YEAST: DESCRIPTION AND STRUCTURE
Montes de Oca, R.,
Zamora, J.L. and
Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma del
Estado de México
Estado de México, Mexico
Dairy Science Department, National Research Centre, 33 Bohouth St. Dokki,
Yeasts are unicellular eukaryotic fungi with completely different properties
from those of bacteria, which are Prokaryotic microorganisms. Yeast contains
almost the same organelles of a mature eukaryotic cell. Nucleus, Golgi
apparatus, mitochondria, endoplasmic reticulum, vacuole, and cytoskeleton are
the most important one. Yeast cell particle size is typically of 5×10
primary method of reproduction is by budding, and occasionally by fission.
Yeast can be identified and characterized based on cell morphology, physiology,
immunology, and using molecular biology techniques. The natural habitat of
yeast may be soil, water, plants, animals, and insects with special habitat of plant
tissues. Many commercial products contain a mixture of varying proportions of
live and dead S. cerevisiaecells are available for using as feed additives in
Key words: Structure, Yeast
Saccharomyces cerevisiae yeast is unicellular fungi that divide asexually by
budding or fission and whose individual cell size with a large diameter of 5–
10μm and a small diameter of 1–7μm. The cells of S. cerevisiae are pigmented,
where cream color may be visualized in surface-grown colonies (Walker and
White, 2011). Yeast cell is completely deferred than bacterial cell in both
structure and function.
S. cerevisiae has an extensive history of uses in the area of food processing.
It is commonly known as baker’s yeast or brewer’s yeast. S. cerevisiaehas been
used for centuries as leavening for bread and as a fermenter of alcoholic
beverages and wine production. Yeast also has a new function as natural feed
additives in ruminant and non-ruminant animals for manipulating the
gastrointestinal tract and the rumen environment.
Description and significance
Yeasts are fungi, whose common characteristics are predominant or
permanent unicellular state. Yeasts are unicellular eukaryotic fungi with
completely different properties than those of bacteria, which are Prokaryotes
(Fig.1). For example, yeasts have a resistant to antibiotics, sulfamides and other
anti-bacterial agents. This resistance is genetically and natural, and not liable to
be modified or transmitted to other microbes. Moreover, yeast particle size
μm) is also significantly higher than bacteria size (0.5×5μm).
The main method of yeast reproduction is primarily by budding, and
occasionally by fission, and these do not form spores in or on a fruiting body.
Identification and characterization of yeast species may be according to a
number of criteria such as 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., DNA reassociation,
ribosomal DNA phylogeny, karyotyping, random amplified polymorphic DNA
(RAPD), DNA base composition and hybridization, 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 (Walker, 2009). Among yeast, S. cerevisiaeis of
industrially important due to its ability to convert sugars (i.e., glucose, maltose)
into ethanol and carbon dioxide (baking, brewing, distillery, liquid fuel
industries). S. cerevisiae breaks down glucose through aerobic respiration in
presence of oxygen. If oxygen is absent, the yeast will then go through anaerobic
fermentation. The net result of this process is two adenosine triphosphate
molecules, in addition to two by products; carbon dioxide and ethanol. Another
common use of yeast is in the rising of bread. The carbon dioxide that is produce
inside the dough causes it to rise and expand. In the baker’s yeast, these have
strains that produce dioxide are more prevalent than ethanol and vice versa for
Ecology and natural habitats
The distribution of yeasts is not as bacteria in the natural environment, but
nevertheless these can be isolated from soil, water, plants, animals, and insects
Fig. 1. Yeast cell (Distillique.co.za. 2015. 'Distillique - Basics of Yeast Nutrients'.
