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.

Morphology

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:

Fig. 22.1 The yeast (blastospore) and hyphal forms of Candida albicans.

• Yeasts are unicellular with spherical or ovoid bodies; all yeasts are similar morphologically on light microscopic examination.

• Moulds are multicellular with a variety of specialized structures that perform specific functions. The size and nature of these structures vary with different genera. Hyphae (singular: hypha or hyphum) are thread-like tubes containing the fungal cytoplasm and its organelles. They can be considered as the structural units of the mould. The hyphae are divided into unit cells by cross walls called septa. The septa have pores that allow the movement of cytoplasm, and even organelles, between cells. The term mycelium is given to the mass of hyphae that forms the mould colony.

Reproduction

Both asexual and sexual modes of reproduction are seen in fungi. It is believed that the sexual forms of fungi are not found in clinical material.

Classification

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:

• yeasts

• filamentous fungi

• dimorphic fungi.

The following methods are used in classification of fungi:

1. Yeasts are identified by biochemical reactions based on the fermentation and assimilation of carbohydrates, utilization of enzyme substrates and other metabolic activities.

2. Moulds are identified by their colour, texture and colonial and microscopic morphology. The specialized asexual reproductive structures of moulds are useful in differentiating various species of moulds.

Cultural requirements

Medically important fungi require different cultural and growth conditions in comparison to bacteria:

1. The vast majority of medically important fungi grow aerobically.

2. Sabouraud dextrose agar (SAB) and variations of it, such as SAB plus antibacterial agents, and potato dextrose agar (PDA) are commonly used for laboratory culture of pathogenic fungi.

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.

Pathogenicity

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:

• the ability to adhere to host tissues and prostheses (e.g. dentures) and form biofilms

• the potential to switch (e.g. rough to smooth colony formation) and modify the surface antigens

• the ability to form hyphae that helps in tissue invasion

• extracellular phospholipase and proteinase, and haemolysin production, which break down physical defence barriers of the host.

Human mycoses

Human infections caused by fungi can be divided into:

• superficial mycoses

• subcutaneous mycoses

• systemic mycoses.

Superficial mycoses

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:

• yeast infections of mucosae, which lead to thrush and similar manifestations (see Chapter 35)

• dermatophyte infection of skin, hair, etc., leading to ringworm or similar diseases.

Subcutaneous mycoses

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.

Systemic mycoses (deep mycoses)

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.

Opportunistic fungal infections

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

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: