Nutrition, Cultivation, Reproduction and Isolation of Fungi

FUNGI

The kingdom of organisms that essentially includes yeast, molds, and mushrooms, is termed as fungi.

It has been duly observed and amply demonstrated that fungi invariably grow as single cells, as in yeast, or as multicellular filamentous colonies, as in molds and mushrooms. Interestingly, fungi do not contain chlorophyll (i.e., the nature’s organic green matter), hence they are saprophytic (i.e., they obtian food from dead organic matter) or parasitic (i.e., they obtain nourishment from the living organ-isms), and above all the body’s normal flora categorically contains several fungi. However, most fungi are not pathogenic in nature.

Importantly, the fungi that essentially cause disease belong to a specific group known as fungi imperfecti. In immunocompetent humans these fungi usually cause minor infections of the hair, nails, mucous membranes, or skin. It is, however, pertinent to mention here that in a person having a com-promised immune system due to AIDS or immunosuppressive drug therapy, fungi critically serve as a source of the viable opportunistic infections that may even cause death ultimately.

Figure 5.6, illustrates the magnified diagramatic representations of yeast, rhizopus, aspergillus, ringworm, and cryptococcus.

Another school of thought defines fungi as — ‘those microorganisms that are invariably nucleated, spore-bearing and do not possess chlorophyll, generally reproduce both asexually and sexually, and have somatic structural features that are essentially surrounded by cell walls com-prising of polysaccharides, cellulose and/or chitin, mannan, and glucan.

In fact, fungi are considered to be mostly saprophytic, making use of dead organic matter as a source of energy, vital natural organic decomposers, and destroyers of food stuffs. While a major segment of species happen to be facultative parasites that specifically able to feed upon either live or dead organic matter, and a relatively minor quantum of species may only survive on the living protoplasms. These fungi are designated as obligate parasites thereby overwhelmingly causing dis-ease of man, animals, and plants. They also prove to be of reasonably great economic and medical importance.

Industrial Research — Certain fungi are intimately associated with the manufacture of bread, beer, and wines (fermentative procedures) ; production of edible varieties of cheese, vitamins, and organic acids (viz., lactic acid, citric acid, acetic acid etc.) ; and several ‘antibiotics’.

Biological Research — Geneticists and Biochemists exploit the fungi profusely by virtue of their extraordinarily unique reproductive cycles, but having a rather relatively simple metabolism.

Reproduction (Cultivation) of Fungi

A large number of fungi invariably get reproduced both asexually and sexually. Nevertheless, the ensuing morphology, and the cycle of these reproductive structures is employed extensively in carrying out their elaborated and logical classification.

i. Asexual Reproduction

The most common procedure of asexual reproduction is usually accomplished by the help of spores. In common practice most of them are found to be colourless (hyaline), while a few of them are duly pigmented as green, yellow, red, orange, black or brown. In fact, their size may invariably range from small to large and their shape from globose via oval, oblong, needle-shaped to helical. Virtually, the ensuing infintie variation in adequate spore appearance and their arrangement prove to be of im-mense utility for proper identification. Asexual reproduction may be borne particularly in a sac-like structure termed as sporangium ; and the spores being referred to as sporangiospores being called as conidia as depicted in Fig. 5.7.

[Adapted From : Hugo and Russell : Pharmaceutical Microbiology, 3rd edn, 1984]

Salient Features : The salient features related to asexual reproduction are as follows :

(1) The simplest form of available fungal spore is known as the zoospore, which possess no rigid cell wall, and is duly propelled by flagella.

(2) Flagellum is usually found to be much more complex than that observed in bacteria.

(3) Flagellum is made up of 11 parallel fibrils, of which 9 forming a cylinder and 2 placed centrally.

(4) Base of flagellum enters the cell and gets attached to the nucleus by a structure termed as rhizoplast.

(5) Flagellum structure (9 + 2 fibrils) is usually found to be fairly consistent with that shown for other flagellated organisms.

(6) Sporangium designates the asexual reproductive structure pertaining to these aquatic fungi. In its early stages it is found to be loaded with nuclei and protoplasm.

(7) Cleavage takes place subsequently whereby the numerous sections invariably get developed into the corresponding uninucleate zoospores.

(8) Finally, following a motile phase, the resulting zoospore encysts, losing its flagellum, and rests quietly just prior to germination.

ii. Sexual Reproduction

Importantly, the sexual reproduction is characterized by the strategical union of two compat-ible nuclei ; and the entire phenomenon may be distinctly divided into three phases, namely :

Phase I : The union of the gametangia (i.e., sex-organs) brings the nuclei into close proximity within the same protoplast. It is also referred to as plasmogamy.

Phase II : It is known as karyogamy, which takes place with the fusion of two nuclei. It has been duly observed that in the lower fungi the said two processes may take place in immediate sequence ; whereas, in the higher fungi they do occur at two altogether different time periods in the course of their life-cycle.

Phase III : It is known as meiosis that essentially takes care of the nuclear fusion whereby the actual number of the chromosomes is distinctly and significantly reduced to its original haploid state.