Yeast: Description And Structure 5
Table 1. Natural yeast habitats (Walker, 2009)
Several non-pathogenic yeasts are associated with the
intestinal tract and skin of warm-blooded animals;yeasts
(e.g., Candida albicans) are opportunistic pathogen to
humans and animals; yeasts are commensally associated with
insects acting as important vectors in the natural distribution
A few viable yeast cells may be expected per cubic meter of
air. Generally, Cryptococcus, Debaryomyces spp.,
Rhodotorula, and Sporobolomyces are dispersed by air from
layers above soil surfaces
Yeasts are fairly ubiquitous in buildings. e.g., Aureobasidium
pullulans is common on damp household wallpaper and S.
cerevisiae is readily isolated from surfaces in wineries
Interface between soluble nutrients of plants and the septic
world are common niches for yeasts; spread of yeasts on the
phyllosphere is aided by insects. The presence of some
organic compounds on the surface and decomposing areas
creates conditions favorable for growth of yeasts
Soil may only be a reservoir for the long-term survival of
yeast, rather than a habitat for growth. Yeasts are ubiquitous
in cultivated soils (nearly 10 000 cells/g of soil) and are
found only in the upper, aerobic soil layers (10–15cm).
Lipomyces and Schwanniomyces are isolated exclusively
Yeasts predominate in surface layers of fresh and salt waters,
but are not present in great numbers (nearly 1000 cells/L).
Most aquatic yeast isolates are of red pigmented genera
(Rhodotorula). The species Debaryomyces hansenii is a
halotolerant yeast that can grow in nearly saturated brine
Plant tissues (i.e. leaves, flowers, and fruits) are preferred yeast habitats, but a
few species are found commensally or in parasitic relationships with animals.
Several species of yeast may be isolated from specialized or extreme
environments, with high sugar or salt concentrations (i.e., low water potential),
with low temperature, and with low oxygen availability.
Types of Saccharomyces
Saccharomyces is a genus in the kingdom of fungi that includes many
species of yeast. The cell of yeast is a saprophytic unicellular fungi cell, where
many members of this genus are considered very important in food production
specially the brewer's yeast or baker's yeast (Table 2).
Taxonomy and characterization
S. cerevisiae is yeast that can exist either as a single-celled organism or as
pseudo-mycelia(Table 3). The yeast cells reproduce by multilateral budding.
Yeast: Description And Structure 6
Each cell produces one to four ellipsoidal, smooth-wall edascospores. Growth
characteristics and physiological traits especially the ability to ferment individual
sugars, is the main differentiation between S. cerevisiaeand other yeasts. This
phenomenon is basis of clinical identification of yeast using commercially
available diagnostic kits, which classify the organism through analysis of the
ability of the yeast to utilize distinct carbohydrates as sole sources of carbon
(Rosini et al., 1982).
Table 2. Saccharomyces species (Walker, 2009)
Table 3. Taxonomic hierarch of yeast S. cerevisiae
The initial classification was based principally on the morphological
characteristics with specific physiological and biochemical traits used to
differentiate between isolates with similar morphological traits. As a result of the
application of newer molecular techniques, the taxonomy of Saccharomyces is
subject to greater scrutiny. In addition, the large heterogeneous species, S.
cerevisiae, may be divided into four distinct species based on DNA homology.
None of the four organisms or other closely related species has been associated
with pathogenicity to humans or has been shown to have adverse effects on the
environment. The four species are S. cerevisiae, Saccharomyces bayanus (also
known as Saccharomyces uvarum), Saccharomyces pasteurianus (also known
as Saccharomyces carlsbergensis), and Saccharomyces paradoxus. All these
four yeast represent industrially important species.
Yeast: Description And Structure 7
Cellular morphology and structure
S. cerevisiae are eukaryotic cells that contain all major organelles that are
also common to animal cells like nucleus, endoplasmic reticulum, mitochondria,
Golgi apparatus, vacuole, cytoskeleton with all three major components, and
many others organelles. Although, the complex-I is absent from S. cerevisiae
cell, the respiratory process can be continued as a results of a simple NADH-
dehydrogenase encoded by the gene NDI1. Generally, yeast is unicellular,
globose with elongate shape. Multilateral budding is typical and pseudohyphae
are rudimentary. True hyphae are absent. Glucan is a major component of cell
walls, as well as mannoproteins. Colonies of Saccharomyces grow rapidly and
mature nearly in three days. Cells are characterized with flat, smooth, moist,
glistening or dull, with cream to tannish cream color. Cell is able to use nitrate
and ability to ferment various carbohydrates. When Saccharomyces grow on
some media such as V-8 medium, Gorodkowa medium, or acetate ascospor agar,
it produces ascospores, which are globose and located in asci that contain 1-4
ascospores. Asci do not rupture at maturity. Most Saccharomyces species are
heterothallic, but a few are homothallic. If occurs, vegetative cells act as asci.
