Download Fungi : mold and yeast Characteristics: • Fungi are eukaryotes

Fungi : mold and yeast
Characteristics:
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Fungi are eukaryotes
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Nearly all multicellular (yeasts are unicellular)
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Distinguished from other kingdoms by:
1. Nutrition:
a. Fungi are heterotrophs that acquire nutrients by absorption
b. Secrete hydrolytic enzymes and acids to decompose complex molecules into simpler ones
that can be absorbed
c. Specialised into three main types:
I.
II.
III.
Saprobes - absorb nutrients from dead organic material
Parasitic fungi - absorb nutrients from cells of living hosts; some are pathogenic
Mutualistic fungi - absorb nutrients from a host, but reciprocate to benefit the host
2. Structural organisation
a. Basic structural unit of fungal vegetative body (mycelium) is the hypha
b. Except for yeast, hyphae are organised around and within food source, which composed
of tubular walls containing chitin
c. Hypha could be septate or aseptate
I.
Hyphae of septate fungi are divided into cells by crosswalls called septa
II.
Hyphae of aseptate fungi lack cross walls (coenocytic)
III.
Parasitic fungi have modified hyphae called haustoria, which penetrate the host tissue
but remain outside cell membrane
3. Reproduction
 Produced either sexually (by meiosis) or asexually (by mitosis)
1. Asexual reproduction: production of various types of spores
o Sporangiophores- upright stalk with an enclosed sac ( bread mold)
o Conidia - upright stalk with no enclosed sac (penicillin)
o Fragmentation – hyphae dry out and shatter releasing individual cells that act like
spores (athlete’s foot)
o Budding – small offspring
2. Sexual reproduction
o “plus and minus” mating types
o Hyphae of different mating types fuse and give rise to a specialized structure that
produces spores ( diploid)
o Most fungi are haploid throughout most of their life cycle
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In favourable conditions, fungi generally clone themselves by producing enormous
numbers of spores asexually
For many fungi, sexual reproduction only occurs as a contingency - results in greater
genetic diversity
Spores are the agent of dispersal responsible for geographic distribution of fungi:
o Carried by wind or water
o Germinate in moist places with appropriate substrata
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3. Growth
Generalised life cycle of fungi
Asexual
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Sexual
Types of Fungi:
o Xerophilic: Molds or yeast capable/prefer of growing at reduced water activity (aw).
o Osmophilic: Yeasts or molds preferring high osmotic pressures for growth.
o Notes:
i.
each osmophilic fungi is xerophilic but not vice versa
ii. water content of medium can be represented by water activity ( aw)
iii. water activity aw can be defined as: the partial vapor pressure of water in a
substance divided by the standard state partial vapor pressure of water
Tips in molds and yeast isolation:
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They tolerate acidic medium
Dichloran rose bengal chloramphenicol agar (DRBC): this media is ideal for mold
culturing as it inhibit the over spreading of mold. This can be advantageous in fungal
counting.
Malt extract agar containing 50% (w/w) sucrose can be used isolate Xerophilic and
Osmophilic Fungi: Its recommended to use sample diluent containing 20% sucrose.
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Try to guess how can we prepare saline containing 20% sucrose, 50% w/w sucrose
containing media as well?
2% solution of sodium citrate, pre-warmed to 45°C, can be used as diluent for highfat foods such as cheese. This aid to emulsify lipid and obtaining a representative
volume to be plated.
Moulds should be enumerated by a surface spread-plate technique rather than with
pour plates, Why?
o This technique provides maximal exposure of the cells to atmospheric oxygen
and avoids heat stress from molten agar.
o Agar spread plates should be dried overnight before being inoculated.
Aflatoxin M1:
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Mycotoxins are those secondary metabolites of fungi which are associated with certain
disorders in animals and humans. The manifestation of toxicity in animals is as diverse as
the fungal species which produce these compounds. In addition to being acutely toxic,
some mycotoxins are now linked with the incidence of certain types of cancer and it is
this aspect which has evoked global concern over feed and food safety, especially for
milk and milk products.
Aflatoxicosis is poisoning resulting from ingestion of aflatoxins in contaminated food or
feed. Aflatoxins are a group of structurally-related toxic compounds produced by certain
strains of the fungi Aspergillus flavus and A. parasiticus. Under favourable conditions of
temperature and humidity, these fungi grow on certain foods and feeds, resulting in the
production of aflatoxins.
Aflatoxin M1 is both a hepatotoxic and hepatocarcinogenic mycotoxin. It also considered
toxic to fungi itself.
Aflatoxin contamination in milk and products is produced in two ways. Either toxins
pass to milk with ingestion of feeds contaminated with aflatoxin, or it results as
subsequent contamination of milk and milk products with fungi.
The major aflatoxins of concern are designated B1, B2, G1 and G2, also M1 and M2 as
metabolic products of AFB. As The aflatoxin M1 (AFLAM1) results from the
hydroxylation of the aflatoxin B1 (AFLAB1). The methods for determining AFM1 can
be classified into two main groups: chromatographic (such as HPLC & TLC) and
immunochemical ( such as quantitative & qualitative ELISA).