Bacterial Infections of the Skin
Jason J. Schafer
Maria C. Foy
Bacterial skin infections range from those that are minor and heal without consequence to those that are more severe and may be disfiguring or even life threatening. Minor infections are quite common and are often self-treated by patients without formal medical care. The majority of wounds seen in health care practice are easily managed with appropriate wound care and antibiotic therapy, if indicated.
Common primary skin infections resulting from bacteria include impetigo, bullous impetigo, folliculitis, felons, paronychias, and cellulitis. (See Box 14.1 for information about associated problems.) These are discussed in this chapter, along with the less common infections erysipelas, ecthyma, furuncles, and carbuncles. This chapter also contains a brief discussion of necrotizing fasciitis, a very serious infection treated in an inpatient setting by specialists. See the accompanying color plates for an illustration of some of these infections.
CAUSES
The bacterial organisms most commonly responsible for causing skin infections are Staphylococcus aureus and beta-hemolytic forms of streptococci such as Streptococcus pyogenes (group A Streptococcus or GAS) and Streptococcus agalactiae (group B Streptococcus) (Tables 14.1 and 14.2).
Impetigo and Ecthyma
In the past, superficial skin infections, such as impetigo, a common infection characterized by scattered vesicular lesions, were caused by GAS. However, a shift in normal skin flora has occurred in the United States, and now impetigo is due primarily to S. aureus alone or less commonly in combination with GAS. Bullous impetigo, a variation of impetigo, is caused primarily by S. aureus.
Ecthyma is a chronic form of impetigo that affects deeper layers of the skin. Usually the causes are the same as those of impetigo; however, gram-negative organisms, such as Pseudomonas aeruginosa, or fungal organisms may also play a role. This is especially true in diabetic or immunocompromised patients. Ecthyma can develop from minor wounds, scabies, insect bites, or any condition that causes itching, scratching, and excoriation.
Impetigo is more common in children but can also be seen in adults. It is most often diagnosed during hot, humid weather when bacterial colonization of the skin occurs more easily. Both impetigo and ecthyma are communicable and can be transmitted through person-to-person contact, often in schools or day care centers. Poor hygiene and crowded living conditions are other factors that can contribute to the development of these infections (Gorbach, 2004).
Cellulitis and Erysipelas
Cellulitis is an infection involving the skin and subcutaneous layers. It has the potential to spread systemically and cause serious illness. It can develop in any type of wound, ranging from a minor break in the skin to more serious laceration, puncture, or burn. Other common precipitants include stasis dermatitis, stasis ulcers, edema of the lower extremity, venous insufficiency, and obesity (Odell, 2003). In intravenous (IV) drug users, cellulitis typically develops at injection sites. The characteristics of infection depend on many factors, including the type of wound, the organisms involved, and the patient.
Most cases of cellulitis are caused by GAS or S. aureus. Patients with certain predisposing factors, however, may be at risk for infections caused by other organisms, including
Escherichia coli, P. aeruginosa, and Klebsiella species (Table 14.2). Pasteurella multocida is the primary cause of cellulitis from animal bites and scratches, although S. aureus and GAS may also be present. Table 14.2 lists additional causes.
Escherichia coli, P. aeruginosa, and Klebsiella species (Table 14.2). Pasteurella multocida is the primary cause of cellulitis from animal bites and scratches, although S. aureus and GAS may also be present. Table 14.2 lists additional causes.
BOX 14.1
Danger: Bites and Other Puncture Wounds
Danger: Bites and Other Puncture Wounds
Human and animal bites and puncture wounds of other sorts are infections waiting to develop. Because these wounds are associated with such a high risk for infection, antibiotic prophylaxis with a broad-spectrum penicillin or quinolone usually begins with the patient’s request for health care. Tetanus is also an important consideration in puncture wounds and bites, and patients should be immunized as needed.
If the wound was created by a clean object and is in an area that is well vascularized, treatment, in addition to antibiotics, may consist simply of washing thoroughly; soaking in warm, soapy water several times a day; and observing the site for a few days.
If the wound was made by an object contaminated with fecal material, soil, or other debris, or if the patient is diabetic or has a compromised circulation, broad-spectrum antibiotics are required based on the probable causative organism.
