Chapter 42: Fungi—Basic Concepts

Fungi are eukaryotes with a higher level of biologic complexity than bacteria. They are spore-bearing, reproducing both sexually and asexually. Fungi may be unicellular or may differentiate and become multicellular by the development of long, branching filaments. They lack the chlorophyll of plants, therefore needing to acquire nutrients from the external environment. The diseases caused by fungi are called mycoses. These infections vary greatly in their manifestations but tend to present with subacute or chronic features, often relapsing over time. Acute disease, such as that produced by many viruses and bacteria, is uncommon with fungal infections.

STRUCTURE

The fungal cell has many typical eukaryotic features, including a nucleus with a nucleolus, nuclear membrane, and linear chromosomes (Figure 42–1). The cytoplasm contains a cytoskeleton with actin microfilaments and tubulin-containing microtubules. Ribosomes and organelles, such as mitochondria, endoplasmic reticulum, and the Golgi apparatus, are also present. Fungal cells have a rigid cell wall external to the cytoplasmic membrane, which differs in its chemical composition from the cell walls of bacteria and plants. In addition to the cell wall, another important difference from mammalian cells is the sterol makeup of the cytoplasmic membrane. In mammalian cells, the dominant membrane sterol is cholesterol; in fungi, it is ergosterol. Fungi are usually haploid in their DNA content, although diploid nuclei are formed through nuclear fusion in the process of sexual reproduction. Interestingly, the generation of polyploid/aneuploid nuclei is a strategy used by some fungi to generate genetic diversity as a response to cell stress, such as antifungal therapy.

The chemical structure of the cell wall in fungi is markedly different from that of bacterial cells in that it does not contain peptidoglycan, glycerol, teichoic acids, or lipopolysaccharide. In their place are complex polysaccharides such as mannans, glucans, and chitins in close association with each other and with structural proteins (Figure 42–2). Mannoproteins are composed of mannose-based polymers (mannan) found on the cell surface and in the structural matrix of the cell wall, where they are linked to various proteins. Mannoproteins are very important since antibodies are readily developed against these molecules on the cell surface. The potential variations in the composition and linkages of the mannan side chains allow fungi to generate a complex and adaptable cell surface to avoid easy immune detection by an infected host. The identification of different antibodies directed against specific mannoproteins also allows laboratories to “serologically” distinguish between individual strains within a fungal species. The alpha- and beta-glucans are polymers of glucose found abundantly throughout the cell wall. Additionally, chitin, composed of long chains of N-acetylglucosamine, provides rigid structural support to the fungal cell in a manner analogous to the chitin in crab shells or cellulose in plants. In addition to their structural roles, these cell wall carbohydrates serve complex cell functions, often simultaneously activating and inhibiting various arms of the host immune response.

METABOLISM

A major difference between fungi and plants is that fungi lack chloroplasts and photosynthetic energy-producing mechanisms. Therefore, fungi must acquire nutrients from exogenous sources. Metabolic diversity among fungi is great, but most are able to grow with very simple carbon and nitrogen sources. In nature, nutrients for free-living fungi are derived from decaying organic matter. Most fungi are strict aerobes, although some can grow under anaerobic conditions.

REPRODUCTION

Fungi may reproduce by either asexual or sexual process. The asexual form is called the anamorph, and its reproductive elements are termed conidia. The sexual form is called the teleomorph, and its reproductive structures are called spores (eg, ascospores, zygospores, basidiospores). Asexual reproduction involves mitotic division of the haploid nucleus and is associated with production by budding, spore-like conidia or, alternatively by the separation of hyphal elements. In sexual reproduction, the haploid nuclei of donor and recipient cells fuse to form a diploid nucleus, which then divides by classic meiosis. Some of the four resulting haploid nuclei may be genetic recombinants and may undergo further division by mitosis. Highly complex specialized structures may be involved. Detailed study of the mating process in fungal species, such as the baker’s yeast Saccharomyces cerevisiae and the red bread mold Neurospora crassa, has been important in gaining an understanding of basic cellular genetic mechanisms.

The size of fungi varies immensely. A single cell without transverse septa may range from bacterial size (2-4 μm) to a macroscopically visible structure. The morphologic forms of growth vary from colonies superficially resembling those of bacteria to some of the most complex, multicellular, colorful, and beautiful structures seen in nature. Mushrooms are an example and can be regarded as a complex organization of cells showing structural differentiation.

Mycology, the science devoted to the study of fungi, has various terms to describe the morphologic components that comprise these structures. The terms and concepts that must be mastered can be limited by considering only the fungi of medical importance and accepting some simplification.

YEASTS AND MOLDS

Fungi that cause human infections can be broadly divided based on their morphological forms. Yeasts are fungi that primarily grow in a round cellular form. Molds are fungi that primarily grow as filamentous, tube-like structures called hyphae (Figure 42–3A and B). Although it is useful to consider this basic distinction based on cell shape, it is important to remember that some fungi can transition between yeast-like and hyphal morphologies. Often, this plasticity of shape is directly related to pathogenesis since different forms may be better suited for different microenvironments. The yeasts tend to have the simplest cellular forms, reproducing by a process of asexual budding, constriction, and cell separation similar to many bacteria. The newly formed daughter cell is often called a blastoconidium.

Fungi may also grow through the development of hyphae (singular, hypha), which are tube-like extensions of the cell with thick, parallel walls. As the hyphae extend, they form an intertwined mass called a mycelium. Most molds form hyphal septa (singular, septum), which are cross-walls perpendicular to the cell walls, dividing the hypha into subunit cells (Figure 42–4). The structure of these septa varies among species and may contain pores and incomplete walls that allow movement of nutrients, organelles, and nuclei between adjacent cells. Some species, including some human pathogens, form septae that are very distant from each other. Because their microscopic appearance therefore suggests a single, continuous hyphal cell, these particular species are often called “aseptate” molds. In both septate and aseptate hyphae, multiple nuclei are often present in each cell.

