MALCOLM BROWN shows off the oil painting above his desk with paternal pride.
But it鈥檚 not the artist or the view of Coulter Lake in Colorado that most
impresses the professor of botany from the University of Texas at Austin. It is
the canvas, the handiwork of bacteria that live in Brown鈥檚 labs, and a product
of his three decades of research into cellulose.
Cellulose is the major component of plant cell walls. It is the most abundant
natural polymer on Earth and forms the basis of the multibillion-dollar paper
and cotton industries. For decades, biotechnologists have dreamt of synthesising
cellulose on a commercial scale without the help of plants. But although the
polymer is chemically no more complicated than a chain of glucose molecules, no
one knows quite how plants produce it. Open up a plant cell to have a look, and
you damage the production mechanism.
Enter Acetobacter xylinum. The cellulose produced by this otherwise
mundane microorganism is remarkably similar in structure to cellulose in higher
plants, but is much easier to study. Brown believes that research with A.
xylinum is close to revealing the secrets of cellulose formation, a
two-stage process in which the glucose molecules are first joined together into
glucan chains (polymerisation) and then aligned to form microfibrils
(crystallisation). 鈥淭he more we know about the fundamental aspects of
polymerisation and crystallisation,鈥� says Brown, 鈥渢he more control we might be
able to have over the process in the natural system鈥攅ither through genetic
manipulation, or in vitro systems in the laboratory, or maybe in a chemical
synthetic fashion.鈥� Now Brown plans to use this information to revolutionise the
cotton and paper industries.
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Microorganisms have been shedding light on cellulose production for two
decades. The breakthrough came in 1976 when Brown, then at the University of
North Carolina at Chapel Hill, and a colleague, David Montezinos, discovered
that a unicellular green alga called Oocystis apiculata produces
cellulose at a site in its cell membrane known as a terminal complex. Higher
plants also have these cellulose production sites, where they assemble glucan
chains and line them up in parallel to form microfibrils. In higher plants, the
fibres then spin out of the membrane making a cocoon around the cell, the cell
wall.
Before long, Brown and his colleagues had found terminal complexes in several
other unicellular organisms, including A. xylinum. With Martin Willison
and Carol Richardson, Brown discovered that each A. xylinum produces a
single strand of cellulose which emerges from the cell and then becomes entwined
with strands from other cells forming a tightly meshed sheet. The end product
resembles plant cellulose more closely than the cellulose produced by any other
unicellular organism. But even after 20 years of research, no one is sure what
A. xylinum uses its cellulose for.
Closer inspection of the terminal complexes in A. xylinum and in
higher plants has revealed that they are structurally quite different. Brown鈥檚
studies show that in the bacterium, the site of cellulose production is linear
and spans the cell membrane. In higher plants, on the other hand, it is shaped
like a rosette with six distinct subunits arranged in a hexagon. And although
the terminal complex is enclosed within the cell membrane, it wanders around the
cell as it spins the fibres that will lie outside the cell membrane and become a
rigid cell wall.
Despite these differences, researchers believe that by understanding how
cellulose is formed in A. xylinum they will be able to exert greater
control over the process in plants such as cotton and trees. 鈥淚f we can work out
what the genes are, and the proteins they code for in bacteria, it seems quite
likely that we can use that knowledge to look for corresponding proteins and
genes in higher plants,鈥� says Chris Brett, a botanist at the University of
Glasgow.
To this end, Brown and others have concentrated their efforts into decoding
the genes that control cellulose production in the bacterium. Thanks to a series
of minor breakthroughs over the past six years, the genetic blueprint for
cellulose synthesis has gradually been deciphered.
In 1989, Brown and a student Fong Chyr Lin purified the enzyme cellulose
synthase from A. xylinum. Other enzymes are required for cellulose
production, but cellulose synthase is thought to be the only one that is unique
to the process. A year later, Brown and a postdoctoral fellow, Inder Saxena,
were the first to isolate and clone the gene for cellulose synthase. At the same
time, a team led by microbial geneticist Hing Wong at the Cetus Corporation in
California, identified the same gene, as well as three more used by the
bacterium to manufacture cellulose. By mutating these genes, Brown and Saxena
have since shown which one codes for crystallisation. Other genes have been
identified since then, the most recent in 1994.
