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Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996.

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Medical Microbiology. 4th edition.

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Chapter 98Microbial Infections of Skin and Nails


General Concepts


Skin diseases can be caused by viruses, bacteria, fungi, or parasites. The most common bacterial skin pathogens are Staphylococcus aureus and group A β-hemolytic streptococci. Herpes simplex is the most common viral skin disease. Of the dermatophytic fungi, Trichophyton rubrum is the most prevalent cause of skin and nail infections.


Primary Infections: Primary skin infections have a characteristic clinical picture and disease course, are caused by a single pathogen, and usually affect normal skin. Impetigo, folliculitis, and boils are common types. The most common primary skin pathogens are S aureus, β-hemolytic streptococci, and coryneform bacteria. These organisms usually enter through a break in the skin such as an insect bite. Many systemic infections involve skin symptoms caused either by the pathogen or by toxins; examples are measles, varicella, gonococcemia, and staphylococcal scalded skin syndrome. Dermatophytic fungi have a strong affinity for keratin and therefore invade keratinized tissue of the nails, hair, and skin.

Secondary Infections: Secondary infections occur in skin that is already diseased. Because of the underlying disease, the clinical picture and course of these infections vary. Intertrigo and toe web infection are examples.

Clinical Manifestations

Most skin infections cause erythema, edema, and other signs of inflammation. Focal accumulations of pus (furuncles) or fluid (vesicles, bullae) may form. Alternatively, lesions may be scaling with no obvious inflammation. Nail infections cause discoloration of the nail and thickening of the nail plate.

Microbiologic Diagnosis

Clinical examination and staining and/or culturing of a specimen of pus or exudate are often adequate for diagnosis. Ultraviolet light (Wood's lamp) is helpful in diagnosing erythrasma and some toe web and fungal infections. Microscopic examination of a KOH preparation of skin scales, nail scrapings, or loose hair is useful for fungal infections. For viral infections, stained smears of vesicle fluid are examined under the microscope for typical cytopathology.

Prevention and Treatment

Cleansing and degerming the skin with a soap or detergent containing an antimicrobial agent may be useful. Drying agents, such as aluminum chloride, and keratinolytic agents, such as topical salicylate, are also helpful. Topical antimicrobial agents can be used for some infections, but systemic therapy may be necessary for patients with extensive disease.


Skin diseases are caused by viruses, rickettsiae, bacteria, fungi, and parasites. This chapter focuses on the common bacterial diseases of skin. Viral infections are also described, but of the cutaneous fungal diseases, only nail infections are included. The other fungal diseases are described in the Mycology section.

Skin Infections

Skin infections may be either primary or secondary (Fig. 98-1). Primary infections have characteristic morphologies and courses, are initiated by single organisms, and usually occur in normal skin. They are most frequently caused by Staphylococcus aureus, Streptococcus pyogenes, and coryneform bacteria. Impetigo, folliculitis, boils, and erythrasma are common examples. Systemic infections may also have skin manifestations. Secondary infections originate in diseased skin as a superimposed condition. Intertrigo and toe web infections are examples of secondary infections.

Figure 98-1. Spread of infections to skin.

Figure 98-1

Spread of infections to skin.

Clinical manifestations vary from disease to disease. Most skin diseases involve erythema, edema, and other signs of inflammation. Focal accumulations of pus (furuncles) or fluid (vesicles and bullae) may form, but lesions may also be scaling without obvious inflammation.

Methods for Laboratory Diagnosis

Specimen Collection


Specimens are collected with a blade or by swabbing the involved areas of the skin. When pustules or vesicles are present, the roof or crust is removed with a sterile surgical blade. The pus or exudate is spread as thinly as possible on a clear glass slide for Gram staining.

For actinomycetes, pus is collected from closed lesions by aspirations with a sterile needle and syringe. Material is collected from draining sinuses by holding a sterile test tube at the edge of the lesion and allowing the pus and granules to run into the tube. Granules are aggregates of inflammatory cells, debris, proteinaceous material and delicate branching filaments. Pus and other exudates are examined microscopically for the presence of granules.


