Bacterial cellulose

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pellicle
being removed from a culture
Nata de coco, a traditional food product from the Philippines made from fermenting coconut water with Komagataeibacter xylinus

Bacterial cellulose is an

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 Komagataeibacter xylinus 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 the jelly-like desserts nata de piña and nata de coco, a Filipino food product.[2][3][4] 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.[1]

History

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.[5] 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.[6] 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.[7]

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.[8] In 1949, the microfibrillar structure of bacterial cellulose was characterized by Muhlethaler.[9] Further bacterial cellulose studies have led to new uses and applications for the material.

Biosynthesis

Chemical structure of cellulose

Bacterial sources

Bacteria that produce cellulose include

Sarcina ventriculi.[10] 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.[11]

General process

Biochemical Pathway for Cellulose Synthesis

The synthesis of bacterial cellulose is a multistep process that involve two main mechanisms: the synthesis of

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[14] or not to involve a lipid intermediate,[12] 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.[15] A. xylinum usually converts carbon compounds into cellulose with around 50% efficiency.[14]

Fermentation production

Bacterial Strains that Produce Cellulose
Micro­organism Carbon source Supple­ment Culture time (h) Yield (g/L)
A. xylinum BRCS glucose ethanol, oxygen 50 15.30
G. hansenii PJK (KCTC 10505 BP) glucose oxygen 48 1.72
glucose ethanol 72 2.50
Aceto­bacter sp. V6 glucose ethanol 192 4.16
Aceto­bacter sp. A9 glucose ethanol 192 15.20
A. xylinum ssp. Sucro­fermentans BPR2001 molasses none 72 7.82
fructose agar oxygen 72 14.10
fructose agar 56 12.00
fructose oxygen 52 10.40
fructose agar oxygen 44 8.70
A. xylinum E25 glucose no 168 3.50
G. xylinus K3 mannitol green tea 168 3.34
G. xylinus IFO 13773 glucose lignosulphonate 168 10.10
A. xylinum NUST4.1 glucose sodium alginate 120 6.00
G. xylinus IFO 13773 sugar cane molasses no 168 5.76
G. xylinus sp. RKY5 glycerol no 144 5.63
Glucon­aceto­bacter sp. St-60-12 and Lacto­bacillus Mali JCM1116 (co-culture) sucrose no 72 4.20

Cellulose production depends heavily on several factors such as the

lignosulfonate.[19] Addition of organic acids, specifically acetic acid, also helped in a higher yield of cellulose.[20] Studies of using molasses medium in a jar fermentor[21] as well as added components of sugarcane molasses[22]
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

sodium alginate[26]
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.[27] For most species, the optimal pH was between 4.0 and 6.0.[17] 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.[28]

Reactor based production

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.[29] 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,[30] the rotary biofilm contactor (RBC),[29] a bioreactor equipped with a spin filter,[31] and a reactor with a silicone membrane.[32]

Structure and properties

Types of cellulose[1]
Genus Cellulose type Biological role
Acetobacter Extracellular pellicle,
ribbons 		Maintain aerobic
environment
Achromobacter Ribbons Flocculation
Aerobacter Fibrils Flocculation
Agrobacterium Short fibrils Attachment to plants
Alcaligenes Fibrils Flocculation
Pseudomonas Non-distinct Flocculation
Rhozobium Short fibrils Attachment to plants
Sarcina Amorphous Unknown

Differences between plant and bacterial cellulose

As the Earth's most common

microfibrils are significantly smaller than those in plant cellulose, making bacterial cellulose much more porous.[9]

Three way branching point mechanism

Macro structure

Cellulose is composed of carbon, oxygen, and hydrogen, and is classified as a polysaccharide, indicating it is a carbohydrate that exhibits polymeric characteristics. Cellulose is composed of straight chain polymers, whose base units of glucose are held together by beta-linkages. The structural role of cellulose in cell walls has been likened to that of the glass strands of fiberglass or to the supporting rods within reinforced concrete.[citation needed] Cellulose fibrils are highly insoluble and inelastic and, because of their molecular configuration, have a tensile strength comparable to that of steel.[citation needed] Consequently, cellulose imparts a unique combination of chemical resilience and mechanical support and flexibility to the tissues in which it resides.[34] Bacterial cellulose, produced by Acetobacter species, displays unique properties, including high mechanical strength, high water absorption capacity, high crystallinity, and an ultra-fine and highly pure fiber network structure.[35] One of the most important features of bacterial cellulose is its chemical purity. In addition to this, bacterial cellulose is stable towards chemicals and at high temperatures.[36] Bacterial cellulose has been suggested to have a construction like a ‘cage' which protects the cell from foreign material and heavy-metal ions, while still allowing nutrients to be supplied easily by diffusion.[2][37] Bacterial cellulose was described by Louis Pasteur as "a sort of moist skin, swollen, gelatinous and slippery." Although the solid portion in the gel is less than one percent, it is almost pure cellulose containing no lignin and other foreign substances.[2] Although bacterial cellulose is obtained in the form of a highly swollen gel, the texture is quite unique and different from typical gels. Cellulose has a high swollen fiber network resulting from the presence of pore structures and tunnels within the wet pellicle. Plant cellulose water retention values 60%, while bacterial cellulose has a water retention value of 1000%.[33] The formation of the cellulose pellicle occurs on the upper surface of the

supernatant film. A large surface area is important for a good productivity. The cellulose formation occurs at the air/cellulose pellicle interface and not at the medium/cellulose interface. Thus oxygen is an important factor for cellulose production.[1] After an inducing and a rapid growth period, the thickness increases steadily. Fibrils appear to be not necessarily linear but contain some "three-way branching points" along their length. This type of branching is considered to be related to the unique characteristics of this material and occurs from branching points produced by binary fission.[38]

