GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

8. FUNGI

8.3. STRUCTURE OF FUNGI

The fungal Cell (Fig. 8.1) consists of a Cell wall, Cytoplasm with a Plasma Membrane, Endoplasmic reticulum, Mitochondria, Ribosomes, inclusions, vacuoles, and a Nucleus (or nuclei). The fungal nucleus is clearly distinct and surrounded by a membrane.

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Fig. 8.1. Diagram of the fungal Cell Structure:

1 — Golgi apparatus (dictyosome); 2 — ribosomes; 3 — lomasomes; 4 — endoplasmic reticulum; 5 — mitochondria; 6 — cytoplasm; 7 — cell wall; 8 — nucleus; 9 — nuclear envelope; 10 — plasma membrane

The fungal cell wall contains about 80–90% Polysaccharides associated with Proteins, Lipids, polyphosphates, and pigments. It consists of multiple layers (outer and inner, or Primary and secondary cell walls). Matrix (skeletal) structures (or the primary wall) are composed of Chitin and Cellulose, while the outer layer (the secondary wall) typically consists of glucans with various types of linkages across different fungal taxa. For example, a cellulose-chitin complex predominates in The Cell walls of many zygomycetes, a cellulose-glucan complex in oomycetes, a chitosan-chitin complex in certain oomycetes, and a chitin-glucan complex in chytridiomycetes, ascomycetes, Basidiomycetes, and deuteromycetes. The cell walls of Fungi have also been found to contain polysaccharides incorporating glucuronic acid, as well as Heteropolysaccharides containing mannose, galactose, glucose, and glucuronic acid.

Lomasomes are vesicular bodies located between The cell wall and The Plasma Membrane. Found in almost all fungi, they are involved in the Synthesis of the cell wall.

Fungal protoplasts are spherical cell formations devoid of a cell wall and sensitive to osmotic Shock. They exhibit metabolic processes and regenerative capacity. Protoplasts are obtained by lysing cell walls using lytic Enzymes or microorganisms, as well as compounds that disrupt cell wall synthesis. For example, the antibiotic griseofulvin inhibits cell wall synthesis in chitin-containing fungi. The antibiotic nystatin binds to the sterols of the plasma membrane and cell wall. The formation of protoplasts in fungi, both during cultivation and in nature, remains insufficiently studied; however, it is likely to play a role in the Pathogenesis of fungal diseases.

The plasma membrane is structurally analogous to the membranes of plant, animal, and Prokaryotic Cells (the so-called universal "unit membrane"), consisting of a lipid bilayer sandwiched between thin protein layers. Protein molecules are located not only on the membrane surface but also span entirely through it.

The Cell Cytoplasm is a colloidal fluid medium of varying viscosity containing cellular Organelles, reserve substance inclusions of various origins, and a system of Cytoplasmic membranes. The cytoplasmic membrane system consists of filaments, strands, tubules, and vesicular networks that envelop individual cell organelles. The membrane systems of the fungal cell include the Golgi apparatus—aggregated vesicles or plates (dictyosomes) of various sizes located near the nuclear membrane, hyphal septa, and conidia.

The endoplasmic reticulum is a dynamic system of cell metabolites located near the plasma membrane and mitochondria. It is formed through the invagination of the plasma membrane or As a result of division.

Vacuoles are clearly visible structures of spherical or irregular shape. They concentrate reserve substances or toxic intermediate metabolites of the cell. It is believed that vacuoles in the cell may originate from the endoplasmic reticulum or the Golgi apparatus.

Lysosomes are cellular organelles represented by bodies of diverse shapes surrounded by a single membrane. They bud off from the Golgi apparatus and reside in the cytoplasm as vesicles. They contain enzymes that break down polymers and are also capable of accumulating metabolic intermediates that are toxic to the cell.

The Nucleus is surrounded by a double-layered, pored envelope formed by the outer and inner nuclear membranes, and contains a nucleolus and Chromosomes. The nucleus contains DNA (in the form of chromosomes), which carries the Genetic information of the cell.

Mitochondria are ellipsoid structures surrounded by a double membrane that constantly move through the cytoplasm, con-

taining internal infoldings known as cristae. They act as energy generators (ATP) in the cell and contain A number of redox enzymes. Mitochondria also contain Mitochondrial DNA.

Ribosomes are numerous small bodies involved in Protein Synthesis.

Inclusions. The main storage substance in fungi is Glycogen, which is distributed as small granules throughout the cytoplasm. Polyphosphates are found in a colloidal state within vacuoles, accounting for up to 22% of all mineral components in the cell. Lipids and fatty substances occur in The Cell as droplets called Liposomes.

The vegetative body of a fungus (thallus) consists of filaments about 5 µm in thick—hyphae (see Chapter 3, Fig. 3.3). Hyphae lack cross-walls (septa) in lower fungi and are divided by such septa into cells in higher fungi. The network of hyphae forming the fungal thallus is called the mycelium.

Mycelial fungi are characterized by dimorphism—the ability of fungi to develop either as Yeast-like cells or to form a mycelium depending on the cultivation conditions.

Modifications of mycelial growth include chlamydospores, strands, rhizomorphs, sclerotia, appressoria, haustoria, and rings.

Chlamydospores are hyphal cells with thickened walls containing fat and glycogen inclusions, and a diameter larger than that of the hypha. They are formed as fragments of hyphae (intercalary), at the ends of hyphae (terminal), or within conidia. They may form during the Aging of a culture following typical sporulation.

Sclerotia are distinct aggregates of septate hyphae forming specialized structures. The hyphal cells and their walls thicken, often becoming melanized and acquiring a dark coloration. The thickened hyphal cells are located in the outer layer of the sclerotium surrounding the thinner-walled, unpigmented Cells of the inner layers. Sclerotium sizes range from a few millimeters to several tens of centimeters in certain fungi, and they can be spherical or irregular in shape. Sclerotia are resistant to unfavorable environmental factors. Two Types of sclerotium formation are described: terminal (at the ends of hyphae) and intercalary (within individual fragments of main hyphae).

Mycelial cords are linearly aggregated hyphae ranging from a few hyphae to cords several centimeters in diameter. They originate from the vegetative mycelium, occurring more frequently in natural environments than under cultivation, and are characteristic of basidiomycetous fungi. More complexly aggregated hyphae (multiply aggregated) are referred to as rhizomorphs. Mycelial cords and rhizomorphs facilitate the spread of the fungus within the substrate, The transport of nutrients along the hyphae to the sites of fruit body formation, and survival under unfavorable conditions.

An appressorium and a haustorium are mycelial branches formed at the tip of a growing hypha.