Saturday, May 20, 2006


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.

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 b-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 disordera 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.

Medical Microbiology

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