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Bacterial, spirochete, and protozoan infections


Bacterial diseases

Impetigo

Key Features

  • Neutrophilic crust

  • Chains or clusters of cocci

Impetigo recruits neutrophils to the stratum corneum. Organisms are commonly visible in hematoxylin and eosin sections. Gram stain and culture may be required.

Differential Diagnosis

Collections of neutrophils within the stratum corneum: psoriasis, tinea, impetigo, Candida, seborrheic dermatitis, syphilis (PTICSS)

Fig. 17.1, Impetigo

Bullous impetigo

Key Features

  • Subcorneal bulla

  • Acantholysis in granular layer

Differential Diagnosis

Staphylococcal scalded-skin syndrome and pemphigus foliaceus demonstrate acantholysis at the same level.

Fig. 17.2, Bullous impetigo

Suppurative folliculitis

Key Features

  • Suppurative inflammation in or around a follicle

  • Focal crusts in the stratum corneum

  • Vertical column of suppurative inflammation in the dermis

The follicle may not be visible in every plane of section. In some sections, only a focus of inflammatory cells may be noted in the dermis. The microscopic differential diagnosis includes bacterial infection (including furunculosis and hot-tub folliculitis), fungal infection, chemical folliculitis, acne, rosacea, and pustular drug eruption.

Botryomycosis

Key Features

  • Large staphylococcal grains in tissue

  • Abscesses and sinus tracts

Clinically, botryomycosis may resemble a mycetoma. The grains represent huge staphylococcal colonies.

Fig. 17.3, Botryomycosis

Pitted keratolysis

Key Features

  • Acral skin

  • Dell or pit in stratum corneum

  • Rods and cocci at base of dell

Pitted keratolysis is rarely biopsied because it is readily distinguished by its appearance and smell (so-called toxic sock syndrome ). Both micrococci and diphtheroids are generally present ( Kytococcus sedentarius and Corynebacterium ).

Fig. 17.4, Pitted keratolysis

Erythrasma

Key Features

  • Rods forming vertical filaments in stratum corneum

  • Inflammation variable

Fig. 17.5, Erythrasma

Ecthyma gangrenosum

Key Features

  • Necrosis of deep dermal vessels

  • Amphophilic bacilli surrounding vessels (light blue haze)

  • Lack of inflammatory infiltrate around vessels

  • Variable hemorrhage and cutaneous necrosis

Ecthyma gangrenosum is usually a manifestation of Pseudomonas sepsis.

Fig. 17.6, Ecthyma gangrenosum

Rhinoscleroma

Key Features

  • Sheets of plasma cells

  • Russell bodies

  • Mikulicz cells (resemble globi of leprosy)

Rhinoscleroma is caused by Klebsiella rhinoscleromatis . The inflammatory infiltrate is mixed and contains many plasma cells. Russell bodies are plasma cells filled with bright pink immunoglobulin (“pregnant plasma cells”). The nucleus may no longer be visible. Mikulicz cells are histiocytes containing large, round collections of bacilli.

Fig. 17.7, Rhinoscleroma

Chancroid

Key Features

  • Ulcer

  • Zone of necrosis, fibrin, and neutrophils at surface

  • Granulation tissue

  • Many plasma cells below granulation tissue

  • Gram stain and culture may demonstrate bacteria

Fig. 17.8, Chancroid

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