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Lec 5 food microbiology Fungi and food spoilage *Introduction: A fungus is any member of the group of eukaryotic organisms that includes unicellular microorganisms such as yeasts and molds, as well as multicellular fungi that produce familiar fruiting forms known as mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from the other eukaryotic life kingdoms of plants and animals. most fungi are not noticeable because of the small size of their structures, lifestyles in soil or on dead matter. Fungi include symbionts of plants, animals, or other fungi and also parasites. They may become noticeable when fruiting, either as mushrooms or as molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange in the environment. They have long been used as a direct source of food. Note : Many species produce bioactive compounds called mycotoxins, such as alkaloids that are toxic to animals including humans. *Characteristics of fungi : Fungal cells have many characteristics are : 1-contain membrane-bound nuclei with chromosomes that contain DNA with noncoding regions called introns and coding regions called exons. 2-Fungi have membrane-bound cytoplasmic organelles such as mitochondria, sterol-containing membranes, and ribosomes of the 80S type. 3-They have a characteristic range of soluble carbohydrates and storage compounds, including sugar alcohols (e.g., mannitol), disaccharides, (e.g., trehalose), and polysaccharides (e.g., glycogen, which is also found in animals. 1 4-With animals: Fungi lack chloroplasts and are heterotrophic organisms and so require preformed organic compounds as energy sources. 5-With plants: Fungi have a cell wall and vacuoles. 6-They reproduce by both sexual and asexual means. 7-The cells of most fungi grow as tubular, elongated, and thread-like (filamentous) structures called hyphae, which may contain multiple nuclei . There are also single-celled fungi (yeasts) that do not form hyphae, 8-The fungal cell wall is composed of glucans and chitin; while glucans are also found in plants and chitin in the exoskeleton of arthropod. Note : fungi cultured on potato-dextrose media or Sabouraud agar . *Mycology :is the branch of biology concerned with the systematic study of fungi, including their genetic and biochemical properties. *Microscopic structures: Most fungi grow as hyphae, which are cylindrical, thread-like structures 2–10 µm in diameter and up to several centimeters in length. Hyphae grow at their tips (apices); new hyphae are typically formed by emergence of new tips along existing hyphae by a process called branching. An environmental isolate of Penicillium 2 1-hypha 2-conidiophore 3-phialide 4-conidia 4-septa *Macroscopic structures: Fungal mycelia can become visible to the naked eye, for example, on various surfaces and substrates, such as damp walls and spoiled food, where they are commonly called molds. Mycelia grown on solid agar media in laboratory petri dishes are usually referred to as colonies. These colonies can exhibit growth shapes and colors (due to spores or pigmentation) that can be used as diagnostic features in the identification of species or groups. *Reproduction Fungal reproduction is complex, reflecting the differences in lifestyles and genetic makeup within this diverse kingdom of organisms. *Asexual reproduction Asexual reproduction occurs via vegetative spores (conidia) or through mycelial fragmentation. Mycelial fragmentation occurs when a fungal mycelium separates into pieces, and each component grows into a separate mycelium. Mycelial fragmentation and vegetative spores maintain clonal populations adapted to a specific niche, and allow more rapid dispersal than sexual reproduction. The "Fungi imperfecti" (fungi lacking the perfect or sexual stage) or Deuteromycota . 3 *Sexual reproduction Sexual reproduction exists in all fungal phyla .It differs in many aspects from sexual reproduction in animals or plants. Differences also exist between fungal groups and can be used to discriminate species by morphological differences in sexual structures and reproductive strategies. Mating experiments between fungal isolates may identify species on the basis of biological species concepts. Some species may allow mating only between individuals of opposite mating type, whereas others can mate and sexually reproduce with any other individual or itself. Species of the former mating system are called heterothallic, and of the latter homothallic. *Mycotoxins Many fungi produce biologically active compounds, several of which are toxic to animals or plants and are therefore called mycotoxins. Of particular relevance to humans are mycotoxins produced by molds causing food spoilage. *Types of Mycotoxins : 1-Aflatoxins: are a type of mycotoxin produced by Aspergillus species of fungi, include types are : B1, B2, G1, and G2.. Aflatoxin B1, the most toxic, is a potent carcinogen and has been directly correlated to adverse health effects, such as liver cancer, in many animal species. Aflatoxins 4 are largely associated with commodities produced in the tropics and subtropics, such as cotton, peanuts, , pistachios, and maize.. 2-Ochratoxin is a mycotoxin that comes in three secondary metabolite effect on urinary system . forms, A, B, and C. All are produced by Penicillium and Aspergillus species. Ochratoxin A is found as a contaminant of a wide range of commodities including beverages such as beer and wine. 3-Citrinin is a toxin that was first isolated from Penicillium citrinum, Citrinin is associated with yellowed rice disease in Japan and acts as a nephrotoxin in all animal species tested. Although it is associated with many human foods (wheat, rice, corn, barley, oats, rye) 4-Ergot :Alkaloids are compounds produced as a toxic mixture of alkaloids of species of Claviceps, which are common pathogens of various grass species.. There are two forms of ergotism: gangrenous, affecting blood supply to extremities, and convulsive, affecting the central nervous system. 5-Patulin : is a toxin produced by the P. expansum, Aspergillus,. P. expansum is especially associated with a range of moldy fruits and vegetables, in particular rotting apples and figs. It is destroyed by the fermentation process and so is not found in apple beverages, such as cider. Although patulin has not been shown to be carcinogenic, it has been reported to damage the immune system. 6-Fusarium :toxins are produced by over 50 species of Fusarium and have a history of infecting the grain of developing cereals such as wheat and maize. 7- mycotoxins, such as: the fumonisins, which affect the nervous systems of horses and may cause cancer in rodents; the trichothecenes, which are most strongly associated with chronic and fatal toxic effects in animals and humans; and zearalenone, that may effect on genital system . 5 Some Types of Medical fungi : 1-Aspergillus flavus: Members of fungi possess the ability to grow where a high osmotic concentration (high sugar, salt, etc.) Aspergillus species are highly aerobic and are found in almost all oxygenrich environments, where they commonly grow as molds on the surface of a substrate, as a result of the high oxygen tension. Commonly, fungi grow on carbon-rich substrates like monosaccharides (such as glucose) and polysaccharides (such as amylose). Aspergillus species are common contaminants of starchy foods (such as bread, peanuts, corn ,grain), and grow in or on many plants and trees . Aspergillus on a tomato in detail 6 2- Pencillium expansum: Penicillium expansum is one of the most prevalent post harvest rots that infects apples. Although it is a major economical problem in apples, this plant pathogen can be isolated from a wide host range, including pears, strawberries, tomatoes, corn, and rice. This mold also produces the carcinogenic metabolite patulin, a neurotoxin that is harmful in apple juice and apple products . Pencillium expansum on agar 7