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Biotechnology could be defined rather nicely as
the use of living organisms or their products to produce goods that
can be sold. The obvious exception of course, are humans –
we call that work!
Fungi are ideally suited for use in biotech for
a number of reasons:
| i) |
Fungi have a massive biochemical toolkit
in the form of their range of different exoenzymes
which can catalyse a great number of useful reactions. |
| ii) |
Fungi are relatively easygoing and most can be grown relatively
easily and cheaply in laboratory conditions in fermentors. |
| iii) |
All molecules in living organisms are optical isomers
(different forms of the same molecule that rotate polarized
light in different ways) of a certain form. Producing chemicals
using chemical engineering produces a random mix of both forms.
This is important when considering pharmaceutical products where
one isomeric form is less effective, or even dangerous. Examples
of this include thalidomide and ethambutol, which have both
highly dangerous and highly useful forms which are expensive
to separate. Enzymes can do this cheaply. |
| iv) |
Many commercially useful products are large molecules of great
structural diversity (e.g. proteins). Synthesising
these chemically would be difficult if not impossible. |
| v) |
Fungi, like us, are eukaryotes which mean
that they can follow human genetic instructions to the letter,
and modify proteins after synthesis in the right way. Bacteria
can’t be trusted to do this correctly, as their molecular
machinery isn’t as sophisticated as a fungus’ protein
synthesis and assembly machinery. Coupled with point ii), this
makes fungi ideally suitable for high-yield production of transgenic
proteins. |
| vi) |
Fungi in their natural environment often face hard times.
To cope with this, they often shut down their biochemical assembly
lines. To do this permanently would mean great difficulty in
getting the metabolic pathways started up again. To avoid this,
fungi employ a skeleton crew of chemicals called secondary
metabolites which keep the enzymes in working order.
The role of secondary metabolites in nature is poorly understood,
but they are thought to prevent the accumulation of intermediates
in normal metabolic pathways. Some secondary metabolites are
important as antibiotics (see point vii), and the diverse nature
of these metabolites means that others may also prove extremely
useful to us. |
| vii) |
Fungi also face competition from other microbes in nature,
and as such, they produce antibiotic substances,
which are often secondary metabolites, to kill
off susceptible competitors. Some of these are of tremendous
importance to modern medicine. |
It is not surprising, therefore, that fungi have
found their way into the production of all kinds of useful things.
We can roughly split these into three areas:
a) Food & drink
b) Pharmaceuticals
c) The rest
We shall also consider, as an appendix, the technology
involved in industrial-scale growth of fungi.
a) Food and drink
The use of fungi to produce food and drink is the
most ancient use of fungi to produce useful substances. Brewing
and baking have been carried out for thousands of years, and utilise
baker’s yeast (Saccharomyces cerevisiae) to catalyse
this reaction (fermentation):
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In baking, the
bubbles of carbon dioxide produced force the dough to rise,
giving bread its light texture. For those of use who perhaps
hoped that the alcohol produced might hang around, disappointment
is in store, as the alcohol evaporates away during the baking
process! |
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The alcohol (ethanol) remains,
of course, when baker’s yeast is used for brewing.
Historically, brewing has had an important role in many cultures.
In the Europe of the Middle Ages, for example, where water
was often unsafe to drink, beer (often the very weak "small
beer") was the staple drink. Brewing was an important
part of daily life, often carried out by women and one of
the few professions open to them.
In our modern world, though, in societies where strongly
alcoholic drinks are freely available, the negative effects
of alcohol are increasingly apparent. Ethanol is a powerful
drug that depresses brain function, causing a wide range of
good and bad effects, and dependence on alcohol frequently
forms a major psychiatric problem. |
While the processes of brewing and baking are simple
enough to be done in the home, there
is a massive industry involving the use of Saccharomyces,
and over 1.5 million tons of Saccharomyces are produced
every year.
Winemaking also relies
on ethanol fermentation, but in this case the yeast is already
present in the skins of the grapes.
For fans of cheese, ascomycete
fungi in the genus Penicillium play an important
role in giving certain cheeses their characteristic flavour
and texture. Penicillium roqueforti gives Roquefort
its characteristic flavour as well as its blue veins, while
Camembert gets its flavour from the surface mould P. camemberti.
