What is Cellulose? 

Cellulose is the Earth's major biopolymer and is of tremendous economic importance globally. Cellulose is the major constituent of cotton (over 94%) and wood (over 50%). Together, cotton and wood are the major resources for all cellulose products such as paper, textiles, construction materials, cardboard, as well as cellulose derivatives such as cellophane, rayon, and cellulose acetate.

Traditionally, cellulose is obtained from plant resources. Bolls from cotton plants are collected, and the fibers are detached from the seeds then processed into bales. In Texas alone, the 1992 cotton crop generated $1.0 billion in revenue and had an economic impact of $4.0 billion according to the Crop Biotechnology Center Datasheet on Cotton, Texas A & M University. The 1985 global production of cotton was approximately 17,540,000 tons.



Wood timber is cut from the forest and sent to the sawmill for cutting and drying. Transported to the paper mill where the wood is shredded into chips  and processed into a thick, watery pulp. This process requires energy and chemicals that are often harmful to the environment since unwanted lignin must be removed and the cellulose must be bleached. 
 
 

Pulp is made into paper and cardboard. In the United States alone, more than two million tons of newsprint and writing paper are produced each year from pulp (Moore, et al, 1995). The forest products industry in the United states is a $70. billion/year industry, not insignificant in relation to other major industries (from the Crop Biotechnology Center Datasheet on the Pine Biotechnology Program, Texas A & M University). In 1985, the world production of pulp reached 140 million metric tons (see Pulp and Paper, August, 1986, p 43)

With increasing population and the quest to continue using cellulose crops from wood and cotton, more land is required to meet the global demand. This has a direct impact on the earth's carbon cycle. In this context, we need to understand cellulose in terms of the global carbon cycle as well as its use by humans. The carbon cycle on earth is a vast interplay between the carbon dioxide of the atmosphere and its sequestration or "fixation" via photosynthesis into organic products, among which cellulose is the most abundant macromolecule on earth. Over 10 11 tons are estimated to be synthesized and destroyed annually on earth (Brown, 1979). 



 

Cellulose can be thought of as a giant carbon "sink" because carbon incorporation into cellulose remains in the product for a rather lengthy time, sometimes for thousands of years. The global warming cycle affected by the emergence of the industrial age is based, in part, by the release of carbon dioxide into the atmosphere through the burning of wood and hydrocarbons. This emerging problem could have immense consequences if the carbon dioxide content continues to rise and trap heat from the sun's irradiation. Thus, one long term alternative is to reverse this conversion by increasing photosynthesis which results in the trapping of more carbon dioxide. Harvesting huge acreage of trees is neither contributing to the ecological management of the photosynthetic potential, nor maintaining a global sink for carbon dioxide.

Sources of Cellulose



Cellulose from such major land plants as forest trees and cotton is assembled from glucose which is produced in the living plant cell from photosynthesis. These are macroscopic, multicellular photosynthetic plants with which we are all familiar. In the oceans, however, most cellulose is produced by unicellular plankton or algae using the same type of carbon dioxide fixation found in photosynthesis of land plants. In fact, it is believed that these organisms, the first in the vast food chain, represent Nature's largest resource for cellulose production. Without photosynthetic microbes, all animal life in the oceans would cease to exist.

Several animals, fungi, and bacteria can assemble cellulose (Brown, 1979); however, these organisms are devoid of photosynthetic capacity and usually require glucose or some organic substrate synthesized by a photosynthetic organism to assemble their cellulose. Some bacteria can utilize methane or sulfur substrates to produce glucose and other organic substrates for cellulose.  

Gluconacetobacter Xylinus formerly known as ACETOBACTER XYLINUM-

A RICH SOURCE OF "Eco-friendly" PURE CELLULOSE 

Among the many bacterias, one of the most advanced types of purple bacteria is the common vinegar bacterium, Acetobacter. This non-photosynthetic organism can procure glucose, sugar, glycerol, or other organic substrates and convert them into pure cellulose (Brown, et al, 1976). 

Acetobacter xylinum is Nature's most prolific cellulose-producing bacterium. A typical single cell can convert up to 108 glucose molecules per hour into cellulose. Consider that as many as a million cells can be packed into a large liquid droplet, and if each one of these "factories" can convert up to 108 glucose molecules per hour into cellulose, the product should virtually be made before one's eyes. 

A single cell of Acetobacter has a linear row of pores from which glucan chain polymer aggregates are spun
  
( Figure 1). Image from R. Malcolm. Brown Lab

  



Acetobacter xylinum is a gram negative bacterium and is unique in its prolific synthesis of cellulose. Rows of pores characteristically secrete mini-crystals of glucan chains which then coalesce into microfibrils. Clusters of microfibrils result in a compound structure known as the ribbon. The ribbon can be observed directly using light microscopy, and the time lapse studies show Acetobacter cells generating cellulose. 

Acetobacter is the model system for study of the enzymes and genes involved in cellulose biosynthesis. The organism also promises to be an important future source for cellulose in the textile, paper, and lumber industries, provided its fermentation can be effectively scaled up. (R. Malcolm Brown Position Paper)

As many as one hundred of these pores can produce a composite cable of glucan polymers resulting in a ribbon. Time lapse analysis of individual Acetobacter cells assembling cellulose ribbons reveals a myriad of activities, each cell acting as a nano-spinneret, producing a bundle of sub-microscopic fibrils. 

Together, the entangled mesh of these fibrils produces a gelatinous membrane known as a pellicle. This membrane of pure cellulose, and cells entrapped within it can be cleaned and dried and the product used for many exciting new applications. One of the unique features of this pure cellulose membrane is that it is very strong in the never dried state, and it can hold hundreds of times its weight in water. This great absorbtivity and strength constitute two of the many novel features of microbial cellulose (Brown, 1989; Brown, 1992; Brown, 1994; White and Brown, 1989).

UNIQUE MICROBIAL CELLULOSE FEATURES AND "Eco-friendly" PRODUCTS

Because the microbial cellulose ribbons are "spun" into the culture medium, membranes and shaped objects can be produced directly during the fermentation process, thus enabling a novel array of non-woven products. Direct dyes can be added during synthesis to alter the cellulose produced. 

Because of the novel features of microbial cellulose, a variety of product applications of microbial cellulose is possible.