MYCOLOGY

By: Aeraaf A K

INTRODUCTION

MYCOLOGY

Fungi represent one of the most important groups of plant kingdom.

The term "mycology" is derived from Greek word "mykes" meaning mushroom. Therefore mycology is the study of fungi.

The ability of fungi to invade plant and animal tissue was observed in early 19th century but the first documented

Animal infection by any fungus was made by Bassi, who in 1835.

INTRODUCTION

According to Alexopoulos (1952), fungi can be defined as:

“Nucleated spore bearing achloroplyllous organisms which generally reproduce sexually and asexually, and whose filamentous, branched, stomatic structures are typically surrounded by cell walls containing cellulose or chitin or both.”

WHAT IS FUNGI???

Diverse group of heterotrophs.– Many are ecologically important saprophytes (consume dead and

decaying matter)– Others are parasites.

Most are multicellular, but yeasts are unicellular. Most are aerobes or facultative anaerobes. Cell walls are made up of chitin (polysaccharide). Over 100,000 fungal species identified. Only about 100 are

human or animal pathogens.

FUNGI

GENERAL CHARACTERS

They are eukaryotes, contain membrane bound cell organelles including nuclei, mitochondria, golgi apparatus, endoplasmic reticulum, lysosomes etc. They also exhibit mitosis.

GENERAL CHARACTERS

2. Have ergosterols in their membranes and possesses 80S ribosomes.

3. Have a rigid cell wall and are therefore non-motile, a feature that separates them from animals. All fungi possess cell wall made of chitin.

4. Are chemoheterotrophs (require organic compounds for both carbon and energy sources) and fungi lack chlorophyll and are therefore not autotrophic.

5. Fungi are osmiotrophic; they obtain their nutrients by absorption.

6. They obtain nutrients as saprophytes (live off of decaying matter) or as parasites (live off of living matter).

7. All fungi require water and oxygen and there are no obligate anaerobes, except yeast.

8. Typically reproduce asexually and/or sexually by producing spores.

9. They grow either reproductively by budding or non-reproductively by hyphal tip elongation.

10. Food storage is generally in the form of lipids and glycogen.

11. Molds & mushrooms are multicellular.

12. Yeasts are unicellular.13. They do not possess

roots, stems or leaves. 14. Few exhibit division of

labour. 15. Typically not motile,

although a few (e.g. Chytrids) have a motile phase.

16. Like plants, fungi have an alternation of generations.

Fungi have a great potentiality for quick adaptation, hence they have colonized all possible types of habitats.

Primitive fungi are aquatic in their habitat.

Higher forms are amphibious and terrestrial.

Majority of terrestrial forms are saprophytic.

 Some live in tissues of plants and animals. Fungi can grow on our food stuffs like bread, jams, pickles and fruits. They are also present all the time in air.

OCCURANCE

There is lack of chlorophyll in fungi hence they are unable to synthesize food. They are heterotrophic in their mode of nutrition and they get food from external source. Thus they live either as parasites or saprophytes. Fungi that get their food from dead organic material are called sabrophytes and the fungi which obtain their prepared food from living plants or animals are called parasites. The living beings on which the fungi parasitize are called hosts. Some fungi grow in association of other plants and are mutually benefited and this process is called symbiosis.

NUTRITION

Fungi exist in two fundamental forms; the filamentous (hyphal) and single celled budding forms (yeast). But, for the classification sake they are studied as moulds, yeasts, yeast like and dimorphic fungi.

MORPHOLOGY OF FUNGI

Fungal cells are typically eukaryotic and have distinguished characteristics than that of bacteria, and algae. The chief components of cell wall appears to be various types of carbohydrate or their mixtures (upto 80-90%) such as cellulose, pectose, callose etc., cellulose predominates in the cell wall of mastigomycotina (lower fungi) while in higher fungi chitin is present.

The living protoplast of the fungal cell is enclosed in a cell membrane called as plasma membrane or plasmalemma. Cytoplasm contains organelles such as nucleus, mitochondria, Golgi apparatus, ribosomes, vacuoles, vesicles, microbodies, endoplasmic reticulum, lysosomes and microtubules. The fungal nucleus has nuclear envelope comprising of two typical unit membrane and a central dense area known as nucleolus, which mainly consist of RNA.

