Bacterial cellulose

Bacterial cellulose is an organic compound with the formula (C6H10O5)n produced by certain types of bacteria. While cellulose is a basic structural material of most plants, it is also produced by bacteria, principally of the genera Acetobacter, Sarcina ventriculi and Agrobacterium. Bacterial, or microbial, cellulose has different properties from plant cellulose and is characterized by high purity, strength, moldability and increased water holding ability. In natural habitats, the majority of bacteria synthesize extracellular polysaccharides, such as cellulose, which form protective envelopes around the cells. While bacterial cellulose is produced in nature, many methods are currently being investigated to enhance cellulose growth from cultures in laboratories as a large-scale process. By controlling synthesis methods, the resulting microbial cellulose can be tailored to have specific desirable properties. For example, attention has been given to the bacteria Acetobacter xylinum due to its cellulose's unique mechanical properties and applications to biotechnology, microbiology, and materials science. Historically, bacterial cellulose has been limited to the manufacture of Nata de coco, a South-East Asian food product. With advances in the ability to synthesize and characterize bacterial cellulose, the material is being used for a wide variety of commercial applications including textiles, cosmetics, and food products, as well as medical applications. Many patents have been issued in microbial cellulose applications and several active areas of research are attempting to better characterize microbial cellulose and utilize it in new areas. Bacterial cellulose is an organic compound with the formula (C6H10O5)n produced by certain types of bacteria. While cellulose is a basic structural material of most plants, it is also produced by bacteria, principally of the genera Acetobacter, Sarcina ventriculi and Agrobacterium. Bacterial, or microbial, cellulose has different properties from plant cellulose and is characterized by high purity, strength, moldability and increased water holding ability. In natural habitats, the majority of bacteria synthesize extracellular polysaccharides, such as cellulose, which form protective envelopes around the cells. While bacterial cellulose is produced in nature, many methods are currently being investigated to enhance cellulose growth from cultures in laboratories as a large-scale process. By controlling synthesis methods, the resulting microbial cellulose can be tailored to have specific desirable properties. For example, attention has been given to the bacteria Acetobacter xylinum due to its cellulose's unique mechanical properties and applications to biotechnology, microbiology, and materials science. Historically, bacterial cellulose has been limited to the manufacture of Nata de coco, a South-East Asian food product. With advances in the ability to synthesize and characterize bacterial cellulose, the material is being used for a wide variety of commercial applications including textiles, cosmetics, and food products, as well as medical applications. Many patents have been issued in microbial cellulose applications and several active areas of research are attempting to better characterize microbial cellulose and utilize it in new areas. As a material, cellulose was first discovered in 1838 by Anselme Payen. Payen was able to isolate the cellulose from the other plant matter and chemically characterize it. In one of its first and most common industrial applications, cellulose from wood pulp was used to manufacture paper. It is ideal for displaying information in print form due to its high reflectivity, high contrast, low cost and flexibility. The discovery of cellulose produced by bacteria, specifically from the Acetobacter xylinum, was accredited to A.J. Brown in 1886 with the synthesis of an extracellular gelatinous mat. However, it was not until the 20th century that more intensive studies on bacterial cellulose were conducted. Several decades after the initial discovery of microbial cellulose, C.A. Browne studied the cellulose material obtained by fermentation of Louisiana sugar cane juice and affirmed the results by A.J. Brown. Other researchers reported the formation of cellulose by other various organisms such as the Acetobacter pasteurianum, Acetobacter rancens, Sarcina ventriculi, and Bacterium xylinoides. In 1931, Tarr and Hibbert published the first detailed study of the formation of bacterial cellulose by conducting a series of experiments to grow A. xylinum on culture mediums. In the mid-1900s, Hestrin et al. proved the necessity of glucose and oxygen in the synthesis of bacterial cellulose. Soon after, Colvin detected cellulose synthesis in samples containing cell-free extract of A. xylinum, glucose and ATP. In 1949, the microfibrillar structure of