Treponema pallidum causes venereal syphilis. By dark-field illumination, T pallidum appears fine and corkscrew shaped, with rigid, uniform spirals. For further discussion of syphilis, see BCSC Section 1, Update on General Medicine, and Section 9, Intraocular Inflammation and Uveitis.

Borrelia burgdorferi

Borrelia species are obligate parasites, best visualized with Giemsa stain. B burgdorferi, the etiologic agent of Lyme disease, is transmitted to humans by the deer tick. The white-footed mouse is considered the natural reservoir of B burgdorferi. Ticks transfer the organism to deer and other mammals, migrating birds, and humans. The pathogenic factors of B burgdorferi include the expression of proteinases that facilitate tissue invasion, induction of proinflammatory cytokines on binding to phagocytes, and activation of the complement cascade. Although the organism can be cultured from biopsies of erythema migrans skin lesions, the diagnosis of Lyme disease is determined by serology and typical clinical findings. See also BCSC Section 1, Update on General Medicine, and Section 9, Intraocular Inflammation and Uveitis.

Mycology

Fungi are eukaryotes that develop branching filaments and reproduce by means of sexually or asexually produced spores. Fungal cell walls are rigid and contain chitin and polysaccharides. Fungi are classically divided into 2 groups: yeasts, which are round or oval fungi that reproduce by budding and sometimes form pseudohyphae by elongation during budding; and molds, which are multicellular fungi composed of tubular hyphae, either septate or nonseptate, that grow by branching and apical extension (Table 5-2). Yeasts may also form hyphae under certain circumstances. The branching hyphae of molds can form a mycelium, an interconnected network of hyphae. Septate fungi are distinguished by walls that divide the filaments into separate cells, each containing one or more nuclei (Fig 5-5). Dimorphic fungi grow in 2 distinct forms as a result of changes in cell-wall synthesis in different environments and may often constitute highly virulent pathogens. Fungal cell walls stain with Gomori methenamine silver but, except for Candida, do not take up Gram stain. Classification of filamentous fungi is based on microscopic features of conidia (fungal elements that form asexually) and conidiophores (the specialized hyphae where conidia are formed). However, the histologic morphology of fungi varies significantly when they are isolated from tissue (corneal scraping/biopsy); fungi therefore require laboratory isolation for definitive identification. Most antifungal medications target the fungal cell wall either through direct toxicity or inhibition of synthesis.

Table 5-2

Figure 5-5 Septate hyphae of filamentous fungus (Fusarium solani). (Diff-Quick stain ×100.) (Courtesy of Elmer Y. Tu, MD.)

Yeasts

The incidence of mycotic infections has increased significantly with the rise of immunosuppressed states, both disease related and pharmacologically induced, as well as with the increase in long-term antibacterial use by immunocompromised patients and the general population. Candida species are ubiquitous in the environment and are ordinarily resident flora of, and recoverable from, the gastrointestinal and genitourinary tracts, the oropharynx, and the skin (with Candida albicans being the most common species at these sites; Fig 5-6). Yeast is a disproportionate cause of fungal keratitis in cooler northern climes. The pathogenesis of this yeast in the cornea is enhanced by the formation of pseudohyphae, which express proteases and phospholipases, facilitating tissue penetration.

Figure 5-6 Yeasts (Candida albicans). (Gram ×1000.) (Courtesy of James Chodosh, MD.)

Cryptococcus neoformans is acquired through inhalation and causes subclinical infection of the pulmonary tract. Clinical cryptococcal disease in the brain and optic nerve, eye, lung, skin, and prostate occurs in immunosuppressed patients.

Rhinosporidium seeberi organisms are present in soil and groundwater and presumably infect humans through contact with these sources. Ocular rhinosporidiosis manifests as sessile or