A portion of the mycelium (vegetative mycelium) usually grows into the medium or organic substrate (eg, soil) and functions like the roots of plants as a collector of nutrients and moisture. The more visible surface growth may assume a fluffy character as the mycelium becomes aerial. The hyphal walls are rigid in order to support this extensive, intertwining network. The aerial hyphae bear the reproductive structures of this class of fungi. These sexual structures are often unique to each species, allowing microbiology laboratories to distinguish among molds based on their morphological features. Some fungi also form pseudohyphae, which are actually elongated yeast cells growing end-to-end. Therefore, pseudohyphae are distinguished from true hyphae by having recurring bud-like constrictions and less rigid cell walls.

Exogenously formed conidia may develop directly from the hyphae or on a special stalk-like structure, the conidiophore (Figure 42–3B). Occasionally, terms such as macroconidia and microconidia are used to indicate the size and complexity of these conidia. Conidia that develop within the hyphae are called either chlamydoconidia or arthroconidia. Chlamydoconidia become larger than the hypha itself; they are round, thick-walled structures that may be borne on the terminal end of the hypha or along its course. Arthroconidia conform more to the shape and size of the hyphal units but are thickened or otherwise differentiated. Arthroconidia may form a series of delicately attached conidia that break off and disseminate when disturbed. Some of the asexual reproductive forms are illustrated in Figure 42–5A–D. The most common sexual spore is termed an ascospore. Four or eight ascospores may be found in a sac-like structure, the ascus.

DIMORPHISM

Although many fungi tend to grow as either yeasts or molds, some species can transition between morphological forms depending on environmental conditions. These species are known as dimorphic fungi. Many fungi, including the most common human fungal pathogen Candida albicans, display a striking ability to modify their cellular shape and structure in order to adapt to new environments. These morphological transitions are often very important for the pathogenesis of human infections.

A distinct group of human pathogens are the thermally dimorphic fungi, shifting from yeast-like to hyphal growth based on temperature. These fungal species tend to grow in the mold form in their environmental reservoir as well as when incubated in culture at ambient temperatures. However, they convert to a yeast-like growth form in the mammalian host or when incubated in culture at 37°C. For most, it is possible to manipulate the culture conditions to demonstrate both yeast and mold phases in vitro. Yeast phase growth requires conditions similar to those of the physiologic in vivo environment, such as 35°C to 37°C incubation and enriched medium. Mold growth requires minimal nutrients and ambient temperatures. Importantly, the conidia produced in the mold phase may be infectious and serve to disseminate the fungus during growth in the environment.

The morphologic and physiologic events associated with the temperature-dependent conversion between the mold and yeast phases have been studied in the human pathogen Histoplasma capsulatum. They are understandably complex, given the dramatic change of milieu encountered by the fungus when its mold conidia float from their soil habitat to the pulmonary alveoli. Conversion to the yeast phase is then triggered by the host temperature (37°C) and other conditions associated with the host microenvironment (eg, iron limitation, pH changes, elevated levels of CO₂). In vitro studies show that the earliest events in this morphological shift involve induction of the heat shock response and uncoupling of oxidative phosphorylation. These early cellular events are followed by a shutdown of RNA synthesis, protein synthesis, and respiratory metabolism. The cells then pass through a metabolically inactive state, emerging with enhanced enzymatic capacities involving sulfhydryl compounds (eg, cysteine, cystine) that are exclusive to the yeast stage. As yeast growth develops, there is recovery of mitochondrial activity and synthetic capacity, but a new constellation of oxidases, polymerases, proteins, cell wall glucans, and other compounds are present, likely favoring host parasitism. In all, more than 500 genes are differentially expressed in the mold and yeast phases.

Dimorphism in fungi is reversible, a feature that distinguishes it from developmental processes such as embryogenesis seen in higher eukaryotes. The importance of the dimorphism in fungal virulence has been demonstrated in several fungi, including C albicans and H capsulatum. Strains that are locked in one growth phase are markedly reduced in their ability to produce disease and persist in the host.

CLASSIFICATION

Fungal classification has historically relied upon observable cellular characteristics such as the septation of hyphae and the appearance of the sexual structures. However, DNA sequence-based classification methods are becoming more common, allowing fungi to be grouped by genetic relatedness. Molecular classification techniques have also demonstrated that several microbial species are actually fungi despite having few fungal growth characteristics (eg, Pneumocystis species and Microsporidia).

Fungi have historically been organized into five phyla: Ascomycota, Basidiomycota, Zygomycota, Chytridiomycota, and Glomeromycota. The medically important genera fall mostly within the Ascomycota, with a few in Basidiomycota, and Zygomycota, as shown in Table 42–1.

The grouping of medically important fungi used in the following chapters is based on the types of tissues they parasitize and the diseases they produce, rather than on the principles of basic mycologic taxonomy. The superficial fungi, such as the dermatophytes, cause indolent lesions of the skin and its appendages, commonly known as ringworm and athlete’s foot, without typically spreading to deeper tissues. The subcutaneous pathogens characteristically cause infection through the skin, followed by subcutaneous or lymphatic spread. The opportunistic fungi are those found in the environment or in the resident flora that produce disease primarily in immunocompromised hosts. The systemic pathogens are the most virulent fungi and may cause serious and progressive systemic disease in previously healthy persons. They are not found in the human microbiota. Although their major potential is to produce deep-seated visceral infections and systemic spread (systemic mycoses), they may also produce superficial infections as part of their disease spectrum or as the initiating event. As with all clinical classifications, overlaps and exceptions occur. In the end, the interplay between host and microorganism defines the disease.