Vesicles are cleaned with 70 percent alcohol followed by sterile saline. Viruses are obtained by unroofing a vesicle with a needle or a scalpel blade. The fluid is collected with a swab or with a tuberculin syringe with a 26- to 27-gauge needle. The fluid obtained from fresh vesicles may contain enough viruses for culture. Direct smears are prepared by scraping cells from the base of the lesions. The cells are smeared on a slide, fixed, and stained with Giemsa or Wright stain or with specific antibodies conjugated to fluorescein or peroxidase.


Cutaneous samples are obtained by scraping skin scales or infected nails into a sterile Petri dish or a clean envelope. For suppurative lesions of deep skin and subcutaneous tissues, aspiration with a sterile needle and syringe is recommended. Direct mounts are made by mixing a small portion of the sample in two or three drops of physiologic saline or KOH on a microscopic slide. A glass coverslip is placed over the preparation before microscopic examination.


Most pathogenic skin bacteria grow on artificial media, and selection of the medium is important. For general use, blood agar plates (preferably 5 percent defibrinated sheep blood) are recommended. In many situations, a selective medium combined with a general-purpose medium is recommended. For example, Staphylococcus aureus may overgrow Streptococcus pyogenes in blood agar medium when both organisms are present. When crystal violet (1 μg/ml) is added to blood agar, S pyogenes is selected over S aureus. Cultures for meningococci, gonocci, and brucellae must be incubated in a CO2 atmosphere. If tuberculosis or fungal infection is suspected, specimens are collected on appropriate media and incubated aerobically. Viruses are cultured on tissue cultures selected for the virus that may be contained in the specimen.

Bacterial Skin Infections

The classification of bacterial skin infections (pyodermas) is an attempt to integrate various clinical entities in an organized manner. An arbitrary but useful classification for primary and secondary bacterial infections is presented in Table 98-1. The list is not complete and includes only the more common skin diseases.

Table 98-1. Classification of Selected Bacterial Skin Infections.

Table 98-1

Classification of Selected Bacterial Skin Infections.

Primary Infections


Three forms of impetigo are recognized on the basis of clinical, bacteriologic, and histologic findings. The lesions of common or superficial impetigo may contain group A β-hemolytic streptococci, S aureus, or both, and controversy exists about which of these organisms is the primary pathogen. The lesions have a thick, adherent, recurrent, dirty yellow crust with an erythematous margin. This form of impetigo is the most common skin infection in children. Impetigo in infants is highly contagious and requires prompt treatment.

The lesions in bullous (staphylococcal) impetigo, which are always caused by S aureus, are superficial, thin-walled, and bullous. When a lesion ruptures, a thin, transparent, varnish-like crust appears which can be distinguished from the stuck-on crust of common impetigo. This distinctive appearance of bullous impetigo results from the local action of the epidermolytic toxin (exfoliation). The lesions most often are found in groups in a single reglon.

Ecthyma is a deeper form of impetigo. Lesions usually occur on the legs and other areas of the body that are generally covered, and they often occur as a complication of debility and infestation. The ulcers have a punched-out appearance when the crust or purulent materials are removed. The lesions heal slowly and leave scars.

Cellulitis and Erysipelas

Streptococcus pyogenes is the most common agent of cellulitis, a diffuse inflammation of loose connective tissue, particularly subcutaneous tissue. The pathogen generally invades through a breach in the skin surface, and infection is fostered by the presence of tissue edema. Cellulitis may arise in normal skin. However, the lesion of cellulitis is erythematous, edematous, brawny, and tender, with borders that are poorly defined.

No absolute distinction can be made between streptococcal cellulitis and erysipelas. Clinically, erysipelas is more superficial, with a sharp margin as opposed to the undefined border of cellulitis. Lesions usually occur on the cheeks.

Staphylococcal Scalded Skin Syndrome

Staphylococcal scalded skin syndrome (SSSS), also called Lyell's disease or toxic epidermal necrolysis, starts as a localized lesion, followed by widespread erythema and exfoliation of the skin. This disorder is caused by phage group II staphylococci which elaborate an epidermolytic toxin. The disease is more common in infants than in adults.


Folliculitis can be divided into two major categories on the basis of histologic location: superficial and deep.

The most superficial form of skin infection is staphylococcal folliculitis, manifested by minute erythematous follicular pustules without involvement of the surrounding skin. The scalp and extremities are favorite sites. Gram-negative folliculitis occurs mainly as a superinfection in acne vulgaris patients receiving long-term, systemic antibiotic therapy. These pustules are often clustered around the nose. The agent is found in the nostril and the pustules. Propionibacterium acnes folliculitis has been misdiagnosed as staphylococcal folliculitis. The primary lesion is a white to yellow follicular pustule, flat or domed. Gram stain of pus reveals numerous intracellular and extracellular Gram-positive pleomorphic rods. The lesions are more common in men than in women. The process may start at the age when acne usually appears, yet most cases occur years later.

In deep folliculitis, infection extends deeply into the follicle, and the resulting perifolliculitis causes a more marked inflammatory response than that seen in superficial folliculitis. In sycosis barbae (barber's itch), the primary lesion is a follicular pustule pierced by a hair. Bearded men may be more prone to this infection than shaven men.

A furuncle (boil) is a staphylococcal infection of a follicle with involvement of subcutaneous tissue. The preferred sites of furuncles are the hairy parts or areas that are exposed to friction and macerations. A carbuncle is a confluence of boils, a large indurated painful lesion with multiple draining sites.


Erysipeloid, a benign infection that occurs most often in fishermen and meat handlers, is characterized by redness of the skin (usually on a finger or the back of a hand), which persists for several days. The infection is caused by Erysipelothrix rhusiopathiae.

Pitted Keratolysis

Pitted keratolysis is a superficial infection of the plantar surface, producing a punched-out appearance. The pits may coalesce into irregularly shaped areas of superficial erosion. The pits are produced by a lytic process that spreads peripherally. The areas most often infected are the heels, the ball of the foot, the volar pads, and the toes. Humidity and high temperature are frequent aggravating factors. Gram-positive coryneform bacteria have been isolated from the lesions.


Erythrasma is a chronic, superficial infection of the pubis, toe web, groin, axilla, and inframammary folds. Most lesions are asymptomatic, but some are mildly symptomatic with burning and itching. The patches are irregular, dry and scaly; initially pink and later turning brown. The widespread, generalized form is more common in warmer climates. Corynebacterium minutissimum is the agent. Because of its small size, the organism is difficult to observe in KOH preparations of infected scales; however, it is readily demonstrable by Gram staining of the stratum corneum. Coral red fluorescence of the infected scales under Wood's light is diagnostic.


Trichomycosis involves the hair in the axillary and pubic regions and is characterized by development of nodules of varying consistency and color. The condition is generally asymptomatic and not contagious. Underlying skin is normal. Infected hairs obtained for microscopic examination are placed on a slide in a drop of 10 percent KOH under a coverslip. The nodules on the hairs are composed of short bacillary forms. Three types of coryneforms are associated with trichomycosis; one resembles C minutissimum, one is lipolytic, and the third is C tenuis.

Secondary Infections


Intertrigo is most commonly seen in chubby infants or obese adults. In the skin fold, heat, moisture, and rubbing produce erythema, maceration, or even erosions. Overgrowth of resident or transient flora may produce this problem.

Acute Infectious Eczematoid Dermatitis

Acute infectious eczematoid dermatitis arises from a primary lesion such as a boil or a draining ear or nose, which are sources of infectious exudate. A hallmark of this disease is a streak of dermatitis along the path of flow of the discharge material. Coagulase-positive staphylococci are the organisms most frequently isolated.

