Introduction to Fungi, Third Edition

SACCHAROMYCES (SACCHAROMYCETACEAE)
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production of ethanol from pentose sugars
in wastes of industrial processes (Jeffries &
Kurtzman, 1994).
2. Bread-making. About 1.5 10 6 tons of
fresh cells of S. cerevisiae are produced worldwide
per annum for use in the production of bread
dough (see p. 274).
3. Single-cell protein (SCP). This term describes
the conversion of low-cost substrates into proteinrich
biomass of unicellular organisms. Yeasts
have a high nutritional value to animals and
man because they are rich in vitamins and
protein, and because they do not generally
produce mycotoxins. Since they also have very
simple growth requirements, yeasts can be used
to convert low-cost substrates such as wastes from
industrial processes into high-value products for
human or animal consumption. While the use of
mineral oil as a substrate was a somewhat
predictable failure, other substrates such as
whey wastes from cheese production, molasses
from sugar cane or pentose-containing wastes
from paper production are promising (Tuse, 1984;
Scrimshaw & Murray, 1995; Paul et al., 2002).
Currently, about 800 000 tons of fodder yeasts are
produced per annum (Kurtzman & Sugiyama,
2001), but an extended application of single-cell
protein technology is hampered by the low
current cost of alternative protein sources such
as soy meal or fish meal (Harrison, 1993;
Scrimshaw & Murray, 1995).
4. Vitamin production. Riboflavin (vitamin
B2) is produced industrially by Eremothecium
spp. (see p. 284).
5. Production of recombinant proteins,
e.g. enzymes, or clinically relevant molecules
such as antigens, insulin and epidermal growth
factor. Expression systems for heterologous
proteins, i.e. proteins of interest whose gene
has been linked to the promoter sequence of
the producing organism, include S. cerevisiae
and Pichia pastoris. The latter holds advantages
because proteins of interest are secreted more
efficiently. Further, the glycosylation (sugar)
chains which are added to the polypeptide
during and after its translation in the rough
endoplasmic reticulum are more similar between
Pichia and mammals than either is to S. cerevisiae.
Therefore, Pichia proteins cause fewer immunological
problems in clinical use (see p. 281).
6. Biological control. Because yeasts do not
produce mycotoxins and because of their ability
to colonize the skin of fruits, they are being
developed as biological control agents against
postharvest losses in fruit crops. Pichia guilliermondii
sprayed onto fruits selectively reduces
development of moulds caused by Penicillium
spp. (Chalutz & Wilson, 1990; McLaughlin et al.,
1990).
10.2 Saccharomyces
(Saccharomycetaceae)
About 10 16 species of Saccharomyces are
currently recognized (Vaughan-Martini &
Martini, 1998b; Barnett et al., 2000; Kirk et al.,
2001). We will focus on S. cerevisiae, which in many
ways is the most important fungus yet discovered.
About 25 strains of S. cerevisiae exist, and these
have different physiological properties which are
relevant to their biotechnological applications.
Many were formerly regarded as different species
(Vaughan-Martini & Martini, 1998b; Rainieri
et al., 2003). Saccharomyces cerevisiae is the brewer’s
and baker’s yeast (see below), although some of
the best brewing yeasts in current use belong
to S. pastorianus (¼ S. carlsbergensis). In nature,
S. cerevisiae is found on ripe fruits, like many
other yeasts. Grape and fruit wines are still often
made by relying on spontaneous fermentations
by yeasts which happen to be growing on the
skin of the fruits used.
The relatively small size of yeast cells
(about 6 8 5 6 mm) has limited their investigation
by light microscopy, but great progress
has been made recently by the use of fluorescent
dyes. Further, by fusing the green fluorescent
protein (GFP) gene to the promoters of diverse
yeast proteins, it has become possible to locate
the site of a defined gene product within the
yeast cell (Kohlwein, 2000). The availability
of freeze-substitution fixation for transmission
electron microscopy has led to the production
of highly resolved ‘natural’ images of