The result of the sexual reproduction is four ascospores, which formed during
meiosis. Once, the ascospores released, these new formed ascospores germinate
produce haploid strains.
Mating between haploid cells must occur to return to the diploid state. Both
of haploid and diploid phases are morphologically similar, but with larger cells
for diploid. In the asexual reproduction, bud grows to reach the size of the
mother cell while nuclear division occurs. The separation occurs after a nucleus
is passed to the daughter cells. Saccharomyces are heterotrophes, obtaining
energy from glucose. They utilize both respiratory and fermentative metabolism.
Approximately, 98% of glucose is metabolized during fermentation, while 2% of
it is made into cell materials. However, the anaerobic metabolism yields more
energy, about 10% of the glucose can be converted to cell material. This
phenomenon is known as the Pasteur’s effect. Saccharomyces have an active
glucose transport system, where glucose metabolization occurred through the
glycolytic pathway. The glycolytic pathway is effective, when glucose present in
low concentrations and will be repressed, when the concentrations are high. In
case of repression, glucose enters the cell via a constitutive facilitated diffusion
system. Moreover, high glucose concentrations may also suppress respiration in
favor of fermentation, even when oxygen is available. This is known as the
Crabtree effect or catabolite repression.
International Collaboration for the Yeast Genome Sequencing stated that S.
cerevisiae was the first eukaryotic genome that was completely sequenced.
Chromosomes of Saccharomyces contain a single linear double-stranded DNA
with few repeated sequences caused mainly by the encoding of ribosomal RNA.
Less than 5% of sequences have introns. In 1996, the Saccharomyces genome
sequence was released in the public domain after that, regular updates have been
maintained at Saccharomyces Genome Database. Another important S.
cerevisiae database is maintained by the Munich Information Center for Protein
Yeast: Description And Structure 8
Sequences (MIPS). The genome has about 12,156,677 base pairs with
6,275 genes about 5,800 are believed to be true functional genes. Genes are
compactly organized on 16 chromosomes. It is estimated that yeast have at least
31% of its genes homologous with that of humans (Herskowitz, 1988). Yeast
genes are classified using gene symbols or systematic names (Fig. 2).
Fig. 2. Yeast chromosome (Millar and Grunstein, 2006)
Because of its unique genetic structure, S. cerevisiae is a useful tool in research
field. At Woolford Laboratory at Carnegie Mellon University, scientists have
used it to study pathways of ribosome assembly with better understanding of the
genetic structure not just of S. cerevisiae, but to certain general genetic
processes. Like other eukaryotes, the 40S ribonucleoprotein contains one 18S
rRNA and 32 ribosomal proteins come from a single 35S transcript synthesized
by polymerase I, on the contrary, pre-RNA is transcribed by polymerase III.
After transcribtion, the pre-RNA is packaged in a 90S RNP. All of these
processes are mediated by enzymes of endoribionucleases and
exoribionucleases. Following these steps, 66S particles are released into the
nucleoplasm, mature, and then are exported to the cytoplasm of the cell. They
have also discovered that there are still other steps before ribosomal subunits are
able to facilitate protein synthesis.
Moreover, they have noted non-ribosomal molecules, which are necessary
for some processes such as rearrangement of rRNA structure as well as RNA
cleavage and processing. Moreover, Alices-Villanueva (1997) studied the TRP1
RI circle plasmid of chromosome IV of S. cerevisiae species. The TRP1 gene in
plasmid (contains Autonomously Replicating Sequence; ARS) codes for an
enzyme required in the synthesis of tryptophan. The ARS allows the plasmid to
replicate independently of chromosomal DNA. Alices-Villanueva created two
versions of this gene, where one of them with only a strong promoter, while the
Yeast: Description And Structure 9
other with both a strong and a weak promoter (Alices-Villanueva, 1997). Rates
of expression are higher for the gene with both promoters, which gives evidence
to the proposed hypothesis. S. cerevisiae contains an acidic cytoplasmic protein
named Gir2. This protein lacks extensive secondary structure (Alves and
Castilho, 2005) with sensitivity to proteolysis. Kelberg (2005) discovered
another new gene in S. cerevisiae called HIM1 on the right arm of chromosome
IV. They stated that when mutations occur in HIM1, there was an increase both
in spontaneous mutation rate and in overall frequencies of mutations.