A unique property of nuclear membrane is that it persists throughout the metaphase of mitosis unlike in plant and animal cells where it dissolves and re-forms. The nucleus possesses paired chromosomes.

In multinucleate hyphae, the nuclei may be interconnected by the endoplasmic reticulum. Vacuoles present inside the cell provide turgidity needed for cell growth and maintenance of cell shape. Beside the osmotic function, they also store reserve materials. The chief storage products of fungi are glycogen and lipid.

ChitinIn the fungal cell wall – polysaccharidesFungi are insensitive to antibiotics e.g.

penicillin that inhibit peptidoglycan.

ErgosterolIn the fungal cell membraneHuman cell membrane has cholesterolSelective action of antifungals based on

difference in sterols

IMPORTANT CELL STRUCTURES

Fungi consists of network of branched filaments called hyphae. Entangled mass of hyphae is myceliym. Hyphae may be segmented or non segmented. Some fungi like yeasts (Saccharomyces) and Syachtrium lack mycelium.

VEGETATIVE STRUCTURES OF FUNGI

Hyphae is a tubular, transparent filament, usually branched, composed of an outer cell wall and a cavity (lumen) lined or filled with protoplasm including cytoplasm. Hyphae are divided into compartments or cells by cross walls called septa and are generally called as septate (with cross wall) or coenocytic (aseptate -without cross wall). Hyphae of most of the fungi measure 5-10 μm.

HYPHAE

Fungi are better able to withstand certain extreme environmental conditions than most other microorganisms.

For example, yeasts and moulds can grow in substrates containing concentrations of sugars that inhibit most bacteria, e.g. jams and jellies are spoiled by moulds but not bacteria.

Yeasts and moulds can generally tolerate more acidic conditions than most other microbes.

Moulds and most yeasts are aerobic. Some yeasts are facultative.

PHYSIOLOGY

Fungi grow over a wide range of temperature, with optimum for most saprophytic species ranging from 22 to 30 oC; pathogenic species have higher temperature optimum, generally 30-37 oC. Some fungi grow at or near 0 oC and thus can cause spoilage of foods (meat, vegetables etc.) in cold storage.

Fungi are heterotrophic, using a wide variety of materials for nutrition. Some species can use inorganic compounds such as N2 and ammonium salts as source of energy. All fungi can use organic N2, hence culture media for fungi contain peptone.

Mushroom in Desert

Mushroom in Snow

REPRODUCTION

Reproduction in fungi takes place by vegetative reproduction, asexual method and sexual method.

It takes place by

(i) Fragmentation:- Hyphae break up into two or more pieces due to some external force and each of them develops into new one.

VEGETATIVE REPRODUCTION

(ii) Fission:- Some unicellular fungi like yeast reproduce by fission. The vegetative cell elongates and divides into two daughter cells by constriction or transverse wall.

(iii) Budding:- The parent cell buds off new cell which separates from parent cell and develop into new individual. Eg. Yeast cells.

(iv) Sclerotia:- In this case the interwoven hyphae of mycelium become compact that the mass becomes rounded and cushion like and is know as sclerotia.

This is modification of mycelium. It is hard, resting storage object. It also serves as a means of preservation and vegetative propagation, on return of favourable conditions it germinates.

It takes place by several types of spores as under:

(i) Motile Zoospore:- Motile zoospores may be uni or biflagellate pear shaped sporangium eg. Albugo and syachyterium.

(ii) Conidia:- These are non motile spores produced exogenously by constriction at the ends of special hyphal branches called conidiophores. They may be produced singly (eg. Phythinum, phytophthora) or in chains (eg. Aspergillus & Penicillium).

ASEXUAL REPRODUCTION

(iii) Oidia:- Some fungi like mucor, the hyphae forming mycelium become divided by transverse walls into large number of short segments each of which is capable of germinating and developing into a new plant. These segments which remain united in chains become free from each other and are known as oidia.

(iv) Chlamyolospores:- They are produced like that of oidia but they differ from the latter being thick walled and coloured black or brown. They may be terminal or produced at irregular intervals along the hyphae.

(v) Spores:- They are simple reproductive units usual unicellular and with certain food reserve. In higher fungi like ascomycetes and basidiomycetes ten spores-asporpores and basidiospore, are produced.