Pseudofolliculitis of the Beard

Pseudofolliculitis of the beard, a common disorder, occurs most often in the beard area of black people who shave. The characteristic lesions are usually erythematous papules or, less commonly, pustules containing buried hairs. This occurs when a strongly curved hair emerging from curved hair follicles reenters the skin to produce an ingrown hair. Gram-positive microorganisms that belong to the resident flora are associated with this disorder—a clear illustration of the opportunism of nonpathogenic bacteria when the host defense is impaired.

Toe Web Infection

The disease commonly referred to as athlete's foot has traditionally been regarded as strictly a fungal infection. This assumption has been revised, however, because fungi often cannot be recovered from the lesions throughout the disease course. Researchers now believe that the dermatophytes, the first invaders, cause skin damage that allows bacterial overgrowth, which promotes maceration and hyperkeratosis. The fungi, through the production of antibiotics, then create an environment that favors the growth of certain coryneform bacteria and Brevibacterium. Proteolytic enzymes, which are produced by some of these bacteria, may aggravate the condition. If the feet become superhydrated, resident Gram-negative rods become the predominant flora, and the toe webs incur further damage. The fungi are then eliminated either by the action of antifungal substances of bacterial origin or by their own inability to compete for nutrients with the vigorously growing bacteria.

Other Bacterial Skin Diseases

Skin Tuberculosis (Localized Form)

Localized skin tuberculosis may follow inoculation of Mycobacterium tuberculosis into a wound in individuals with no previous immunologic experience with the disease. The course starts as an inflammatory nodule (chancre) and is accompanied by regional lymphangitis and lymphadenitis. The course of the disease depends on the patient's resistance and the effectiveness of treatment. In an immune or partially immune host, two major groups of skin lesions are distinguished: tuberculosis verrucosa and lupus vulgaris.

Mycobacterium marinum Skin Disease

Many cases of M marinum skin disease occur in children and adolescents who have a history of using swimming pools or cleaning fish tanks. Often, there is a history of trauma, but even in the absence of trauma the lesions appear frequently on the sites most exposed to injury. The usually solitary lesions are tuberculoid granulomata that rarely show acid-fast organisms. The skin tuberculin test is positive.

Mycobacterium ulcerans Skin Disease

Lesions in M ulcerans skin disease occur most often on the arms or legs and occasionally elsewhere, but not on the palms or soles. Most patients have a single, painless cutaneous ulcer with characteristic undermined edges. Geographic association of the disease with swamps and watercourses has been reported. In some tropical areas, chronic ulcers caused by this organism are common.

In scrofuloderma, tuberculosis of lymph nodes or bones is extended into the skin, resulting in the development of ulcers.

A disseminated form of the disease occurs when bacteria are spread through the bloodstream in patients who have fulminating tuberculosis of the skin. When hypersensitivity to tubercle bacilli is present, hematogenously disseminated antigen produces uninfected tuberculous skin lesions such as lichen scrofilloslls.


There are several agents of actinomycetoma. About half of the cases are due to actinomycetes (actinomycetoma); the rest are due to fungi (eumycetoma). The most common causes of mycetoma in the United States are Pseudallescheria (Petriel lidium) boydii (a fungus) and Actinomyces israelii (a bacterium). Regardless of the organism involved, the clinical picture is the same. Causative organisms are introduced into the skin by trauma. The disease is characterized by cutaneous swelling that slowly enlarges and becomes softer. Tunnel-like sinus tracts form in the deeper tissues, producing swelling and distortion, usually of the foot. The draining material contains granules of various sizes and colors, depending on the agent.


Actinomyces israelii usually is the agent of human actinomycosis; Arachnia propionica (Actinomyces propinicus) is the second most common cause. The characteristic appearance of the lesion is a hard, red, slowly developing swelling. The hard masses soften and eventually drain, forming chronic sinus tracts with little tendency to heal. The sinus tracts discharge purulent material containing “sulfur” granules. In about 50 percent of cases, the initial lesion is cervicofacial, involving the tissues of the face, neck, tongue, and mandible. About 20 percent of cases show thoracic actinomycosis, which may result from direct extension of the disease from the neck or from the abdomen or as a primary infection from oral aspiration of the organism. In abdominal actinomycosis, the primary lesion is in the cecum, the appendix, or the pelvic organs.

