Fungi are eukaryotes that develop branching filaments. Their cell walls are rigid and contain chitin and polysaccharides. Fungi are classically divided into 2 groups: yeasts, round or oval fungi that reproduce by budding and sometimes form pseudohyphae by elongation during budding; and molds, multicellular fungi composed of tubular hyphae, either septate or nonseptate, that grow by branching and apical extension (Table 10-2). 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 the presence of walls that divide the filaments into separate cells, each containing one or more nuclei (Fig 10-5). 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 thus require laboratory isolation for definitive identification. Most antifungal medications target the fungal cell wall either through direct toxicity or interference with cell wall synthesis.
The incidence of mycotic infections has increased significantly with the rise of immuno-suppressed 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 10-6). 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 10-6 Yeasts (Candida albicans). Budding yeast form (lower arrow) and solitary yeast (upper arrow). (Gram stain, original magnification ×1000.)
(Courtesy of James Chodosh, MD.)
Cryptococcus neoformans infection is acquired through inhalation and causes clinical disease in the brain and optic nerve, eye, lung, skin, and prostate in immunosuppressed patients.
Rhinosporidium seeberi is present in soil and groundwater and presumably infects humans through contact with these sources. Ocular rhinosporidiosis manifests as sessile or pedunculated papillomatous or polypoid lesions in the conjunctiva, which may be associated with similar lesions in the nose and nasopharynx.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.