Many other moulds grow on different cheeses, and form integral
parts of the flavour and texture. |
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Soy sauce is also fermented as
part of its production processes. The fungus used is Aspergillus
orzyae, a harmless relative of the Aspergillosis
pathogens. A. orzyae is also a source of many industrially
important enzymes. Tempe, another traditional food,
from Indonesia, is produced by the action of Rhizopus oligosporus
on soybeans. Soybeans themselves are edible, but have a high fat
and protein content, which can make for, let’s say, a rather
smelly situation! The advantage of fermentation is that the fat
and protein content is reduced, and the beans can be enjoyed without
such adverse effects.
Carbonated soft drinks
are usually at a low pH, due to the carbonation
process and the presence of other acids. However, using a weak acid
that will react with the stronger acids present acts as a buffer
or an acidity regulator that holds the drink’s acidity at
pH 3 or 4, which is suitable for drinking. Citric acid is
an ideally suited acidity regulator. Citric acid is found in the
citrus fruits, and it would seem to be the simplest thing to extract
citric acid from the fruits. However, to produce citric acid in
the quantities needed for soft drink production would be costly
beyond belief. The ascomycete
fungus Aspergillus niger also produces citric acid, and
can be grown in large quantites in fermentors.
Agaricus bisporus
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At the most basic level, though,
many fungi are themselves highly valued food sources, and
the fruiting
bodies (mushrooms) of edible basidiomycetes
are often collected for cooking.
The common edible mushroom, Agaricus bisporus, is
grown commercially by mixing the mycelium with rye grains
and compost, allowing it to grow, and then “casing”
with peat or chalk, and the fruiting bodies can be harvested
from this. |
Other fungi can be eaten, but rarely with the simplicity
and ease of mushrooms. One example is the use of the fungus
Fusarium venenatum, isolated from the soil, which is grown
in 155,000 litre fermentors to produce the mycoprotein
which is a major ingredient in the QuornTM
range of meat alternatives.
b) Pharmaceuticals
We shall split our discussion of pharmaceutical
production using fungi into two sections for reasons of clarity:
i) Natural fungal products
ii) Genetically manipulated products grown in fungi
i) Natural Fungal Products
| Fungi are renowned for the production
of antibiotics (substances which kill or stop
the growth of other microbes). A number of extremely important
antibiotics are produced by fungi, including penicillin,
produced by Penicillium chrysogenum. Other antibiotic-producing
fungi include Acremonium chrysogenum, which produces
cephalosporins, and Tolypocladium inflatum
which makes cyclosporins. Fungi even make antibiotics
which are highly lethal against other fungi – an example
being griseofulvin from the fungus Penicillium
griseofulvum which can be used to treat fungal infections. |
"This plate has a lawn of staphylococci
growing around the periphery. Their growth is inhibited in
the vicinity of the fungus, Penicillium chrysogenum."
Microbiology at Leeds © University of Leeds.
Image courtesy LTSN
Bioscience ImageBank . |
While some fungal products can help boost our defences
against infection, others can weaken our immune system. This might
not sound very beneficial, but when organ transplants are conducted,
the patient’s immune system often recognizes the transplant
as non-self and tries to attack it. To prevent this, transplant
teams use drugs called immunosuppressants to temporarily
prevent organ rejection. Fungi provide two of the most useful immunosuppressants,
with a cyclosporin (Cyclosporin A) which was first developed as
an antibiotic, and gliotoxin, produced by some Aspergillus
species.
One fungus has been found to produce an important
anticancer drug. Paclitaxel (trade name: Taxol), was first isolated
from the Pacific Yew, Taxus brevifolia, and found to be
useful in treating cancer. Concerns were raised over the potential
for the Pacific Yew to be cut down to extinction, so research has
focused on using other methods to produce paclitaxel. Luckily, an
endophyte fungus isolated from yew, Pestalotiopsis,
produces paclitaxel in viable amounts.
Since it is estimated that only 5% of all fungal
species have been identified by mycologists, it likely that some
of the remaining 95% also produce metabolites of great interest.
ii) Genetically Manipulated Products Produced in Fungi
Fungi can also be used as hosts for genes from
other organisms, and this area of biotechnology has received much
attention in recent years.
