Bacterial cellulose is an
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...
with the formula produced by certain types of
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
. While
cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
is a basic structural material of most plants, it is also produced by bacteria, principally of the genera ''
Acetobacter
''Acetobacter'' is a genus of acetic acid bacteria. Acetic acid bacteria are characterized by the ability to convert ethanol to acetic acid in the presence of oxygen. Of these, the genus ''Acetobacter'' is distinguished by the ability to oxidize ...
'',
''Sarcina ventriculi'' and ''
Agrobacterium
''Agrobacterium'' is a genus of Gram-negative bacteria established by H. J. Conn that uses horizontal gene transfer to cause tumors in plants. ''Agrobacterium tumefaciens'' is the most commonly studied species in this genus. ''Agrobacterium'' is ...
''. 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
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...
, 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 xylinum'' due to its cellulose's unique mechanical properties and applications to
biotechnology
Biotechnology is the integration of natural sciences and engineering sciences in order to achieve the application of organisms, cells, parts thereof and molecular analogues for products and services. The term ''biotechnology'' was first used b ...
,
microbiology
Microbiology () is the scientific study of microorganisms, those being unicellular (single cell), multicellular (cell colony), or acellular (lacking cells). Microbiology encompasses numerous sub-disciplines including virology, bacteriology, prot ...
, and
materials science. Historically, bacterial cellulose has been limited to the manufacture of
Nata de coco
''Nata de coco'', also marketed as coconut gel, is a chewy, translucent, jelly-like food produced by the fermentation of coconut water, which gels through the production of microbial cellulose by '' Komagataeibacter xylinus''.
Originating i ...
, 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.
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
''Komagataeibacter xylinus'' is a species of bacteria best known for its ability to produce cellulose, specifically bacterial cellulose.
History and taxonomy
The species was first described in 1886 by Adrian John Brown, who identified the bacter ...
'', was accredited to A.J. Brown in 1886 with the synthesis of an extracellular gelatinous mat.
[Brown, A.J. J. Chem. Soc.,49,172, 432(1886);51,643(1887)] 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.
[Browne, C.A., ''J. Chem. Soc.'', 28, 453 (1906)] 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.
[Tarr, H.L.A., Hibbery, H. ''Can. J. Research'', 4, 372 (1931)]
In the mid-1900s, Hestrin et al. proved the necessity of
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
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.
[A. Steinbuhel, "Bacterial Cellulose." Biopolymers. Weinheim: Wiley-VCH, 2001. Print.] 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.
Biosynthesis
Bacterial sources
Bacteria that produce cellulose include
Gram-negative bacteria
Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...
species such as ''
Acetobacter
''Acetobacter'' is a genus of acetic acid bacteria. Acetic acid bacteria are characterized by the ability to convert ethanol to acetic acid in the presence of oxygen. Of these, the genus ''Acetobacter'' is distinguished by the ability to oxidize ...
'', ''
Azotobacter
''Azotobacter'' is a genus of usually motile, oval or spherical bacteria that form thick-walled cysts (and also has hard crust) and may produce large quantities of capsular slime. They are aerobic, free-living soil microbes that play an importan ...
'', ''
Rhizobium
''Rhizobium'' is a genus of Gram-negative soil bacteria that fix nitrogen. ''Rhizobium'' species form an endosymbiotic nitrogen-fixing association with roots of (primarily) legumes and other flowering plants.
The bacteria colonize plant cells ...
'', ''
Pseudomonas
''Pseudomonas'' is a genus of Gram-negative, Gammaproteobacteria, belonging to the family Pseudomonadaceae and containing 191 described species. The members of the genus demonstrate a great deal of metabolic diversity and consequently are able ...
'', ''
Salmonella
''Salmonella'' is a genus of rod-shaped (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' is the type species and is fur ...
'', ''
Alcaligenes
''Alcaligenes'' is a genus of Gram-negative, Aerobic organism, aerobic, Bacillus (shape), rod-shaped bacteria. The species are motile with amphitrichous flagella and rarely nonmotile. It is a genus of Non-fermenter, non-fermenting bacteria (in t ...
'', and
Gram-positive bacteria
In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.
Gram-positive bact ...
species such as
''Sarcina ventriculi''.
[M. Shoda, Y. Sugano (2005) Recent advances in bacterial cellulose production, Biotechnol. Bioprocess Eng. 10 1-8] 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.
[S. Bielecki, A. Krystynowicz, M. Turkiewicz, H. Kalinowska: Bacterial Cellulose. In: Polysaccharaides and Polyamides in the Food Industry, A. Steinbuchel, S.K. Rhee (Eds.), Wiley-VCH Verlag, Weinhein, Germany (2005) pp. 31-85]
General process
![Biochemical Pathway for Cellulose Synthesis](https://upload.wikimedia.org/wikipedia/commons/b/bc/Biochemical_Pathway_for_Cellulose_Synthesis.jpg)
The synthesis of bacterial cellulose is a multistep process that involve two main mechanisms: the synthesis of
uridine diphosphoglucose
Uridine diphosphate glucose (uracil-diphosphate glucose, UDP-glucose) is a nucleotide sugar. It is involved in glycosyltransferase reactions in metabolism.
