, connective tissue, or be present in other areas. Anaerobic cellulitis may also occur as a postoperative condition. The onset and spread of the lesion is usually slow and patients may not immediately show obvious systemic effects. The causative agents in deep tissue infections are almost always a mixture of aerobic/facultative and anaerobic organisms. Common aerobic/facultative organisms include Escherichia coli, alpha-hemolytic and nonhemolytic streptococci, and S. aureus, but group A streptococci and other members of the Enterobacteriaceae may be encountered as well. The anaerobes are typically found in greater numbers and in more variety, and include Peptostreptococcus spp., Bacillus fragilis group strains, Prevotella, Porphyromonas, other anaerobic gram-negative bacilli, and clostridia. Bacteremia is not usually present.

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