Eukaryotic Organisms: Fungi Lecture Notes PDF

Eukaryotic Organisms: Fungi Lecture: 12 All texts and images are subject to copyright Fungi Of the more than 100,000 species of fungi, only about 200 are pathogenic to humans and animals. However, over the last 10 years, the incidence of serious fungal infecti...

Eukaryotic Organisms: Fungi Lecture: 12 All texts and images are subject to copyright Fungi Of the more than 100,000 species of fungi, only about 200 are pathogenic to humans and animals. However, over the last 10 years, the incidence of serious fungal infections has been increasing. These infections are occurring in health care settings and in people with compromised immune systems, and often involve fungi normally found in and on the human body. In addition, thousands of fungal diseases afflict economically important plants. Fungi are also beneficial. They’re important in the food chain because they decompose dead plant matter, thereby recycling vital elements. The hard parts of plants, which animals can’t digest, are decomposed primarily by fungi through the use of extracellular enzymes, such as cellulases. Nearly all plants depend on symbiotic fungi, known as mycorrhizae, which help their roots absorb minerals and water from the soil..The study of fungi is called mycology. Characteristics of Fungi ▪ Multicellular fungi are identified on the basis of physical appearance, including colony characteristics and reproductive spores. Vegetative Structures ▪ Fungal colonies are described as vegetative structures because they’re composed of the cells involved in catabolism and growth. ▪ Some of the vegetative cells may develop into reproductive cells called spores. Molds and Fleshy Fungi The thallus (body) of a mold or fleshy fungus (mushroom) consists of long filaments of cells joined together; these filaments are called hyphae (singular: hypha). Hyphae can grow to immense proportions. A mushroom was found to extend over 4 square miles. ▪ Septate hyphae: in most molds, the hyphae contain cross-walls called septa (singular: septum), which divide them into distinct, uninucleate (one-nucleus) cell-like units. These hyphae are called septate hyphae. ▪ Coenocytic hyphae: in a few classes of fungi, the hyphae contain no septa and appear as long, continuous cells with many nuclei. These are called coenocytic hyphae. Characteristics of Fungi ▪ Even in fungi with septate hyphae, there are usually openings in the septa that make the cytoplasm of adjacent “cells” continuous. ▪ Hyphae grow by elongating at the tips. ▪ Each part of a hypha is capable of growth, and when a fragment breaks off, it can elongate to form a new hypha. ▪ Thus, the fragmentation of hyphae is a form of asexual reproduction. ▪ In the laboratory, fungi are usually grown from fragments obtained from a fungal thallus. ▪ Vegetative hypha: The portion of a hypha that obtains nutrients is called the vegetative hypha ▪ Reproductive or aerial hypha: of a hypha that is involved in reproduction is the reproductive or aerial hypha. ▪ Aerial hyphae often bear reproductive spores. ▪ When environmental conditions are suitable, the hyphae grow to form a filamentous mass called a mycelium, which is visible to the unaided eye.  Figure: Fungal thallus consist of a mass of hyphae. (a) A photomicrograph of aerial hyphae, showing reproductive spores. (b) A colony of Aspergillus niger grown on a glucose agar plate, showing both vegetative and aerial hyphae. Characteristics of Fungi Yeasts ▪ Yeasts are non-filamentous, unicellular fungi that are typically spherical or oval. ▪ Like molds, yeasts are widely distributed in nature; they are frequently found as a white powdery coating on fruits and leaves. ▪ Yeasts can be distinguished by their method of asexual reproduction as either 1. budding yeasts or 2. fission yeasts. Budding yeasts: ▪Example: Saccharomyces ▪They divide unevenly. ▪In budding, the parent cell forms a protuberance (bud) on its outer surface. ▪As the bud elongates, the parent cell’s nucleus divides, and one nucleus migrates into the bud. ▪Cell wall material is then laid down between the bud and parent cell, and the bud eventually breaks away. Figure: A budding yeast. Characteristics of Fungi ▪ Some yeasts produce buds that fail to detach; these buds form a short chain of cells called a pseudohypha. ▪ Candida albicans attaches to human epithelial cells as a yeast but usually requires pseudohyphae to invade deeper tissues. Fission yeasts ▪ Example: Schizosaccharomyces ▪ They divide evenly to produce two new cells. ▪ During fission, the parent cell elongates, its nucleus divides, and two offspring cells are produced. ▪ Increases in the number of yeast cells on a solid medium produce a colony similar to a bacterial colony. Figure: Colonies of yeast. Characteristics of Fungi ▪ Yeasts are capable of facultative anaerobic growth, which allows these fungi to survive in various environments. ▪ If given access to oxygen, yeasts perform aerobic respiration to metabolize carbohydrates into carbon dioxide and water; denied oxygen, they ferment carbohydrates and produce ethanol and carbon dioxide. ▪ This fermentation is used in the brewing, wine-making, and baking industries. ▪ Saccharomyces species produce ethanol in brewed beverages and carbon dioxide for leavening bread dough. Dimorphic Fungi ▪ Some fungi, most notably the pathogenic species, exhibit dimorphism—two forms of growth: mold or as a yeast. ▪ The moldlike forms produce vegetative and aerial hyphae; the yeastlike forms reproduce by budding. Figure Representative asexual spores Life Cycle of Fungi ▪ Filamentous fungi can reproduce asexually by fragmentation of their hyphae, and yeasts can reproduce asexually by budding or fission. ▪ In addition, both sexual and asexual reproduction in fungi occur by the formation of spores. ▪ After a fungus forms a spore, the spore detaches from the parent and germinates. ▪ Unlike the bacterial endospore, this is a true reproductive spore; a second organism grows from the spore. ▪ Fungal spores are formed in a number of different ways, depending on the species. ▪ In some yeast, spores are formed by cell division of the parent cell. ▪ Fungal spores can be either asexual or sexual. ▪ Asexual spores are formed by the hyphae of a single organism. ▪ When these spores germinate, they become organisms that are genetically identical to the parent. Life Cycle of Fungi ▪ Sexual spores result from the fusion of nuclei from two separate organisms of opposite mating strains of the same species of fungus. ▪ Because sexual spores require two opposite mating strains, they are made less frequently than asexual spores. ▪ Organisms that grow from sexual spores will have genetic characteristics of both parental strains. ▪ Because spores are of considerable importance in identifying fungi. Figure: The life cycle of Rhizopus, a mucoromycete. This fungus will reproduce asexually most of the time. Two opposite mating strains (designated + and −) are necessary for sexual reproduction. Fungal Diseases ▪ Any fungal infection is called a mycosis. ▪ Mycoses are generally chronic (long-lasting) infections because fungi grow slowly. ▪ Mycoses are classified into five groups according to the degree of tissue involvement and mode of entry into the host: 1. Systemic, 2. Subcutaneous, 3. Cutaneous, 4. Superficial, or 5. Opportunistic. ▪ Some fungi cause disease by producing toxins. Fungal Diseases Systemic mycoses ▪ They are fungal infections deep within the body. ▪ They are not restricted to any particular region of the body but can affect a number of tissues and organs. ▪ Systemic mycoses are usually caused by fungi that live in the soil. ▪ The spores are transmitted by inhalation; these infections typically begin in the lungs and then spread to other body tissues. ▪ They are not contagious from animal to human or from human to human. ▪ Example: histoplasmosis and coccidioidomycosis. Subcutaneous mycoses ▪ They are fungal infections beneath the skin caused by saprophytic fungi that live in soil and on vegetation. Fungal Diseases Cutaneous mycoses ▪ Fungi that infect only the epidermis, hair, and nails are called dermatophytes, and their infections are called dermatomycoses or cutaneous mycoses. ▪ Dermatophytes secrete keratinase, an enzyme that degrades keratin, a protein found in hair, skin, and nails. ▪ Infection is transmitted from human to human or from animal to human by direct contact or by contact with infected hairs and epidermal cells. Superficial mycoses ▪ The fungi that cause superficial mycoses are localized along hair shafts and in superficial (surface) epidermal cells. ▪ These infections are prevalent in tropical climates. Fungal Diseases Opportunistic mycoses ▪ An opportunistic pathogen is generally harmless in its normal habitat—which may include in or on the human body—but can become pathogenic in a host who is seriously debilitated or traumatized, who is under treatment with broad-spectrum antibiotics, whose immune system is suppressed by drugs or by an immune disorder, or who has a lung disease. Opportunistic mycoses ▪ Pneumocystis is an opportunistic pathogen in individuals with compromised immune systems and causes the most commonly threatening infection in people with AIDS. ▪ Yeast infection, or candidiasis, is most frequently caused by C. albicans and may occur as vulvovaginal candidiasis or thrush, a mucocutaneous candidiasis. ▪ Candidiasis frequently occurs in newborns, in people with AIDS, and in people being treated with broad-spectrum antibiotics. Economic Effects of Fungi ▪ Fungi have been used in biotechnology for many years. Here are some examples: ✔ Aspergillus niger has been used to produce citric acid for foods and beverages. ✔ The commonly used drugs called statins that inhibit cholesterol synthesis are produced by Aspergillus terreus. ✔ The yeast S. cerevisiae is used to make bread and wine. It is also genetically modified to produce a variety of helpful proteins, including hepatitis B vaccine. ✔ Cyclosporine, used to prevent transplant rejections, is produced by Tolypocladium inflatum. ✔ Trichoderma is used commercially to produce the enzyme cellulase, which is used to remove plant cell walls to produce a clear fruit juice. ✔ The fungus Taxomyces produces taxol, an anticancer drug. ✔ Penicillin is produced by Penicillium chrysogenum, and cephanosporins are produced by Acremonium chrysogenum.