Brown believes that the handful of key genes involved have all been isolated.
鈥淣ot only have we isolated them, we can now put them into other kinds of
bacteria like Escherichia coli, and it will make cellulose in vitro.鈥�
His current challenge, however, is a bit more ambitious. Over the next three
years, Brown鈥檚 team will be transferring a variety of the cellulose-producing
genes from A. xylinum into Texas upland cotton. His aim is to improve
cotton production creating stronger material and cotton that absorbs dyes
differently.
Although cotton yields the purest natural cellulose, it still contains up to
five per cent impurities and removing them in the manufacture of fabrics can be
an expensive operation. But the cellulose produced in the lab by feeding
concentrated sugars to A. xylinum is almost 100 per cent pure. Brown is
hoping this property may be transferable to cotton plants, slashing the cost of
manufacture.
The other alternative is to make cotton-like fabrics directly from bacterial
cellulose. This is not a commercial proposition at the moment. Bacterial
cellulose is currently manufactured on a small scale in closed vessels, and
although it grows fast鈥攁 sheet one metre square takes only 60 hours to
grow鈥攖he isolation of enzymes and the cleaning and drying of bacterial
cellulose are very expensive. But as techniques improve, industrial scale
production becomes more feasible. Brown predicts that bacterial cellulose could
become competitive with some cotton products in seven years or so.
Meanwhile, other researchers have had some success in bringing cellulose
manufacture to the lab, if not the production line. In 1991, Shiro Kobayashi of
Tohoku University in Japan reversed the action of a cellulose-degrading enzyme
to produce cellulose in a test tube. Collaborating with Kobayashi, Brown and his
team first isolated nature鈥檚 equivalent of this enzyme and then used it to
synthesise cellulose. Last year Brown and his colleagues became the first to
make cellulose in the lab using enzymes extracted directly from plants.
Then, in March, Fumiaki Nakatsubo and a team from Kyoto University, Japan,
published their recipe for making cellulose from glucose in a test tube via a
purely synthetic chemical pathway. They used a modified ester and an acid
catalyst to get their glucose rings lining up in the stepped strands
characteristic of glucan (快猫短视频, Science, 30 March, p 17). The
Kyoto team hopes eventually to use the technique to make high-value polymers for
medical use.
There is no immediate prospect, though, of manufactured cellulose replacing
plant-produced cellulose in bulk industries such as paper. At present, trees are
the cheapest source of cellulose, even though wood contains between 50 and 60
per cent lignin鈥� another polymer鈥攁nd other impurities which must be
removed in costly pulping operations to make paper. But Brown believes his work
with A. xylinum presents the prospect of a radical change in paper
manufacture.
A. xylinum is not photosynthetic, but Brown鈥檚 idea is to insert its
cellulose-producing genes into a bacterium that is. 鈥淚 think in the next ten
years you are going to see photosynthetic bacteria that can be genetically
engineered and grown in the ocean,鈥� he says. If he succeeds, bacteria producing
cellulose could be cultivated in vast shallow natural lakes such as California鈥檚
Salten Sea. And commercial forestry would be drastically cut.
* * *
A sound investment
Cellulose produced by the bacterium Acetobacter xylinum has some unusual
physical properties that have already inspired a variety of commercial
applications. For example, doctors in the US are testing it as a medical
dressing, attracted by the fact that it is non-adhesive, flexible and
transparent.
Bacterial cellulose is also fine, strong and maintains its shape when
moulded. Furthermore, it propagates sound at about the same speed as aluminium,
with a fraction of the resonance. Sony has been exploiting these properties to
produce high quality 鈥渂iocellulose鈥� headphones. When the company first started
production in 1989, these cost 拢2500 a pair. Now, with improved
fermentation methods, they sell for 拢200.
In the Philippines, bacterial cellulose soaked in coconut milk and sugar is
served as a pudding. But the cellulose itself contains no calories. Could this
be the next wonder slimming aid?