Saccharomyces cerevisiae life cycle
Growth in yeast cells is synchronized with the growth of buds.The buds
reach the size of the mature cells by the time it separates from the parent cell. In
case of rapidly growing yeast cultures, yeast cells can be seen to have buds,
where bud formation occupies the whole yeast cell cycle. Both of mother and
daughter cells can start budding before the cell separation occurs (Fig. 3).
S. cerevisiae can reproduce as sexually or asexually. It can indefinitely
reproduce both as diploids (2n) and as haploids (1n) in which new daughter cells
arise mitotically as buds that grow in size and eventually split from the mother
cell. This fact greatly facilitates genetic analysis. The transition between haploid
and diploid phases of the life cycle is accomplished by mating of two haploids to
form a diploid zygote and by meiosis, where one diploid cell undergoes
premeiotic S-phase and two meiotic divisions resulting in four haploid cells,
which are enclosed in ascospore walls. Haploid cells occur in two mating types,
a and ά (Fig. 4). Both of them can reproduce mitotically as stable haploid cells.
Or they can engage in sexual reproduction, in which cells of opposite mating
types communicate with each another by proteins known as pheromones.
Both mating and meiosis are controlled genetically by the mating type locus
of which two alleles exist, corresponding to the two sexes. Tetrad analysis of the
meiotic products, which is impossible to perform in higher organisms, is one of
the most convenient ways of genetic analysis. The cell life cycle in yeast
normally consists of the following stages – G1, S, G2, and M – which are the
normal stages. In the G1 phase of the cell cycle, S. cerevisiae cells have the
options for cell differentiation, where haploid cells can mate with partner cells of
the opposite sex or form stationary (G0) cells, and these have the ability to age.
Diploid cells can undergo meiosis or form stationary cells. Furthermore, these
can be transformed into pseudohyphae and those can age.
Yeast: Description And Structure 10
The pheromones induce dissimilar cells to undergo cell fusion followed by
nuclear fusion. The new formed zygote has a single diploid nucleus and buds to
produce diploid progeny. Diploid yeast cells also propagate as stable diploid
cells by mitotic division. Starvation, however, induces those to undergo meiosis
and sporulation, which allows the yeast cells to ‘reshuffle’ their genes, when
Fig. 3. Saccharomyces cerevisiae mitotic cell cycle (Cosma, 2004)
Fig. 4. Haploid yeast cells be ‘a’ or ‘ά’ mating type (Lodish et al., 2000)
Yeast: Description And Structure 11
conditions are poor, perhaps enabling those to find a combination more suitable
for survival in the environment. Generally, meiosis reduces the diploid nucleus
to four haploid nuclei, which become encapsulated in four haploid spores. The
nutrient depletion induces meiosis and sporulation, while the subsequent
availability of nutrients promotes spore germination and gamete production. Sex
in yeast is determined by the mating type locus (designated as MAT) on
chromosome III. There are two mating types: ‘a’ and Mating ability segregates
2a: 2α in tetrads derived from MATa/MATα heterozygous diploids, indicating
that the ‘a’and mating types are specified by alleles of a single locus (MAT).
MATa or MATα cells mate efficiently with cells of the opposite sex.
Heterozygous MATa/MATa diploids are sterile, but it is possible to derive
MATa/MATa or MAT α/MATα diploid cells. These diploid cells will mate with
other cells of the opposite mating type, either haploids or diploids. The ability to
mate is thus determined by the genetic configuration at the MAT locus and as
such is not related to ploidy.