In fungi like other organisms sexual reproduction takes place by the union of two compatible nuclei and consists of three phases.

(1) Plasmogamy:- It involves the fusion of two protoplasts bringing two haploid nuclei class together in the same cell.

(2) Karyogamy:- It involves the fusion of two haploid nuclei brought together during plasmogamy. This results in the formation of a diploid structure the zygote.

(3) Meiosis:- It follows nuclear fusion which takes place in all sexually reproducing fungi. It again reduces the number of chromosomes from diploid zygote nucleus to original haploid number in the daughter nuclei.

In fungi sexual reproduction is of following types:- (a) Planoganetic Copulation:- Planoganetes are motile gametes.

This involved the fusion of two naked gametes. The fusing gametes may be isogametes, anisogametes or heterogametes.

SEXUAL REPRODUCTION

(b) Gametangial Contact:- In this case the gametes are transferred directly from one gametangium to the other. These gametes are undifferentiated protoplasts and are represented by nucleus. They never fuse.

(c) Ganetangial Copultion:- it involves the fusion of entire contents of two gametangia.

(d) Spermatization:- In some cases like ascomycetes and basidionycetes there is the formation of certain non motile male cells the spermatia. They are brought in contact with female. Gametangia, receptive hyphae or somatic hyphae to which they are attached. Partition wall dissolves and contents of spermatia pass into females organ.

(e) Somatogamy:- In most of the member of ascomycetes and basidiomycetes there are no sexual organs and the sexual act is a accompanied by fusion of two nuclei from somatic cells. This fusion results the formation of zygote, zygospore or oospore. They produce spores which in their turn develop into new mycellium.

CLASSIFICATION

Classification is done based on (i) Cell Morphology (ii) Sexual Spore Formation

CLASSIFICATION

1. Yeasts2. Yeast like fungi 3. Molds 4. Dimorphic fungi

(i) Based on Cell Morphology

Unicellular fungi which reproduce by budding

On culture - produce smooth, creamy colonies

e. g Cryptococcus neoformans (capsulated yeast).

YEAST

Grow partly as yeasts and partly as elongated cells resembling hyphae which are called pseudo hyphae.

e.g. Candida albicans

YEAST LIKE FUNGI

Form true mycelia & reproduce by formation of different types of spores.

Vegetative/ aerial hyphae

e.g. Rhizopus, Mucor

MOLD OR FILAMENTOUS FUNGI

Occur in 2 forms Molds (Filaments) – 25C

(soil)

Yeasts – 37C (in host tissue)

Most fungi causing systemic infections are dimorphic: Histoplasma capsulatum Blastomyces dermatidis Paracoccidioides brasiliensis Coccidioides immitis Penicillium marneffei Sporothrix schenkii

DIMORPHIC FUNGI

Based on sexual spore formation: 4 classes

1. Zygomycetes 2. Ascomycetes Reproduce

Sexually3. Basidiomycetes4. Deuteromycetes (fungi

imperfectii)

(ii) BASED ON SEXUAL SPORE FORMATION

Commonly known as bread moulds, these are fast growing, terrestrial, largely saprophytic fungi.

Hyphae are coenocytic and mostly aseptate. Asexual spores include chlamydoconidia, conidia and sporangiospores.

Sporangiophores may be simple or branched.

Sexual reproduction involves producing a thick-walled sexual resting spore called a zygospore.

Eg: Rhizopus, Mucor, Rhizomucor, Absidia, Cunninghamella.

Zygomycetes

They exist as saprophytes and parasites of plants.

Hyphae are septate with simple septal pores.

Asexual reproduction is by conidia. Sexual reproduction is by the formation of endogenous ascospores, typically eight, in an ascus.

Ascomycetes

They exist as saprobes and parasites of plants. Hyphae are dikaryotic and can often be distinguished by the presence of clamp connections over the septa. Sexual reproduction is by the formation of exogenous basidiospores, typically four, on a basidium. Occasional species produce conidia but most are sterile.

Basidiomycetes

Deuteromycetes are also known as Fungi Imperfecti because of absence of sexually reproducing forms (teleomorph or perfect stage). As their teleomorph continue to be discovered, they would be classified among the previous categories, until then this remains an artificial and heterogeneous group.

Deuteromycetes

DONE BY:Aeraaf A K

II Sem B.Sc CBZ ‘B’