Treatment of the Pyodermas

General Considerations

Debriding superficial pyoderma and then repeatedly cleansing the exposed lesions with topical antiseptics such as chlorhexidine removes the source of infection and minimizes its spread to adjacent skin sites or to other patients. Many secondary superficial skin infections, such as the web infections, will clear with simple twice-daily cleansing. For foot infections, the patient should wear open shoes or sandals, which permit air circulation. Aluminum chloride, a drying agent, inhibits overgrowth of opportunistic bacteria in foot, perineal, and axillary areas. Keratinolytic agents (e.g., topical salicylates) remove hyperkeratotic lesions that harbor pathogens, improving the exposure of the infected skin surface to other topical treatments.

Topical Treatment

Topical antibiotics contain a combination of neomycin, bacitracin, and polymyxin. Some newer preparations contain mupirocin, gramicidin, or erythromycin, and others combine these antibiotics with steroids. For an informed, cooperative patient suffering only minimal disease, topical antibiotics are often preferred to oral antibiotics because of the adverse reactions associated with systemic therapy.

Systemic Therapy

Systemic treatment with antibiotics is mandatory for extensive pyoderma. Systemic antibiotics can be administered orally or parenterally. Oral therapy is sufficient for most extensive dermal infections, but the parenteral route is preferred for severe infections.

A wide range of antibiotics for systemic therapy of pyoderma is available (Table 98-2). The choice of a specific antibiotic should be based on two factors: isolation and identification of the pathogen, and the depth and extent of infection. In this costconscious world one must also relate efficacy to consumer cost. Many less expensive antibiotics are just as effective against a given pathogen as the most expensive drugs with wider spectra.

Table 98-2. Acceptable Antibacterial Agents for Treatment of Bacterial Skin Infections.

Table 98-2

Acceptable Antibacterial Agents for Treatment of Bacterial Skin Infections.

Viral Skin Diseases

Viral skin diseases can produce both localized and generalized skin infections (Table 98-3). Viruses from several major groups cause skin lesions.

Table 98-3. Viruses Associated with Skin Infections.

Table 98-3

Viruses Associated with Skin Infections.

Herpes Simplex Virus

Herpes simplex virus infection is probably the most common viral skin disease (see Ch. 68). Almost the entire adult population has had herpes simplex at one time or another. Herpes simplex virus, a DNA virus, is the agent. There are two types of herpes simplex virus. Type 1 is usually associated with nongenital lesions, whereas type 2 is recovered from genital lesions. The incidence of type 1 genital infections in young patients has recently increased.


The viruses that cause smallpox, vaccinia, and cowpox are closely related; all are large DNA viruses (see Ch. 69). The smallpox virus is now extinct. Cowpox virus causes an infection of cattle that is acquired by handling infected animals. Vaccinia viruses are vaccine strains developed in the laboratory and adapted to grow in the skin of humans, rabbits, and calves. Several clinical manifestations may occur in individuals who were vaccinated against smallpox with vaccinia virus. The main problem with vaccinia virus arose when it became desirable to vaccinate a person already suffering from eczema or other skin diseases. Vaccination may produce eczema vaccinatum. Molluscum contagiosum also is caused by a poxvirus and is characterized by numerous small, pink nodules, most often on the face, genitalia, or the rectal area. Lesions also occur on the back, arms, buttocks, and inner thighs. The disease is generally harmless and self-limiting.


Human papillomaviruses cause warts (see Ch. 66). Verruca vulgaris occurs commonly on hands and fingers as single or multiple lesions. These warts are generally painless, firm, dry, and rough. They may remain stable or regress spontaneously. Verruca plantaris (plantar wart) is a clinical variety of verruca vulgaris that occurs on the sole of the foot.

During standing, walking, and running, these warts push into the skin and may be painful. Genital warts appear as large lesions of red, soft masses that may coalesce. Verruca plana juvenilis (also known as juvenile flat warts) occurs most commonly in children. The lesions are in groups and may appear on the face, neck, back of the hands, and arms. These warts may also occur in adults.


