Study of Fungi
Contents
• Fungi morphological types
• Structure of yeasts
• Structure of molds
• Classification of fungi
• Growth of fungi
• Life cycle of fungi
• Cultivation of fungi
Intended
Learning objectives
At the end of this lecture the student will be able to
• Explain the general features of fungi
• Explain morphological types of fungi
• Outline the fungal classification
• Explain the conditions required for fungal growth
• Explain certain fungal infections
Introduction
• Classified as Kingdom ‘Fungi’
• Includes microorganisms such as yeasts and molds
• Fungi feed by absorption of nutrients from the environment
around them
• Grow through and within a moist substrate on which they
are feeding
• Most fungi are saprophytes, feeding on dead or decaying material
• Many fungi are parastitic, feeding on living organisms
without killing them
Morphology
of fungi
General
• Vary widely in size and shape, from unicellular, microscopic
organisms to multicelluleye
• Individual cells range from 1 μ to 30 μ
• Microscopic fungi exist as either molds or yeasts or both
• Internally, fungal cells are fairly typical eukaryotic
cells
Molds
• The body of a mold or fleshy fungus is called ‘Thallus’
• Thallus consists of large multicellular aggregates of long
branching filaments, called ‘hyphae’
• The tube-like hyphae are responsible for the fluffy
appearance of the macroscopic mold colony
• The hyphae and other structures combine to form an
elaborate network called a ‘mycelium’
Morphology of molds
• There are vegetative hyphae and reproductive hyphae
• Spores are borne on the reproductive hyphae
• Spore size, shape and structure are used in the
classification and identification of fungi
Morphology of yeast
Yeast
• These are large (5 to 8 μ)
• Single-celled organisms that rarely form filaments
• Most yeasts reproduce by the asexual process of budding
• Colonies are usually characterized by a smooth surface
Nutrition
• Fungi contain complex enzymes
• Enzymes enter host cells
• Break down the complex substances available –
wood, vegetation, leather, bread, and so forth
• The fungus absorbs the end products
Reproduction in
fungi
• Fungi reproduce sexually or asexually, or both, depending
upon the species and the environmental conditions
• Those that produce only asexual spores are
known as Deuteromycetes – Fungi imperfecti
• The group contains most of the pathogenic fungi
• Yeasts reproduce both by spores and by a process known as budding
Budding in fungi
• 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
• Bud eventually breaks away.
• One yeast cell can in time produce up to 24 daughter cells
by budding.
• Some yeasts produce buds that fail to detach themselves; 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
Growth
• Favourable conditions – warmth and moisture
• As the temperature decreases, fungal activity also
decreases
• Spores are very resistant to cold, some surviving freezing
temperatures for long periods of time
• Fungi are easily killed at high temperatures
Nutritional
adaptations in fungi
Fungi differ from bacteria in certain environmental
requirements and in the following nutritional characteristics:
• Fungi usually grow better in an environment with a pH of
about 5, which is too acidic for the growth of most common bacteria.
• Almost all molds are aerobic. Most yeasts are facultative anaerobes.
• Most fungi are more resistant to osmotic pressure than bacteria;
most can therefore grow in relatively high sugar or salt concentrations.
• Fungi can grow on substances with a very low moisture content,
generally too low too support the growth of bacteria.
• Fungi require somewhat less nitrogen than bacteria for an
equivalent amount of growth.
• Fungi are often capable of metabolizing complex
carbohydrates, such as lignin (a component of wood), that most bacteria cannot
use for nutrients.
Life cycle
of fungi
• Filamentous fungi
can reproduce asexually by fragmentation of their hyphae.
• Both sexual and asexual reproduction in fungi occurs by
the formation of spores
• Spores are formed from aerial hyphae
• Fungal spores can be either asexual or sexual.
• Asexual spores are formed by the hyphae of one organism.
When these spores germinate, they become organisms that are genetically
identical to the parent.
Types of asexual spores produced by fungi.
