Chapter 22 Fungi of relevance to dentistry
The study of fungi is called mycology. Fungi are eukaryotic microorganisms, as opposed to bacteria together with Archaea that are prokaryotic (Chapter 2). By far the most important fungus of relevance in dentistry is a yeast belonging to the genus Candida. It is an oral commensal in about half of the general population. In this chapter, the general characteristics of some medically important fungi will be given, but the emphasis will be on fungal infections of the oral cavity – the oral mycoses, especially those caused by the Candida species.
Fungi exhibit two basic structural forms: the yeast form (Fig. 22.1) and the mould form. While some fungi are capable of existing as both forms (dimorphic) at different times, others exist in one form only. This morphological switching depends on factors such as the environment and nutrient supply. Generally, dimorphic fungi exist as moulds in the natural environment (and in laboratory culture) and as yeasts in tissue:
Taxonomy of fungi is a complex subject not dealt with here. Most of the medically important fungi are classified as fungi imperfecti as their sexual forms have not been identified. Fungi of medical importance are classified into:
These mycological media differ from conventional bacteriological media in having a high carbohydrate content (SAB usually contains 3% dextrose or sucrose) and an acidic pH (approximately 4.0). Both these conditions are inhibitory to most bacteria. The SAB medium may also be supplemented with antibiotics to suppress bacterial growth.
In general, medically important fungi do not possess the virulent attributes of bacteria such as exotoxins and endotoxins (an exception is the exotoxin, aflatoxin, produced by Aspergillus species); hence, they cause slowly progressive chronic infections rather than the acute disease commonly seen in bacterial or viral diseases. However, they may cause life-threatening acute infections in immunocompromised patients (e.g. those with acquired immune deficiency syndrome (AIDS)). The oral fungal pathogen Candida possesses a number of virulent attributes, including:
Superficial mycoses involve the mucosal surfaces and keratin-containing structures of the body (skin, nails and hair). These infections, relatively common in western countries, are in general cosmetic problems and are not life-threatening. Superficial mycoses include:
Subcutaneous mycoses involve the subcutaneous tissue and rarely disseminate. They are the result of traumatic implantation of environmental fungi leading to chronic progressive disease, tissue destruction and sinus formation. Examples include sporotrichosis and mycetoma (Madura foot), which are common in the tropics and rare in the West.
By far the most serious, and often fatal, systemic mycoses involve the internal organ systems of the body. The organisms are generally acquired through the respiratory tract and spread haematogenously. In the developed world, they are increasingly seen in compromised patients with impaired defence systems when the organisms behave as opportunistic pathogens. In the developing world, systemic mycoses (e.g. histoplasmosis, blastomycosis and coccidioidomycosis) occur in otherwise healthy individuals.
When fungi (such as Candida albicans) that are generally innocuous for healthy humans cause disease in compromised patient groups, they are called opportunistic pathogens. Such opportunistic mycoses are increasingly common owing to a global rise in compromised individuals such as human immunodeficiency virus (HIV)-infected patients, organ transplant recipients on immunosuppressive therapy and cancer patients on cytotoxic and radiation therapy.
Yeasts are unicellular, oval or spherical organisms, 2–5 µm in diameter, and stain positively by the Gram method (Fig. 22.2). They are commonly seen to have lateral projections or buds called daughter cells. These gradually enlarge in size until they split off from the parent or mother cell to produce the next generation. Most yeasts develop pseudohyphae (chains of elongated budding cells devoid of septa or cross walls) but only a few form true hyphae (septate hyphae). Yeasts of the genus Candida, the most important fungal pathogen in the oral cavity, also form pseudohyphae. It is a common yeast that lives in the oral cavity of about half of the population and is also a resident commensal of the gut. It can cause either superficial or systemic candidiasis (synonym: candidosis). The superficial disease affects:
The infection is usually endogenous in origin. Several species in the genus Candida are found in humans, including C. albicans, C. glabrata, C. krusei and C. tropicalis (Fig. 22.3), but C. albicans is responsible for the vast majority of infections (>90%). Candida dubliniensis is a newly recognized species of Candida very similar to C. albicans. First isolated from the oral cavity/>