Saccharomyces, Neurospora, Agaricus and Aspergillus

Saccharomyces (Class Ascomycetes)

There are about thirty species of Saccharomyces. The best known is S. cerevisiae, strains of which are used in the fermentation of beer and wine and in baking. It is found in nature on ripe fruit. Grape wines are often made by spontaneous fermentation by yeasts growing on the surface of the fruit. Thus S. cerevisiae is a yeast of great economic importance. Its cells are elliptical, measuring about 6 to 8 by 5. They multiply asexually by budding. Where a bud had formed on a cell, a raised scar remains. As many as 23 bud scars have been seen in a single cell. During budding, the nucleus divides by construction and a portion of it enter the bud along with other organelles. The cytoplasmic connection is closed by the laying down of cell-wall material. Under appropriate conditions, S. cerevisiae forms asci. The cytoplasm of the cell differentiates into four thick-walled spherical spores, although the number of spores can be fewer. The cells from which asci develop are diploid and the nuclear divisions which precede spore formation are meiotic. It may be noted that many strains of the yeast are heterothallic, and the ascospores are of two mating types. Mating type is specifically controlled by a single gene which exists in two allelic states a and , and segregation at reduction-division preceding ascospore formation gives rise to two a and two ascospores. Fusion normally occurs only between cells of differing mating types, a process termed legitimate copulation. Such fusions result in diploid cells which form asci containing viable ascospores. Many studies have been made of yeast genetics. For instance, by means of hybridization (crossing of different yeasts), it is possible to develop strains of yeasts (hybrids) with desirable characteristics from two genetically different strains.

Neurospora (Class Ascomycetes)

This genus of Ascomycetes is of particular interest to biologists because of its wide use in the study of genetics and metabolic pathways. Two well-known species are N. crassa and N. sitophila. Some species are responsible for food spoilage, and some are used in industrial fermentations. Certain species produce ascospores, and some species are heterothallic. Morphologically, Neurospora produces a loose network of long strands of septate aerial mycelia. Conidia, usually oval, form branched chains at the tips of the aerial hyphae. Because of the characteristic pink or red color of the conidia and the common occurrence of Neurospora species on farinaceous foods, they are often called pink bread molds. The ascospores of Neurospora are produced in perithecia.

Agaricus (Class Basidiomycetes)

The best-known species are A. Campestris, the field mushroom, and A. diasporas, the cultivated mushroom. The pink coloration of the young gills is due to the cytoplasmic pigment in the spores. The gills later turn a purplish brown because of the deposition of dark pigments in the spore wall most of the larger species of Agaricus are edible.

Filobasidiella neoformans (Class Basdiomycetes)

Of the approximately 12,000 species of Basidiomycetes, none was implicated in human disease until recent times. The perfect stage of Cryptococcus neoformations was discovered in 1975; it is now called Filobasidiella neoformations. It is an important basidiomycetes pathogen of humans, causing cryptococcosis, a generalized (systemic) mycotic infection involving the bloodstream as well as the lungs, central nervous system, and other organs.

Aspergillus (Class Deuteromycetes)

The aspergilli are widespread in nature, being found in fruits, vegetables, and another substrate which may provide nutriment. Some species are involved in food spoilage. They are important economically because they are used in a number of industrial fermentations, including the production of citric acid and gluconic acid, both produced in abundance by A. niger. Morphologically, the aspergilla produce septate, branching mycelia with the vegetative portions sub-merged in a nutrient. The conidiophores, or fertile hyphae, arise from foot cells which may also be submerged. Conidiophores may be septate or nonseptate. At the apex, the conidiophore inflates to form a vesicle. This, in turn, gives rise to sterigmata, which may be single-layered (primary) or double-layered (primary and secondary). Conidia arise from the sterigmata and are borne in chains. The vesicles vary in size and shape, depending on the species. Conidia are produced within the tubular sterigmata and are extruded to form spore chains. Conidia are of various colors and are quite characteristic of the species; the most common colors are black, brown, and green. Aspergilla grow in high concentrations of sugar and salt, indicating that they can extract water required for their growth from relatively dry substances.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.