Care for bite wounds depends on several factors, including whether the biter was a human or an animal, the location of the wound, and whether the wound is primarily a puncture or a laceration. All bite wounds should be cleaned thoroughly with soap and water. Puncture bites should be irrigated with normal saline solution. Extensive wounds may require surgical debridement, tendon repair, or suturing. If the bite is located on an extremity, elevation of the extremity will help prevent swelling.
All human bites that break the skin should be treated with antibiotics. Appropriate choices include amoxicillin-clavulanate or ampicillin-sulbactam for IV treatment. Oral doxycycline can be used for patients who are allergic to penicillin. Treatment should be given for a full 7 to 10 days.
Minor animal bites may not require antibiotic therapy unless the wound is on the hand, foot, or face. However, the infection rate in animal bites can range from 2% to 20%, and it may be prudent to treat the wound with a course of antibiotics. The possibility of rabies must also be addressed. The same agents used for human bites are also appropriate for animal bites.
Although most puncture wounds heal without incident, patients should be instructed to observe for signs of infection, including inflammation, persistent pain, swelling, or purulent drainage. If a puncture wound becomes infected, further systemic antibiotic therapy is required and should be based on Gram stain and culture results. A follow-up visit should be scheduled within days to ensure that the wound is healing without further infection.
Staphylococcus aureus organisms that are resistant to commonly used antibiotics have recently emerged as a major cause of community-acquired skin infections. These organisms are resistant to all penicillins and cephalosporin agents and are referred to as community-associated methicillin-resistant S. aureus (CA-MRSA) (DeLeo et al., 2010). Infections resulting from these organisms can be particularly challenging to manage because of their resistance to commonly used antibiotics. Also, as opposed to other skin infections, CA-MRSA infections may occur in otherwise healthy individuals but are most common among patients with close contact with others colonized or infected with CA-MRSA. Transmission of this pathogen
appears to occur most commonly when there are crowded living conditions (e.g., military bases, prisons) or close physical contact between individuals, particularly when injury to the skin is common (e.g., athletes).
appears to occur most commonly when there are crowded living conditions (e.g., military bases, prisons) or close physical contact between individuals, particularly when injury to the skin is common (e.g., athletes).
TABLE 14.1 Selected Organisms That Cause Skin Infections | ||||||||||||||||||||||||||||||||||||||||||||||
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TABLE 14.2 Organisms That Cause Cellulitis | ||||||||||||||||||||||||||||||
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Erysipelas, predominantly caused by S. pyogenes, is a superficial form of cellulitis and is seen more often in children, especially infants, and the elderly. Also known as St. Anthony’s fire, it is an acute condition that can spread rapidly through the skin and lymphatics, causing significant mortality (up to 5%) if left untreated.
Pustular Infections
Pustular infections include folliculitis, furunculosis, and carbunculosis. Folliculitis is a superficial infection of the hair follicle commonly caused by S. aureus. Patients who have been taking prolonged antibiotic therapy for acne, however, may acquire folliculitis from gram-negative organisms such as Klebsiella, Enterobacter, or Proteus species. Lesions associated with folliculitis are usually found on the cheek or chin, under the nose, or on the central facial areas (Trent et al., 2001). Pseudomonas aeruginosa can also cause folliculitis, particularly in those who frequently use hot tubs, as a result of inadequate chlorination.
Furunculosis (furuncles) and carbunculosis (carbuncles) are also pustular infections usually caused by S. aureus. Both conditions involve deeper areas of the skin and can develop from unresolved cases of folliculitis.
Irritation from shaving, plucking, and waxing of hair may contribute to folliculitis. Other predisposing factors include humid conditions, tight clothing, diabetes, occlusion of the hair follicles from cosmetics or sunscreens, poor hygiene, and occupational exposure to heavy grease or solvents.
Other common skin infections, such as paronychia (an infection surrounding a nail bed) and a felon (an infection affecting the tip of a digit), are usually caused by S. aureus, S. pyogenes, or Pseudomonas species and occasionally other gram-negative bacilli.