Saccharomyces cerevisiae commercial applications
Most commercial products contain a mixture of varying proportions of live
and dead S. cerevisiae cells. Those with a predominance of live cells are sold as
live yeasts, while others containing more dead cells and the growth medium are
sold as yeast cultures (Newbold and Rode, 2006). Examples include Yea-sacc
(Alltech Inc.), Diamond V Yeast culture (Diamond V, Mills Inc.), and Levucell
SC-20 (Lallemand Animal Nutrition). In addition to its use in food processing,
S. cerevisiae is widely used for the production of macromolecular cellular
components such as lipids, proteins, enzymes, and vitamins (Bigelis, 1985;
Stewart and Russell, 1985). S. cerevisiae has been regarded having GRAS status
by FDA. Furthermore, the National Institutes of Health in its Guidelines for
Research Involving Recombinant DNA Molecules considers S. cerevisiae a safe
organism. The abundance of information on S. cerevisiae, derived from its role
in industrial applications, has positioned S. cerevisiae as a primary model for the
Yeast, as a eukaryotic cell can be used in many different applications rather
than the use in bread backing and wine industries. Utilization of yeast as feed
additives in animal nutrition as safe and natural feed additives is an area of
research interest, where, it can be proved to be efficient in improving animal
Alices-Villanueva, H. 1997. Investigation of The Influence of Selected Gene
Promoter Strength on Yeast Acentric Ring Plasmid Copy Number (Doctoral
dissertation).University of Hawaii.
Alves, V. S. and Castilho, B. A. 2005. Gir2 is an intrinsically unstructured
protein that is present in Saccharomyces cerevisiae as a group of
heterogeneously electrophoretic migrating forms. Biochemical and
Biophysical Research Communications, 332(2), 450-5.
Yeast: Description And Structure 12
Bigelis, R. 1985. Primary metabolism and industrial fermentations. In: J. W.
Bennet, L. L. Lasure (Eds.),Gene Manipulations in Fungi. (pp. 357), New
York, NY: Academic Press.
Cosma, M. P. 2004. Daughter‐specific repression of Saccharomyces cerevisiae
HO: Ash1 is the commander. EMBO reports, 5, 921 – 1013.
Herskowitz, I. 1988. Life cycle of the budding yeast Saccharomyces cerevisiae.
Microbiolgy Reviews, 52: 536–53.
Kelberg, E. P. 2005. HIM1, a new yeast Saccharomyces cerevisiae gene playing
a role in control of spontaneous and induced mutagenesis.Mutat
Research, 578, 64-78.
Lodish, H., Berk, A., Zipursky, S. L., Matsudaira, P., Baltimore, D., and Darnell,
J. 2000.Mutations: Types and Causes. In: W. H. Freeman, Molecular Cell
Biology. New York: Available from: http://www.ncbi.nlm.nih.gov/
Millar, C. B. and Grunstein, M. 2006. Genome-wide patterns of histone
modifications in yeast. Nature Reviews Molecular Cell Biology, 7, 657-666.
Newbold, C. J., and Rode, L. M. 2006.Dietary additives to control
methanogenesis in the rumen. International Congress Series, 1293, 138–147.
Rosini, G., Federici, F., Vaughn, A. E., and Martini, A. 1982. Systematics of the
species of the yeast genus Saccharomyces associated with the fermentation
industry. European Journal of Applied Microbiology and Biotechnology, 15,
Stewart, G. C., and Russell, I. 1985. The biology of Saccharomyces. In: A. L.
Demain, N. A. Solomon, (Eds.), Biology of industrial organisms (pp. 511-
536). Menlo Park, California: Benjamin Cummins Publishers.
Walker, G. M. 2009. Yeasts. In: M. Schaechter (Ed.) Desk Encyclopedia of
Microbiology. (pp. 1174-1187) 2nd ed. London: Elsevier/Academic Press.
Walker, G. M., and White, N. A. 2011.Introduction to Fungal Physiology. In:
Kavanagh, K. (ed), Fungi: Biology and Applications (pp. 1-36). West
Sussex, UK: John Wiley and Sons Ltd.
Yeast: Description And Structure 13