General Characteristics of Fungi

■ Fungi are eukaryotic microorganisms (domain eucarya) that occur ubiquitously in nature. Only about 200 of the thousands of species have been identified as human pathogens, and among these known pathogenic species fewer than a dozen are responsible for more than 90% of all human fungal infections.

The basic morphological element of filamentous fungi is the hypha and a web of intertwined hyphae is called a mycelium. The basic form of a unicellular fungus is the yeast cell. Dimorphic fungi usually assume the form of yeasts in the parasitic stage and the form of mycelia in the saprophytic stage. The cell walls of fungi consist of nearly 90% carbohydrate (chitin, glucans, mannans) and fungal membranes are rich in sterol types not found in other biological membranes (e.g., ergosterol). Filamentous fungi reproduce either asexually (mitosis), by hyphal growth and tip extension, or with the help of asexual spores. Yeasts reproduce by a process of budding. Sexual reproduction (meiosis) on the other hand, produces sexual spores. Fungi imperfecti or deuteromycetes are the designation for a type of fungi in which the fructification forms are either unknown or missing entirely. ■

Definition and Taxonomy

Fungi are microorganisms in the domain eucarya (see. p. 5). They show less differentiation than plants, but a higher degree of organization than the pro-karyotes bacteria (Table 5.1). The kingdom of the fungi (Mycota) comprises over 50 000 different species, only about 200 of which have been identified as human pathogens. Only about a dozen of these "pathogenic" species cause 90% of all human mycoses. Many mycotic infections are relatively harmless, for instance the dermatomycoses. In recent years, however, the increasing numbers of patients with various kinds of immune defects have resulted in more life-threatening mycoses.

Table 5.1 Some Differences between Fungi and Bacteria

Properties

Fungi

Bacteria

Nucleus

Cytoplasm

Cytoplasmic membrane

Cell wall

Metabolism

Eukaryotic; nuclear membrane; more than one chromosome; mitosis

Mitochondria; endoplasmic reticulum; 80S ribosomes

Sterols (ergosterol)

Glucans, mannans, chitin, chitosan

Heterotrophic; mostly aerobes; no photosynthesis

Size, mean diameter Yeast cells: 3-5-10 im.

Molds: indefinable

Dimorphism

In some species

Prokaryotic; no membrane; nucleoid; only one "chromosome"

No mitochondria; no endoplasmic reticulum; 70S ribosomes

No sterols

Murein, teichoic acids (Gram-positive), proteins

Heterotrophic; obligate aerobes and anaerobes, facultative anaerobes

1-5 im None

The taxonomy of the fungi is essentially based on their morphology. In medical mycology, fungi are classified according to practical aspects as dermatophytes, yeasts, molds, and dimorphic fungi. Molds grow in filamentous structures, yeasts as single cells and dermatophytes cause infections of the keratinized tissues (skin, hair, nails, etc.). Dimorphic fungi can appear in both of the two forms, as yeast cells or as mycelia (see the following pages).

Fungi are carbon heterotrophs. The saprobic or saprophytic fungi take carbon compounds from dead organic material whereas biotrophic fungi (parasites or symbionts) require living host organisms. Some fungi can exist in both saprophytic and biotrophic forms.

Morphology

Two morphological forms of fungi are observed (Fig. 5.1):

■ Hypha: this is the basic element of filamentous fungi with a branched, tubular structure, 2-10 im in width.

— Basic Morphological Elements of Fungi

— Basic Morphological Elements of Fungi

Pseudomycelium

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Fig. 5.1 There are two basic morphological forms: hypha and yeast. a Hypha, septate, or nonseptate. b Mycelium: web of branched hyphae. c Yeast form, budding (diameter of individual cell 3-5 im). d Pseudomycelium.

■ Mycelium: this is the web or matlike structure of hyphae. Substrate my-celia (specialized for nutrition) penetrate into the nutrient substrate, whereas aerial mycelia (for asexual propagation) develop above the nutrient medium.

■ Fungal thallus: this is the entirety of the mycelia and is also called the fungal body or colony.

■ Yeast: the basic element of the unicellular fungi. It is round to oval and 310 im in diameter. Several elongated yeast cells chained together and resembling true hyphae are called pseudohyphae.

■ Dimorphism: some fungal species can develop either the yeast or the mycelium form depending on the environmental conditions, a property called dimorphism. Dimorphic pathogenic fungi take the form of yeast cells in the parasitic stage and appear as mycelia in the saprophytic stage.

Metabolism

All fungi are carbon heterotrophs, which means they are dependent on exogenous nutrient substrates as sources of organic carbon, and with a few exceptions, fungi are obligate aerobes. Many species are capable of maintaining metabolic activity in the most basic of nutrient mediums. The known metabolic types of fungi include thermophilic, psychrophilic, acidophilic, and ha-lophilic species. The metabolic capabilities of fungi are exploited in the food industry (e.g., in the production of bread, wine, beer, cheese, or single-cell proteins) and in the pharmaceutical industry (e.g., in the production of antibiotic substances, enzymes, citric acid, etc.). The metabolic activity of fungi can also be a damaging factor. Fungal infestation can destroy foods, wooden structures, textiles, etc. Fungi also cause numerous plant diseases, in particular diseases of crops.

Reproduction in Fungi

Asexual reproduction. This category includes the vegetative propagation of hyphae and yeasts as well as vegetative fructification, i.e., formation of asexual spores.

■ Hyphae elongate in a zone just short of the tip in which the cell wall is particularly elastic. This apical growth process can also include formation of swellings that develop into lateral hyphae, which can in turn also branch out.

■ Yeasts reproduce by budding. This process begins with an outgrowth on the mother cell wall that develops into a daughter cell or blastoconidium. The isthmus between the two is finally cut off by formation of a septum. Some yeasts propagate in both the yeast and hypha forms (Fig. 6.2, p. 362).

■ Vegetative fructification. A type of propagative form, the asexual spores, is formed in this process. These structures show considerable resistance to exogenous noxae and help fungi spread in the natural environment. Asexual spores come in a number of morphological types: conidia, sporangiospores, arthrospores, and blastospores. These forms rarely develop during the parasitic stages in hosts, but they are observed in cultures. The morphology of the asexual spores of fungi is an important identification characteristic.

Sexual fructification. Sexual reproduction in fungi perfecti (eumycetes) follows essentially the same patterns as in the higher eukaryotes. The nuclei of two haploid partners fuse to form a diploid zygote. The diploid nucleus then undergoes meiosis to form the haploid nuclei, finally resulting in the haploid sexual spores: zygospores, ascospores, and basidiospores. Sexual spores are only rarely produced in the types of fungi that parasitize human tissues.

Sexual reproduction structures are either unknown or not present in many species of pathogenic fungi, known as fungi imperfecti (deuteromy-cetes).

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