Necrotizing Fasciitis
Necrotizing fasciitis is an extremely serious infection of the subcutaneous tissues that can be life threatening if not diagnosed early and treated appropriately. Management often requires emergent surgical interventions to remove infected tissue in combination with antibiotic therapy. It is most likely to occur in middle-aged, elderly, or seriously debilitated patients (Sadick, 1997). The mortality rate is typically 20% to 30% but can approach up to 50% in patients with underlying vascular disease. Often, the initial lesion is minor, such as an insect bite or boil; it is rarely associated with Bartholin gland or perianal abscesses (Gorbach, 2004). However, 20% of cases have no visible lesions (Gorbach, 2004). Patients with diabetes and alcoholism may have no evidence of trauma (Odell, 2003).
Necrotizing fasciitis is often a polymicrobial infection caused by combinations of pathogens that include S. pyogenes, S. aureus, and anaerobic bacteria such as Bacteroides fragilis and Peptostreptococci. GAS and S. aureus can also cause necrotizing fasciitis alone as a monomicrobial infection. Clostridium perfringens, an anaerobic bacterium commonly found in soil, can cause “gas gangrene,” a type of necrotizing fasciitis that can occur following deep penetrating trauma injuries (Ustin & Malangoni, 2011). Varicella infection is often considered a risk factor for invasive skin infections, including necrotizing fasciitis (Wilhelm & Edson, 2001).
Patients with necrotizing fasciitis are very ill and require intensive care. Surgical debridement of infected areas and IV antibiotic therapy are also commonly indicated. Patients who are elderly or debilitated or who have predisposing medical conditions, such as diabetes, advanced atherosclerotic disease, and lesions starting in an extremity and progressing into the back, chest wall, or buttock muscles, have a poor prognosis (Gorbach, 2004).
PATHOPHYSIOLOGY
The skin is composed of three layers. The outer layer is the epidermis, the first line of defense against infection. Nail tissue is part of the epidermis. Underneath is the dermis, which contains connective tissue, blood vessels, nerves, hair follicles, sweat glands, and sebaceous glands. The innermost layer is composed of subcutaneous tissue. Skin infections may be classified according to the depth of penetration and the layer and skin structure affected.
BOX 14.2
Predisposing Factors in Skin Infections
Predisposing Factors in Skin Infections
Chronic carriers of Staphylococcus aureus
Diabetes mellitus
Peripheral vascular disease
Venous stasis
Alcoholism (malnutrition)
Immune deficiency
Corticosteroid therapy
Obesity
Trauma or burns
Poor hygiene
Warm, humid conditions
Topical irritants
Tight clothing
Under normal circumstances, bacteria present on the skin as normal flora cause no harm. However, a break in the skin can allow these organisms to penetrate and proliferate, resulting in a skin infection. Some people are persistent carriers of S. aureus in the nasal, perineal, or axillary areas. These individuals may be more prone to developing skin infections and more likely to experience recurrences.
Patients with predisposing medical conditions (Box 14.2), such as diabetes, immune system disorders, and malnutrition from alcoholism or other causes, are more prone to skin infection because of poor wound healing. Also at a higher risk for skin infection are those with circulatory compromise of the arterial, venous, or lymphatic systems. Wound infections in patients with these conditions have the potential to become more serious and invasive, requiring IV antibiotic therapy, hospitalization, or referral to a specialist. Wound infections can also become more serious when treatment is delayed. The practitioner must be alert for these situations and act promptly.
In addition, many organisms aside from GAS or S. aureus may cause skin infections in patients with chronic conditions like diabetes. These organisms include E. coli, Klebsiella species, and P. aeruginosa. Infections in these patients are often more difficult to manage clinically and have the tendency to become chronic. Infections that do resolve with appropriate therapy have a high risk of recurrence. A common example of this scenario is a diabetic foot infection.
DIAGNOSTIC CRITERIA
Impetigo and Ecthyma
Impetigo, a highly contagious, common primary skin infection in children, is most frequently found on the face, scalp, or extremities. It begins as scattered, discrete macules that itch and are spread by scratching. These macules then develop into vesicles and pustules on an erythematous base that eventually rupture, oozing a purulent liquid. Once dried, the lesions appear thick, with a characteristic honey-colored crust on the surface. Once healed, scarring is rare. Regional lymphadenopathy may be present, and lesions may itch; however, fever or other systemic complaints are uncommon. The infection is diagnosed clinically by the appearance of hallmark honey-colored crusts. A Gram stain of the vesicular fluid can confirm the diagnosis if there is clinical uncertainty.