Functions
UDP-glucose is used in nucleotide sugar metabolism as an activated form of glu ...
(UDPGIc), followed by the
polymerization of glucose into long and unbranched chains (the β-1→4 glucan chain) by
cellulose synthase
The UDP-forming form of cellulose synthase () is the main enzyme that produces cellulose. Systematically, it is known as ''UDP-glucose:(1→4)-β-D-glucan 4-β-D-glucosyltransferase'' in enzymology. It catalyzes the chemical reaction:
: UDP-g ...
. 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
In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms. The chemical formula for all hexoses is C6H12O6, and their molecular weight is 180.156 g/mol.
Hexoses exist in two forms, open-chain or cyclic, that easily conver ...
,
glycerol
Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
,
dihydroxyacetone
Dihydroxyacetone (; DHA), also known as glycerone, is a simple saccharide (a triose) with formula .
DHA is primarily used as an ingredient in sunless tanning products. It is often derived from plant sources such as sugar beets and sugar cane, an ...
,
pyruvate
Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell.
Pyruvic aci ...
, and
dicarboxylic acids
In organic chemistry, a dicarboxylic acid is an organic compound containing two carboxyl groups (). The general molecular formula for dicarboxylic acids can be written as , where R can be aliphatic or aromatic. In general, dicarboxylic acids show ...
) entering the
Krebs cycle
The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and protein ...
,
gluconeogenesis
Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrat ...
, or the
pentose phosphate cycle depending on what carbon source is available. It then goes through
phosphorylation
In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
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.
Fermentation production
Cellulose production depends heavily on several factors such as the
growth medium
A growth medium or culture medium is a solid, liquid, or semi-solid designed to support the growth of a population of microorganisms or cells via the process of cell proliferation or small plants like the moss ''Physcomitrella patens''. Differen ...
, environmental conditions, and the formation of byproducts. The fermentation medium contains
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
,
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
, 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
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
and
sucrose
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula .
For human consumption, sucrose is extracted and refined ...
are the most commonly used carbon sources for cellulose production, while
fructose
Fructose, or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galacto ...
,
maltose
}
Maltose ( or ), also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the two- ...
,
xylose
Xylose ( grc, ξύλον, , "wood") is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional gro ...
,
starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...
, and
glycerol
Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
have been tried.
Sometimes,
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
may be used to increase cellulose production.
The problem with using glucose is that
gluconic acid
Gluconic acid is an organic compound with molecular formula C6H12O7 and condensed structural formula HOCH2(CHOH)4COOH. It is one of the 16 stereoisomers of 2,3,4,5,6-pentahydroxyhexanoic acid.
In aqueous solution at neutral pH, gluconic acid fo ...
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
Lignosulfonates (LS) are water-soluble anionic polyelectrolyte polymers: they are byproducts from the production of wood pulp using sulfite process, sulfite pulping. Most delignification in sulfite pulping involves acidic cleavage of ether bonds, ...
.
Addition of organic acids, specifically
acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...
, 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
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
and
methionine
Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical ro ...
improved yield.
Pyridoxine
Pyridoxine, is a form of vitamin B6 found commonly in food and used as a dietary supplement. As a supplement it is used to treat and prevent pyridoxine deficiency, sideroblastic anaemia, pyridoxine-dependent epilepsy, certain metabolic disorde ...
,
nicotinic acid
Niacin, also known as nicotinic acid, is an organic compound and a form of vitamin B3, an essential human nutrient. It can be manufactured by plants and animals from the amino acid tryptophan. Niacin is obtained in the diet from a variet ...
,
p-aminobenzoic acid
4-Aminobenzoic acid (also known as ''para''-aminobenzoic acid or PABA because the two functional groups are attached to the benzene ring across from one another in the ''para'' position) is an organic compound with the formula H2NC6H4CO2H. PABA i ...
and
biotin
Biotin (or vitamin B7) is one of the B vitamins. It is involved in a wide range of metabolic processes, both in humans and in other organisms, primarily related to the utilization of fats, carbohydrates, and amino acids. The name ''biotin'', bor ...
are vitamins important for cellulose production whereas
pantothenate
Pantothenic acid, also called vitamin B5 is a water-soluble B vitamin and therefore an essential nutrient. All animals require pantothenic acid in order to synthesize coenzyme A (CoA) – essential for fatty acid metabolism – as well as to, i ...
and
riboflavin
Riboflavin, also known as vitamin B2, is a vitamin found in food and sold as a dietary supplement. It is essential to the formation of two major coenzymes, flavin mononucleotide and flavin adenine dinucleotide. These coenzymes are involved in ...
have opposing effects.
