Introduction to Fungi, Third Edition

262 HEMIASCOMYCETES
(Yarrow, 1998; Barnett et al., 2000). The analysis of
DNA sequences (e.g. 18S rDNA) is now performed
routinely in many laboratories, and a comparison
with the extensive databases of appropriate
sequences should afford identification at least to
genus level. In this way, hemiascomycete yeasts
can be distinguished from Archiascomycetes
and also from basidiomycete yeasts. Such a distinction
should be unequivocal since it utilizes
the very same characters by which the classes
Hemi- and Archiascomycetes were established.
10.1.1 Occurrence and isolation of
Hemiascomycetes
Hemiascomycete yeasts are prominent as epiphytic
saprotrophic colonizers of plant organs,
especially where sugars are present, e.g. in the
nectar of flowers, on fruits, and on wounded
or exposed surfaces of plants. Between 10 5 and
10 7 yeast cells g 1 plant material (fresh weight)
may be present (Phaff & Starmer, 1987). Yeasts
also occur in the soil, although only a few exclusively
soil-borne species have been described.
Most yeasts are probably introduced into the soil
with the plant material with which they were
originally associated (Phaff & Starmer, 1987).
Yeasts also occur in freshwater and marine
situations. Some species are associated with
insects and other animals, including the guts of
vertebrates which have a thriving yeast mycota.
Yeasts may grow on skin surfaces and one
species Candida albicans can, under certain
circumstances, turn into a mild or severe
pathogen of humans, especially of immunocompromised
patients (see p. 276). Hemiascomycetes
are of little importance as plant pathogens with
the exception of Eremothecium spp. which cause
lesions on citrus fruits, cotton and other
crop plants, and are spread by sucking insects
(see p. 284).
Many species of Hemiascomycetes can grow
under conditions of reduced water availability
corresponding to about 50% glucose or a nearsaturated
NaCl solution. Consequently, they can
colonize most types of preserved foods, whereby
the type of preservative determines the species
composition (Pitt & Hocking, 1985; Fleet, 1990).
Fortunately, food spoilage by yeasts does not
normally result in the production of toxins,
in contrast to bacteria or certain filamentous
fungi. However, the economic losses of food
spoilage due to yeasts are still considerable.
Hemiascomycete yeasts are easily isolated
onto most standard agar media augmented
with a suitable antibiotic to suppress bacteria,
e.g. a mixture of penicillin G and streptomycin
sulphate (100 200 mg l 1 each), added to the
cooling agar after autoclaving. Plant or soil samples
can be plated either directly, or the yeasts
can be suspended by shaking the sample in
sterile distilled water containing a detergent
such as 0.01% (v/v) Triton X-100 or Tween 80.
The undiluted sample or a dilution series in
water can be plated out, and the density of
colony-forming units (CFU) g 1 soil or leaves
can be calculated. Yeasts are just large enough
to be resolved as individual cells when a Petri
dish is inverted and viewed with a 10 objective,
whereas bacterial cells are not resolved at that
magnification.
10.1.2 The importance of
Hemiascomycetes
A very small number of species is of immense
importance to biotechnology, and an adequate
discussion is beyond the scope of this book.
Below is a mention of the most important
aspects; some further applications and the
yeast species involved have been summarized
by J. F. T. Spencer et al. (2002).
1. Alcoholic fermentation mainly by
Saccharomyces cerevisiae. This is the oldest and yet
still the most important area of biotechnology,
with about 10 11 l of beer and 3 10 10 l of wine
produced worldwide each year (Oliver, 1991;
Kurtzman & Sugiyama, 2001). The discovery of
alcoholic fermentations has been made several
times independently in the history of mankind.
Details of fermentation processes are given in
on pp. 274 276. Industrial alcohol (ethanol) is
often obtained from fermentations of corn
starch hydrolysate by S. cerevisiae, but there is
an ongoing interest in using other yeasts
(Pachysolen tamophilus, Pichia stipitis) for the