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Yeast Cells Microscopic MorphologyYeasts are nonfilamentous, unicellular fungi that are typically spherical or oval.  Budding yeasts, such as Saccharomyces, divide unevenly.  In budding, 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, and the 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 attached to human epithelial cells as a yeast but usually requires pseudohyphae to invade deeper tissues.

Fission yeasts, such as Scllizosacc/wromyces, divide evenly to produce two new cells. During fission, the parent cell elongates, its nucleus divides, and two daughter cells arc produced. Increases in the number of yeast cells on a solid medium produce a colony similar to a bacterial colony.

Yeasts are capable of facultative anaerobic growth. Yeasts can use oxygen or an organic compound as the final electron acceptor; this is a valuable attribute because it allows these fungi to survive in various environments. If given access to oxygen, yeasts perform aerobic respiration to metabolize carbohydrates to carbon dioxide and water; denied oxygen, they ferment carbohydrates and produce ethanol and carbon dioxide. This fermentation is used in the brewing, wine-making, and baking industries.  Saccharomyces species produce ethanol in brewed beverages and carbon dioxide for leavening bread dough.

Dimorphic Fungi are fungi, most notably the pathogenic species, exhibit dimorphism-two forms of growth. Such fungi can grow either as a mold or as a yeast. The moldlike forms produce vegetative and aerial hyphae; the yeastlike forms reproduce by budding. Dimorphism in pathogenic fungi is temperature-dependent:  at 37 °C, the fungus is yeastlike, and at 25°C, it is moldlike. However, the appearance of the dimorphic (in this instance, nonpathogenic) fungus changes with CO2 concentration.

Yeast Taxonomy

The most commercially exploited yeast species, S. cerevisiae (baker’s yeast), belongs to the fungal kingdom subdivision Ascomycotina. Other yeast genera are categorized under Basidiomycotina (e.g., Cryptococcusspp. and Rhodotorula spp.) and Deuteromycotina (e.g., Candida spp. And Brettanomyces spp.). There are around 100 recognized yeast genera and we recommend The Yeasts Taxonomic Guide by Kurtzman for additional information on yeast taxonomy.

Natural Habitats of Yeast Communities

Yeasts are not as ubiquitous as bacteria in the natural environment, but nevertheless they can be isolated from soil, water, plants, animals, and insects. Preferred yeast habitats are plant tissues (leaves, flowers, and fruits), but a few species are found in commensal or parasitic relationships with animals. Some yeasts, most notably Candida albicans, are opportunistic human pathogens. Several species of yeast may be isolated from specialized or extreme environments, such as those with low water potential (i.e., high sugar or salt concentrations), low temperature (e.g., some psychrophilic yeasts have been isolated from Polar Regions), and low oxygen availability (e.g., intestinal tracts of animals).

General Cellular Characteristics of Yeast Cells

Yeasts are unicellular eukaryotes that have ultrastructural features similar to that of higher eukaryotic cells. This, together with their ease of growth, and amenability to biochemical, genetic, and molecular biological analyses, makes yeasts model organisms in studies of eukaryotic cell biology. Yeast cell size can vary widely, depending on the species and conditions of growth. Some yeasts may be only 2–3mm in length, whereas others may attain lengths of 20–50mm. Cell width appears less variable, between 1 and 10mm.S. cerevisiae is generally ellipsoid in shape with a large diameter of 5–10mm and a small diameter of 1–7mm. The table below summarizes the diversity of yeast cell shapes:

Cell shape
Description
Examples of yeast genera
EllipsoidOvoid-shaped cellsSaccharomyces
CylindricalElongated cells with hemispherical endsSchizosaccharomyces
ApiculateLemon shapedHanseniaspora,Saccharomycodes
OgivalElongated cell rounded at one end and pointed at otherDekkera,Brettanomyces
Flask shapedCells dividing by bud fissionPityrosporum
PseudohyphalChains of budding yeast cells, which have elongated without detachment. Pseudohyphal morphology is intermediate between a chain of yeast cells and a hypha.Occasionally found in starved cells of Saccharomyces Cerevisiae and frequently in Candida albicans (filamentous cells form from ‘germ tubes’, and hyphae may give rise to buds called blastospores)
HyphalBasidiomycetous yeast cells grow lengthwise to form branched or unbranched threads or true hyphae, occasionally with septa (cross walls) to make up mycelia. Septa may be laid down by the continuously extending hyphal tipSaccharomycopsisspp.
DimorphicYeasts that grow vegetatively in either yeast or filamentous formsC. albicans, Saccharomycopsis fibuligera, Kluyveromyces marxianus, Malassezia furfur,Yarrowia lipolytica,Ophiostoma novo-ulmi,Sporothrix schenkii,
MiscellaneousTriangularTrigonopsis
CurvedCryptococcus
StalkedSterigmatomyces
SphericalDebaryomyces

Several yeast species are pigmented and various colors may be visualized in surface-grown colonies, for example, cream (e.g.,S. cerevisiae), white (e.g., Geotrichum spp.), black (e.g.,Aureobasidium pullulans), pink (e.g.,Phaffia rhodozyma), red (e.g., Rhodotorula spp.), orange (e.g., Rhodosporidiumspp.), and yellow (e.g.,Bulleraspp.). Some pigmented yeasts have applications in biotechnology. For example, the astaxanthin pigments of P. rhodozymahave applications as fish feed colorants for farmed salmonids, which have no means of synthesizing these red compounds

Yeast Identification

Yeast species may be identified and characterized according to various criteria based on cell morphology (e.g., mode of cell division and spore shape), physiology (e.g., sugar fermentation tests), immunology (e.g., immunofluorescence), and molecular biology (e.g., ribosomal DNA phylogeny, DNA reassociation, DNA base composition and hybridization, karyotyping, random amplified polymorphic DNA (RAPD), and amplified fragment length polymorphism (AFLP) of D1/D2 domain sequences of 26S rDNA). Molecular sequence analyses are being increasingly used by yeast taxonomists to categorize new species.