bacterial cellulose was characterized by Muhlethaler. Further bacterial cellulose studies have led to new uses and applications for the material. Bacteria that produce cellulose include Gram-negative bacteria species such as Acetobacter, Azotobacter, Rhizobium, Pseudomonas, Salmonella, Alcaligenes, and Gram-positive bacteria species such as Sarcina ventriculi. The most effective producers of cellulose are A. xylinum, A. hansenii, and A. pasteurianus. Of these, A. xylinum is the model microorganism for basic and applied studies on cellulose due to its ability to produce relatively high levels of polymer from a wide range of carbon and nitrogen sources. The synthesis of bacterial cellulose is a multistep process that involve two main mechanisms: the synthesis of uridine diphosphoglucose (UDPGIc), followed by the polymerization of glucose into long and unbranched chains (the β-1→4 glucan chain). Specifics on the cellulose synthesis has been extensively documented. The former mechanism is well known while the latter still needs exploring. The production of UDPGIc starts with carbon compounds (such as hexoses, glycerol, dihydroxyacetone, pyruvate, and dicarboxylic acids) entering the Krebs cycle, gluconeogenesis, or the pentose phosphate cycle depending on what carbon source is available. It then goes through phosphorylation along with catalysis, followed by isomerization of the intermediate, and a process known as UDPGIc pyrophosphorylase to convert the compounds into UDPGIc, a precursor to the production of cellulose. The polymerization of glucose into the β-1→4 glucan chain has been hypothesized to either involve a lipid intermediate or not to involve a lipid intermediate, though structural enzymology studies and in vitro experiments indicate that polymerization can occur by direct enzymatic transfer of a glucosyl moiety from a nucleotide sugar to the growing polysaccharide. A. xylinum usually converts carbon compounds into cellulose with around 50% efficiency. Cellulose production depends heavily on several factors such as the growth medium, environmental conditions, and the formation of byproducts. The fermentation medium contains carbon, nitrogen, and other macro and micro nutrients required for bacteria growth. Bacteria are most efficient when supplied with an abundant carbon source and minimal nitrogen source. Glucose and sucrose are the most commonly used carbon sources for cellulose production, while fructose, maltose, xylose, starch, and glycerol have been tried. Sometimes, ethanol may be used to increase cellulose production. The problem with using glucose is that gluconic acid is formed as a byproduct which decreases the pH of the culture and in turn, decreases the production of cellulose. Studies have shown that gluconic acid production can be decreased in the presence of lignosulfonate. Addition of organic acids, specifically acetic acid, also helped in a higher yield of cellulose. Studies of using molasses medium in a jar fermentor as well as added components of sugarcane molasses on certain strains of bacteria have been studied with results showing increases in cellulose production. Addition of extra nitrogen generally decreases cellulose production while addition of precursor molecules such as amino acids and methionine improved yield. Pyridoxine, nicotinic acid, p-aminobenzoic acid and biotin are vitamins important for cellulose production whereas pantothenate and riboflavin have opposing effects. In reactors where the process is more complex, water-soluble polysaccharides such as agar, acetan, and sodium alginate are added to prevent clumping or coagulation of bacterial cellulose. The other main environmental factors affecting cellulose production are pH, temperature, and dissolved oxygen. According to experimental studies, the optimal temperature for maximum production was between 28 and 30 °C. For most species, the optimal pH was between 4.0-6.0. Controlling pH is especially important in static cultures as the accumulation of gluconic, acetic, or lactic acid decreases the pH far lower than the optimal range. Dissolved oxygen content can be varied with stirrer speed as it is needed for static cultures where substrates need to be transported by diffusion. Static and agitated cultures are conventional ways to produce bacterial cellulose. Both static and agitated cultures are not feasible for large-scale production as static cultures have a long culture period as well as intensive manpower and agitated cultures produce cellulose-negative mutants alongside its reactions due to rapid growth. Thus, reactors are designed to lessen culture time and inhibit the conversion of bacterial cellulose-producing strains into cellulose-negative mutants. Common reactors used are the rotating disk reactor, the rotary biofilm contactor (RBC), a bioreactor equipped with a spin filter, and a reactor with a silicone membrane.