Impetigo may also present with bullous lesions and can be referred to as bullous impetigo. Found on the face, scalp, extremities, trunk, and intertriginous areas, it affects primarily newborns and children ages 3 to 5 (Wilhelm & Edson, 2001). Bullous impetigo is characterized by the formation of superficial, flaccid bullae on the skin. The brownish-gray lesions are sometimes crusted or have an erythematous halo. They also appear to be smooth and shiny, as if they were coated with lacquer.
In the most severe form of bullous impetigo, exfoliation of large areas of the skin can occur in what is called “scalded skin” syndrome. This presentation is most common in infants but can also occur in those who are immunosuppressed. It is thought to occur more often in patients who are sensitive to toxins produced by staphylococcal organisms (Barg et al., 1998). Such cases carry a greater risk for more invasive infection because of the loss of large amounts of the skin, the body’s protective barrier.
Ecthyma occurs when a case of impetigo worsens and spreads deeply to the dermis. Much less common than impetigo, ecthyma usually affects debilitated individuals and the elderly (Odell, 2003). Signs of ecthyma are usually found on the lower extremities, beginning with the formation of vesicles that then develop into shallow ulcerations. The ulcerations enlarge over several days and are surrounded by an erythematous halo. Because the infection affects the deeper layers of the skin, scarring is often seen after ulcerations heal (Wilhelm & Edson, 2001). Lesions are usually painful and may persist for weeks to months.
Cellulitis and Erysipelas
Cellulitis is a potentially serious infection involving the skin and subcutaneous tissue. In adults, it most commonly affects the lower extremities and begins with a skin break resulting from a localized trauma that may not be apparent. In children, cellulitis usually results from an insect bite or wound. The disease can spread through the superficial layers of skin and cause painful erythema, with the affected area warm and tender to the touch. Pitting edema can also be present, and the skin may be shiny and have an “orange peel” appearance. The margins of cellulitis are diffuse, not sharply demarcated, and the affected area is flat and usually edematous. In open wounds, purulent drainage and necrosis may be present. Red streaks may develop proximal to the area of infection, indicating lymphatic spread or lymphangitis. Crepitus may be present, suggesting involvement of anaerobic organisms. Systemic symptoms of fever, chills, and malaise and regional adenitis are also common and can indicate bacteremia. In fact, these symptoms may be present before cellulitis is clearly evident.
Cellulitis due to CA-MRSA may be accompanied by the presence of an abscess or a furuncle that contains a necrotic center (DeLeo et al., 2010). These infections may progress rapidly and cause local tissue destruction, possibly leading to systemic infection. In addition to painful erythema, the abscess present may spontaneously drain but often will require an incision and drainage procedure for proper management.
Erysipelas, a type of cellulitis limited to the superficial layers of the skin, is most common in children, especially infants and the elderly, but it can occur in healthy individuals who have sustained only minor wounds. Other patients who are at risk include those with venous insufficiency or underlying skin ulcers, diabetics, alcoholics, or those with nephritic syndrome. Erysipelas is most commonly found on the lower extremities but can also be present on the face and scalp. Erysipelas begins as an area of sharply demarcated erythema that spreads rapidly over a period of minutes to hours. The affected area is slightly raised, firm, warm, and tender to the touch. Erythema spreads along local lymphatic channels, which gives the skin a typical “orange peel” appearance due to lymphatic obstruction.
Common systemic symptoms include pain, malaise, chills, and fever. In more serious cases, patients may appear seriously ill. Erysipelas occurring on the face often follows a respiratory infection; this presentation can be particularly serious due to the potential occurrence of cavernous sinus thrombosis (Odell, 2003). Erysipelas can recur, usually in the same area as a previous infection, especially in patients with venous insufficiency or lymphedema (Wilhelm & Edson, 2001).
Pustular Infections
Folliculitis is usually superficial and occurs on hairy areas of the skin, especially the bearded parts of the face and the intertriginous areas. Early lesions appear as erythematous papules that turn into small pustules within approximately 48 hours. As a result, it is common to see various lesions at different stages of development. These lesions may itch initially and then rupture and form crusts. Folliculitis is often recurrent and can represent over months or even years.
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