– Blastospores – yeast, some molds
– Arthrospores – molds
– Chlamydospores – molds
– Conidiospores – molds
– Sporangiospores – molds
Asexual spores in
fungi
Conidiospore, or conidium (plural: conidia), a unicellular
or multicellular spore that is not enclosed in a sac
Sporangiospore, formed within a sporangium, or sac, at the end of an aerial
hypha called a sporangiophore
• Sexual spores result from the fusion of nuclei from two
opposite mating strains of the same species of fungus.
• Fungi produce sexual spores less frequently than asexual
spores.
• Organisms that grow from sexual spores will have genetic
characteristics of both parental strains.
• The sexual spores produced by fungi characterize the phyla
• Types – Ascospores (yeast, some molds), Zygospores –
(molds – filamentous), Basidiospores – (complex molds – mushrooms)
Classification
of fungi
• Fungi are usually classified according to biological
taxonomy based upon the type of hypha, spore, and reproduction
The four Phyla of fungi are:
1. Zygomycota
2. Ascomycota
3. Basidiomycota
4. Deuteromycota
Zygomycota
• Saprophytic molds that have coenocytic hyphae
• An example is Rhizopus stolonifer, the common black bread
mold.
• Asexual spores – Sporangiospores
• Sexual spores – Zygospores
Ascomycota
• Molds with septate hyphae and some yeasts
• The sac fungi
• Asexual spores – Conidiospores
• Sexual spores – Ascospores
Basidiomycota
• Club fungi, also possess septate hyphae.
• Mushrooms, toadstools, rusts, and smuts.
• Sexual spores – Basidiospores
Deuteromycota
• Fungi imperfecti: a heterogeneous collection of fungi
without sexual reproduction.
• Fungi whose sexual cycle had not been observed were put in
a “holding category” called Deuteromycota Conidiospores
Fungal
Infections (Mycosis)
A. Growth in, on tissues
à tissue destruction
B. Production of toxins (mycotoxins)
C. Allergies
A. Growth in, on
tissues à tissue destruction
1. Superficial (cutaneous) infections
a) Dermatophytes
b) Involve keratinized tissues (hair, skin, nails)
c) Transmitted by direct, indirect contact
2. Subcutaneous (Intermediate) infections
a) Involve tissues underlying skin à lymphatics
b) Introduced into cuts, wounds
c) Source – soil
3. Systemic infections
a) Involve internal organs: lungs à other organs
b) Inhale spores
4. Opportunistic infections
a) Produced by fungi that generally do not cause disease
b) Occur in individuals whose resistance (immunity)
impaired.
B. Production of
toxins (mycotoxins)
• Some fungi incapable of causing infectious diseases
produce toxic substances that poison the person who ingests them.
• These substances are collectively called mycotoxins.
• The most commonly known mycotoxin poisoning is from
certain mushrooms
• Mycotoxins may be produced by fungi growing on grain,
nuts, and other agricultural products.
Aflatoxin
• A potent human carcinogen
• A naturally occurring toxic metabolite produced by certain
fungi (Aspergillus flavis), a mold found on food products such as corn and
peanuts, peanut butter
• Acts as a potent liver carcinogen in rodents (and,
presumably, humans).
Some fungal
infections
Aspergillosis
Caused by the fungus Aspergillus and usually occurs in
people with lung diseases or weakened immune systems
Candidiasis
Caused by the yeast Candida. Candidiasis can occur in the
mouth and throat, vagina, or the bloodstream.
Neoformans infection
Caused by Cryptococcus neoformans, which can infect the
brain (meningitis) in people with HIV/AIDS.
Mucormycosis
A rare infection that mainly affects people with weakened
immune systems.
Summary
• Fungi classified as yeasts and molds
• Grow on moist substrate and obtain nutrition by absorption
• Yeasts are single celled eukaryotic organisms
• Molds are multicellular having complex morphology
• Fungal diseases are called mycosis
• Fungal toxins are known as mycotoxins
• The four classes of fungi are:
1.
Class Phycomycetes
2.
Class Ascomycetes
3.
Class Basidiomycetes
4.
Class Deuteromyceters
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