The Biotechnology Revolution that began in the second
half of the last century saw massive interest in the use of molecular
cloning which can be used to replicate DNA fragments
from one organism as part of the DNA of another organism. A spinoff
of this is that if a functional gene is cloned using an expression
vector, proteins can be made by the transgenic
organism. As well as the ethical considerations, microbes are much
more suited than higher organisms (plants and animals) for this
kind of production since they can be grown on a large scale in fermentors,
allowing high yields of pharmaceutical products to be achieved.
The techniques of molecular cloning are also more easily carried
out in microbes than in higher organisms.
Early efforts focused on producing large amounts
of protein in harmless strains of the gut bacterium Escherichia
coli. This is simple enough to do, but there is a problem with
using bacteria. While the genetic
code is largely universal, and the machinery used to
decipher it is mostly the same in E. coli as in humans,
there is one critical difference. Human proteins can be modified
within the cell to add different sugars on the end. This is known
as glycosylation. Bacteria, however, lack the molecular
hardware to be able to do this. While prokaryotes
such as E. coli or other bacteria may be able to replicate
human DNA, and to synthesise the protein correctly, therefore, they
are unlikely to be able to modify the protein by adding sugars,
or to fold the protein into precisely the right conformation
(shape).
This is where fungi come into the story. Fungi
are eukaryotic, like humans, and thus have the
right protein-modifying machinery to be able to fold and glycosylate
the proteins. In short, fungi can put the sugar icing on a protein
cake, and having a nicely iced protein is essential for pharmaceutical
production. We mentioned above how having the right optical
isomer is important; imagine if entire chemical groups
drop off the protein. It may have no theraputic benefit at all,
and possibly it could even be harmful.
The first genetically modified
(or recombinant) protein approved for use as a
drug in humans was insulin. Before its approval,
all insulin had to be isolated from animal pancreas. Diabetics often
became allergic to the protein. As a result, researchers at a company
called Genentech isolated the mRNA transcript produced
by the human insulin gene from pancreas tissue, where it is active,
and converted it to DNA by means of an enzyme called reverse
transcriptase. The DNA was then stuck to a molecule known
as a vector, which allows the foreign DNA to be
replicated and expressed as part of the host’s own DNA. The
recombinant DNA could then be inserted into Saccharomyces
ceresvisiae yeast cells. After growing normally, the recombinant
human insulin could be extracted, and used to treat diabetics without
any risk of allergic reactions.
Since the use of recombinant human insulin from
yeast, a number of other important drugs have been produced in fungi,
including interferon, a peptide drug that has powerful
antiviral and anticancer effects.
c) The Rest
This is a loose grouping that is intended to give
just a flavour of the sorts of applications fungal biotechnology
have outside the areas of food and medical biotechnology, rather
than an encyclopaedia of all the areas of fungal biotech.
i) Enzymes
As has been mentioned quite often already, fungi
have an impressive enzymic toolkit. One area where extracted fungal
enzymes are used is in biological washing powders.
Such detergents have to be quite heat resistant to remain functional
at high temperatures. Lipases (fat-digesting enzymes) from Thermomyces
can withstand high temperatures, as the fungus in question is thermophilic
(likes it hot). Therefore, these lipases have been used in biological
washing powders, and can degrade grease stains for a brighter, whiter
finish.
Fungal enzymes are also used in the textile
industry. Until recently, stone-washed jeans actually were
stone-washed with pumice. This unfortunately tended to damage the
garment, so damaged jeans were an unavoidable by-product. However,
it was realized that cellulase enzymes from Trichoderma
fungi could be used to degrade the denim a certain amount to give
it just the right amount of colour.
So fungi taketh away colour, but they also giveth
colour as well. Fermentation of textile dyes by fungi can produce
new colours, and fungi themselves produce pigments during their
growth.
Differing
pigmentation of wild-type (far left) and mutant growth of Aspergillus
nidulans.
© Gareth W. Griffith
White rot fungi could also have
uses in the paper industry, by replacing some chemical
treatments used to make paper. This could reduce pollution associated
with making paper.
ii) Bioremediation
Fungi can also bring their exoenzymes to bear on
some unusual substrates that aren’t found in nature (xenobiotics).
Such xenobiotics are likely to persist in the environment for a
long time, as most decomposers
can’t handle them. Xenobiotics (which include substances such
as pesticides and effluents) frequently pose a hazard to health,
and the first step is to use expensive methods to remediate contaminated
land and water. These methods may themselves be environmentally
damaging, but bioremediation using fungi (and bacteria)
may provide a far more environmentally-friendly solution.
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