Study of Fungi

Study of Fungi


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• Saprophytic molds that have coenocytic hyphae

• An example is Rhizopus stolonifer, the common black bread mold.

• Asexual spores – Sporangiospores

• Sexual spores - Zygospores


• Molds with septate hyphae and some yeasts

• The sac fungi

• Asexual spores – Conidiospores

• Sexual spores - Ascospores


• Club fungi, also possess septate hyphae.

• Mushrooms, toadstools, rusts, and smuts.

• Sexual spores - Basidiospores  


• 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.


• 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


Caused by the fungus Aspergillus and usually occurs in people with lung diseases or weakened immune systems


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.


A rare infection that mainly affects people with weakened immune systems.


• 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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