Sizes of synthetic and naturally occurring fibers[39]

Properties and characterization

Sheet-shaped material prepared from bacterial cellulose has remarkable mechanical properties. According to Brown, the

hydrogen bonds. This Young's modulus does not vary with temperature nor the cultivation process used. The very high Young's modulus of this material must be ascribed to its super-molecular structure.[37][38]

This property arises from adjacently aligned glucan chains participating in inter- and intrachain hydrogen bonding.[34] Bacterial cellulose subfibrils are crystallized into microfibrils which group to form bundles, that then form 'ribbons'. These fibers are two orders of magnitude thinner than cellulose fibers produced by pulping wood.[8] Today, it is known that the pellicle comprises a random assembly of fibrils (< 130 nm wide), which are composed of a bundle of much finer microfibrils (2 to 4 nm diameter). It is also known that the pellicle gives a film or sheet when dried if the shrinkage across the plane is restricted.[38] The ultrafine ribbons of microbial cellulose form a dense reticulated structure, stabilized by extensive hydrogen bonding. Bacterial cellulose is also distinguished from its plant counterpart by a high crystallinity index (above 60%). Two common crystalline forms of cellulose, designated as I and II, are distinguishable by X-ray, nuclear magnetic resonance (NMR), Raman spectroscopy, and infrared analysis.[8] Bacterial cellulose belongs crystallographically to Cellulose I, common with natural cellulose of vegetable origin, in which two cellulose units are arranged parallel in a unit cell.[2][40] The term Cellulose I is used for this parallel arrangement, whereas crystalline fibrils bearing antiparallel

X-ray diffraction, was such that the molecular chain axis lay randomly perpendicular to the thickness such that the (1 1 0) plane was oriented parallel to the surface.[38]

Although cellulose forms a distinct crystalline structure, cellulose fibers in nature are not purely crystalline. In addition to the crystalline and

heterogeneity within the fiber is that the fibers are at least partially hydrated by water when immersed in aqueous media, and some micropores and capillaries are sufficiently spacious to permit penetration.[37]

Scanning electron microscopy of a fractured edge has revealed a pile of very thin layers. It is suggested that these fibrils in layers are bound through interfibrillar hydrogen bonds, just as in pulp-papers, but the density of the interfibrillar hydrogen bonds must be much higher as the fibrils are finer, hence the contact area is larger.[38]

Applications

Bacterial cellulose has a wide variety of current and potential future applications. Due to its many unique properties, it has been used in the food industry, the medical field, commercial and industrial products, and other technical areas. Bacterial cellulose is a versatile structural material, allowing it to be shaped in a variety of ways to accommodate different uses. A number of

patents have been issued for processes involving this material.[41]
. Bacterial cellulose pellicles were proposed as a temporary skin substitute in case of human burns and other dermal injuries [44. Fontana, J.D. et al (1990) "Acetobacter cellulose pellicle as a temporary skin substituite". .Applie d Biochemistry and Biotechnology (Humana Press) 24-25 : 253-264].

Food

The oldest known use of bacterial cellulose is as the raw material of

bulking agent used as a food ingredient to act as a thickener, texturizer, and/or calorie reducer.[42] Microbial cellulose has also been used as an additive in diet beverages in Japan since 1992, specifically kombucha, a fermented tea drink.[9]

Commercial products

Bacterial cellulose also has wide applications in commercial industries. In papermaking, it is used as an ultra-strength paper and as a reticulated fine fibre network with coating, binding, thickening and suspending characteristics.

Sony Corporation.[2] Bacterial cellulose is also used as an additive in the cosmetic industry. Furthermore, it is being tested in the textile industry, with the possibility of manufacturing cellulose based clothing.[35]

Medical

In more modern applications, microbial cellulose has become relevant in the

ulcers.[43] Studies have also been performed where traditional gauze dressings are treated with a microbial cellulose biopolymer to enhance the properties of the gauze. In addition to increasing the drying time and water holding abilities, liquid medicines were able to be absorbed by the microbial cellulose coated gauze, allowing them to work at the injury site.[44]

Microbial cellulose has also been used for internal treatments, such as

tissue regeneration.[43] Bioprocess ® and Gengiflex ® are some of the common trademarked products of microbial cellulose that now have wide applications in surgery and dental implants. One example involves the recovery of periodontal tissues by separating oral epithelial cells and gingival connective tissues from the treated root surface.[1]

Current research/future applications

An area of active research on microbial cellulose is in the area of

organic light-emitting diodes (OLEDs).[35]

Challenges/limitations

Due to the inefficient production process, the current price of bacterial cellulose remains too high to make it commercially attractive and viable on a large scale.[35] Traditional production methods cannot produce microbial cellulose in commercial quantities, so further advancements with reactor based production must be achieved to be able to market many microbial cellulose products.[29]

See also

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