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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.
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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
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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 .
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*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
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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 .
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 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
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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
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