On this basis, it may be possible to identify pathogen-specific molecular targets from investigations of the biosynthesis of fungal wall components. The composition of fungal cell walls is relatively simple and includes substances not typically found in animal and plant hosts (e.g., chitin). Even a small lesion in the cell wall can result in extrusion of cytoplasm as a result of the internal (turgor) pressure of the protoplast. The fungal cell wall is also essential to prevent osmotic lysis. This filtering effect may be especially important in protecting fungal pathogens against certain fungicidal products of the host. The fungal wall also protects cells against mechanical injury and blocks the ingress of toxic macromolecules. Thus, our knowledge of fungal morphogenesis has evolved in parallel with our understanding of fungal cell wall biosynthesis. Not all species of fungi have cell walls, but in those that do, cell wall synthesis is an important factor in determining the final morphology of fungal elements. Fungal pathogens rely on these digestive enzymes to penetrate natural host barriers. This process involves release of specific proteolytic, glycolytic, or lipolytic enzymes from the hypha or yeast, extracellular breakdown of the substrate(s), and diffusion of the products of digestion through the fungal cell envelope ( Fig. Macromolecules and insoluble polymers (e.g., proteins, glycogen, starch, and cellulose), on the other hand, must undergo preliminary digestion before they can be absorbed by the fungal cell. Small molecules (e.g., simple sugars and amino acids) accumulate in a watery film surrounding the hyphae or yeast and simply diffuse through the cell wall. The true fungi obtain their carbon compounds from nonliving organic substrates (saprophytes) or living organic material (parasites) by absorption of nutrients through their cell wall. Academic Press, San Diego, 1981, with permission.) (X8,980) (From Cole, GT, Kendrick B: Biology of Conidial Fungi. DYC, Daughter yeast cell GT, germ tube H, hypha Ph, pseudohypha YMC, yeast mother cell. It can undergo rapid transformation from the yeast to the hyphal phase in vivo, which partly contributes to its success in invading host tissue.ĭimorphism in C albicans. Candida is an example of such a dimorphic fungus ( Fig. Some of the opportunistic fungal pathogens of humans are dimorphic, growing as a mycelium in nature and as a vegetatively reproducing yeast in the body. Many fungi occur not as hyphae but as unicellular forms called yeasts, which reproduce vegetatively by budding. The septum, however, still provides for cytoplasmic communication, including intercellular migration of nuclei. The more phylogenetically primitive molds (e.g., water molds, bread molds, and other sporangial-saclike-forms) produce cenocytic filaments (multinucleate cells without cross-walls), while the more advanced forms produce hyphae with cross-walls (septa) that subdivide the filament into uninucleate and multinucleate compartments. A mass of hyphae forms the thallus (vegetative body) of the fungus, composed of mycelium. They are characterized by filamentous, vegetative cells called hyphae. The molds, for example, are a large group of microscopic fungi that include many of the economically important plant parasites, allergenic species, and opportunistic pathogens of humans and other animals. Macroscopic fungi such as morels, mushrooms, puffballs, and the cultivated agarics available in grocery stores represent only a small fraction of the diversity in the kingdom Fungi.
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