Because of the limited number of effective antiviral agents, prevention is important. Oral and intravenous acyclovir is effective for treatment of primary herpesvirus infection and for recurrent genital herpes and herpes zoster in immunosuppressed persons.

Fungal Skin Diseases

Several genera of fungi are responsible for diseases of the skin. This group of fungi, known collectively as dermatophytes, is discussed in the chapters on mycology. Some nondermatophytes, including yeasts, can also cause skin infections.


The nail consists of four epidermal components: the matrix, proximal nailfold, nailbed, and hyponychium (Fig. 98-2). The matrix is close to the bony phalanx. The horny end product of the matrix is the nail plate, which migrates distally over the nailbed. The distal portion of the matrix, the lunula, is visible as a white, crescent-shaped structure. The proximal nailfold is a modified extension of the epidermis of the dorsum of the finger, which forms a fold over the matrix; its horny end product is the cuticle. The nailbed is an epidermal structure that begins at the distal margin of the lunula and terminates in the hyponychium, which is the extension of the volar epidermis under the nail plate. It ends adjacent to the nailbed.

Figure 98-2. Longitudinal section (diagrammatic sketch) of fingernail.

Figure 98-2

Longitudinal section (diagrammatic sketch) of fingernail.

Fungal Infections of the Nails

Onychomycoses are infections of the nails by fungi. Universally recognized agents of these diseases are species of Trichophyton, Microsporum (rarely), and Epidermophyton (Table 98-4). These dermatophytes are commonly called ringworm fungi. Nondermatophytic fungi also occasionally cause onychomycoses, but usually cause only toenail problems; they rarely affect the fingernails.

Table 98-4. Fungi Associated with Onychomycosis.

Table 98-4

Fungi Associated with Onychomycosis.

Conventionally, onychomycosis is classified into four types:


Distal subungual onychomycosis primarily involves the distal nailbed and hyponychium, with secondary involvement of the underside of the nail plate. Trichophyton rubrum is one of the organisms that cause this clinical type.


White superficial onychomycosis involves the toenail plate on the surface of the nail. It is caused by T mentagrophytes and by species of Cephalosporium, Aspergillus, and Fusarium.


Proximal subungual onychomycosis is an invasion of the nail plate from the proximal nailfold producing a specific nail condition. It is caused by T rubrum and T megninii. This is a rare type of onychomycosis, but in patients with AIDS proximal white subungual onychomycosis is common.


Candida onychomycosis involves all of the nail plate. It is caused by C albicans and is seen in patients who have chronic cutaneous candidiasis, a syndrome associated with cellular and humoral immune abnormalities.

Treatment of Nail Diseases


Superficial types of onychomycosis may be successfully treated. Mechanical scraping of the chalky white material on the nail plate and application of topical antifungal agents such as miconazole, ciclopirox olamine, or clotrimazole are recommended. Newer therapeutic nail lacquers are being tested in the United States. Distal subungual and proximal subungual onychomycosis infections are much more difficult to treat. Oral griseofulvin may be required to bring about clearing of the fingernail. For toenails with extensive involvement, oral itraconazole, fluconazole and terbinafine are effective. No oral or topical medication is effective in eliminating nondermatophyte mold infection of the nails.

Bacterial Nail Infections

Pseudomonas aeruginosa is associated with green nail syndrome, which is essentially a greenish discoloration of the nail plate. Attempts to culture Pseudomonas from the deep section of the nail have not been successful; however, P aeruginosa has been isolated on cultures of specimens from the paronychia (inflammatory lesion around the margin of a nail). Whether there is true invasion of the nail plate by the bacteria or just diffusion of the pigment into the nail plate is not certain. Black paronychia is associated with Proteus species. Staphylococci and streptococci may be found as secondary invaders.


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Copyright © 1996, The University of Texas Medical Branch at Galveston.
Bookshelf ID: NBK8301PMID: 21413322


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