Introduction

The skin provides an efficient, significant barrier against invading microbes and thus represents the first line of defense against infection (see Chapter 20, The Immune System). The human skin and mucous membranes host many bacterial species as part of their normal flora (see Chapter 9, Infection and Disease). Although these organisms normally do not cause infections in intact skin, any break in the skin can lead to a range of infections, from treatable to life-threatening skin conditions. There are many types of skin infections caused by different organisms such as bacteria, viruses, fungi, and parasites. Microbial disease of the skin may result from the following:

Persons with certain conditions or diseases can be at greater risk for such infections; such people include but are not limited to those with:

To understand the different types of skin infections and infections of the underlying tissue it is important to understand tissue structure. The skin is composed of three layers: the epidermis, dermis, and subcutaneous tissue or hypodermis (Figure 10.1).

Bacterial Infections

Bacterial infections of skin, soft tissue, and muscle can be classified on an anatomical basis, depending on the layers of skin and soft tissue involved:

Staphylococcal Infections

Staphylococcal infections are among the most common bacterial skin infections. Staphylococci are part of the normal flora on the skin and in the nose of healthy adults. In general, these bacteria do not cause harm, except when there is a break in the skin, burns, or other injuries allowing the organism to enter through the mechanical skin barrier and cause infection. Staphylococcal infections range from mild to life-threatening. People susceptible to staphylococcal infections include newborns, breastfeeding women, people with skin disorders, and people with a weakened immune system, such as those with diabetes and cancer.

Furuncles (Boils)

Staphylococcus aureus is the most common cause of boils and persons who are carriers of the virulent strain of Staphylococcus aureus often go through recurrent boils. Exposure to Pseudomonas aeruginosa and/or Pseudomonas folliculitis in hot tubs or swimming pools may also lead to furuncles. The infection begins in a hair follicle (folliculitis) and subsequently spreads into the surrounding dermis (Figure 10.2). Furuncles can occur in the hair follicles anywhere on the body but are most commonly found on the face, neck, armpit, buttocks, and thighs. At first the lesion presents itself as a firm, red, painful nodule and then develops into a large, painful mass that often drains large amounts of pyogenic exudate (pus). Collections of furuncles can fuse to form carbuncles, a large infected mass, which may drain through several sinuses or develop into an abscess. Drainage to the inside can result in access of the bacteria to underlying sites, which can then be the cause of serious infections such as peritonitis, empyema, meningitis, or systemic poisoning (septicemia). Furuncles may heal on their own if they drain properly. Warm moist compresses encourage furuncles to drain and therefore speed up the healing processes. Deep or large lesions most often need to be lanced or drained surgically.

Cellulitis

Cellulitis is an acute infection of the dermis and subcutaneous tissue usually caused by Staphylococcus aureus, streptococci, or other bacteria. The origin of the infection is either a superficial skin lesion such as a boil or ulcers resulting from trauma. Cellulitis is characterized by redness, swelling, warmth, and pain or tenderness and develops within a few hours or days of the trauma. It occurs most commonly on the lower legs (Figure 10.3) and the arms or hands, but other areas of the body may be involved. Regional lymph nodes become enlarged and the patient suffers malaise, chills, and fever. Systemic antibiotics as well as local compresses and analgesics are usually necessary.

Impetigo (Pyoderma)

Impetigo is a superficial skin infection limited to the epidermis, common in infants and children. People who play contact sports (e.g., football, wrestling) are also susceptible, regardless of age. Staphylococcus aureus is an organism that can cause highly contagious infections such as impetigo in neonates, causing a threat in neonatal care units. In older children impetigo may also be caused by group A β-hemolytic streptococci. Scratching, direct contact with hands, eating utensils, or towels can easily be vectors to spread the infection. Lesions start with red or pimple-like sores most commonly on the face, arms, and legs. The small vesicles rapidly enlarge, fill with pus, and subsequently rupture to form yellowish-brown crusty masses (Figure 10.4). Because of autoinoculation with hands, towels, and clothes, additional vesicles develop around the primary site of infection. The treatment of impetigo depends on the age of the child and the severity of the infections and may include hygienic measures, topical treatment, and oral antibiotics in more severe cases.

Streptococcal Infections

Most often streptococcal skin infections are secondary to a primary lesion caused by another organism. Streptococci can cause cellulitis and impetigo as described previously. Although uncommon because of antibiotic treatment, glomerulonephritis may occur after streptococcal infections.

Erysipelas

Erysipelas is a type of acute infection generally caused by group A Streptococcus. Historically, the face was the most commonly involved site of infection, but now it accounts for approximately 20% of cases and the legs are most often affected (Figure 10.5). Erysipelas can be distinguished from cellulitis by the raised advancing edges and sharp borders. In general, oral antibiotics such as penicillin (see Chapter 21, Pharmacology, and Chapter 22, Antimicrobial Drugs) are used to treat the infection; however, severe cases, with complications such as bacteremia (bacteria in the blood), might require the use of intravenous antibiotics.

Acute Necrotizing Fasciitis

Necrotizing fasciitis (“flesh-eating bacteria”; see Medical Highlights) is an uncommon infection of the deep layers of the skin and subcutaneous tissue. Although a mixture of aerobic and anaerobic bacteria is often present at the original site of infection, the severe inflammation and tissue necrosis seem to be due primarily to the actions of a highly virulent strain of Streptococcus pyogenes (gram-positive, group A β-hemolytic Streptococcus), the organism responsible for “strep throat.” These infections typically originate in the mucous membranes (e.g., the throat) or skin. Necrotizing fasciitis is a progressive, rapidly spreading, inflammatory infection causing secondary necrosis of subcutaneous tissue and adjacent fascia. Proteases, which are tissue-destroying enzymes released by the pathogen, are the cause of the necrosis. The infected area becomes noticeably inflamed, painful, and enlarged, and symptoms of dermal gangrene are apparent. Systemic toxicity accompanied by fever, tachycardia, hypotension, mental confusion, disorientation, and possibly organ failure occur. Immediate and aggressive treatment is essential, including surgical removal of all infected tissue, accompanied by aggressive antimicrobial therapy, fluid replacement, and possible amputation to prevent further spread of the infection. The mortality rate is estimated to be 40% to 60%.

MEDICAL HIGHLIGHTS

Flesh-eating Bacteria

The expression “flesh-eating bacteria” is somewhat sensational, but the fact remains that bacteria causing the condition of necrotizing fasciitis do attack and destroy the subcutaneous (soft) tissue, which then becomes gangrenous. Although this condition is not new, it is rare, and has recently captured the attention and imagination of the public mostly because of coverage generated by the various news media.

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