In reactors where the process is more complex, water-soluble polysaccharides such as
agar
Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from ogonori (''Gracilaria'') and "tengusa" (''Gelidiaceae''). As found in nature, agar is ...
,
acetan, and
sodium alginate
Alginic acid, also called algin, is a naturally occurring, edible polysaccharide found in brown algae. It is hydrophilic and forms a viscous gum when hydrated. With metals such as sodium and calcium, its salts are known as alginates. Its colou ...
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 and 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.
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.
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
A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular ...
contactor (RBC),
a
bioreactor
A bioreactor refers to any manufactured device or system that supports a biologically active environment. In one case, a bioreactor is a vessel in which a chemical reaction, chemical process is carried out which involves organisms or biochemistry, ...
equipped with a spin filter,
and a reactor with a
silicone
A silicone or polysiloxane is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cooking ...
membrane.
Structure and properties
Differences between plant and bacterial cellulose
As the Earth's most common
organic
Organic may refer to:
* Organic, of or relating to an organism, a living entity
* Organic, of or relating to an anatomical organ
Chemistry
* Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
material, cellulose can be classified into plant cellulose and bacterial cellulose, both of which are naturally occurring. Plant cellulose, which makes up the cell walls of most plants, is a tough, mesh-like bulkwork in which cellulose fibrils are the primary architectural elements. While bacterial cellulose has the same molecular formula as plant cellulose, it has significantly different macromolecular properties and characteristics.
In general, microbial cellulose is more chemically pure, containing no
hemicellulose
A hemicellulose (also known as polyose) is one of a number of heteropolymer, heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls.Scheller HV, Ulvskov H ...
or
lignin
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity ...
, has a higher water holding capacity and
hydrophilicity
A hydrophile is a molecule or other molecular entity that is intermolecular force, attracted to water molecules and tends to be dissolution (chemistry), dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clar ...
, greater
tensile strength
Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...
resulting from a larger amount of
polymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are ...
, ultrafine network architecture. Furthermore, bacterial cellulose can be produced on a variety of substrates and can be grown to virtually any shape due to the high moldability during formation.
Additionally, bacterial cellulose has a more crystalline structure compared to plant cellulose and forms characteristic ribbon-like
microfibrils A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and sperm tail. Its most frequently o ...
.
A hallmark of microbial cellulose, these thin
microfibrils A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and sperm tail. Its most frequently o ...
are significantly smaller than those in plant cellulose, making bacterial cellulose much more porous.
Macro structure
Cellulose is composed of
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
,
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
, and
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
, and is classified as a
polysaccharide
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...
, indicating it is a
carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...
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. Cellulose fibrils are highly insoluble and inelastic and, because of their molecular configuration, have a tensile strength comparable to that of steel. Consequently, cellulose imparts a unique combination of chemical resilience and mechanical support and flexibility to the tissues in which it resides.
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.
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.
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
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
.
Bacterial cellulose was described by
Louis Pasteur
Louis Pasteur (, ; 27 December 1822 – 28 September 1895) was a French chemist and microbiologist renowned for his discoveries of the principles of vaccination, microbial fermentation and pasteurization, the latter of which was named afte ...
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
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity ...
and other foreign substances.
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%.
The formation of the cellulose pellicle occurs on the upper surface of the
supernatant
In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...
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.
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.
Properties and characterization
Sheet-shaped material prepared from bacterial cellulose has remarkable mechanical properties. According to Brown, the
pellicle of bacterial cellulose was "very tough, especially if an attempt was made to tear it across its plane of growth".
The
Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied leng ...
for bacterial cellulose has been reported to be as high as 15 GPa across the plane of the sheet, whereas the highest values attained in the past by polymeric films or sheets is < 10GPa at most. The sheet's high Young's modulus has been attributed to the unique super-molecular structure in which fibrils of biological origin are preserved and bound tightly by
hydrogen bonds
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
. 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.
This property arises from adjacently aligned glucan chains participating in inter- and intrachain hydrogen bonding.
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.
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.
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
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
,
nuclear magnetic resonance
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...
(NMR),
Raman spectroscopy
Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman sp ...
, and
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
analysis.
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
In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...
.
The term Cellulose I is used for this parallel arrangement, whereas crystalline fibrils bearing antiparallel
polyglucan chains arise forming the thermodynamically stable Cellulose II.
The molecular arrangement in the sheet, confirmed by
X-ray diffraction
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
, 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.
Although cellulose forms a distinct crystalline structure, cellulose fibers in nature are not purely crystalline. In addition to the crystalline and
amorphous
In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal.
Etymology
The term comes from the Greek ''a'' ("wi ...
regions, cellulose fibers contain various types of irregularities, such as kinks or twists of the microfibrils, or voids such as surface micropores, large pits, and
capillaries
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
. Thus, the total surface area of a cellulose fiber is much greater than the surface area of an ideally smooth fiber of the same dimension. The net effect of structural
heterogeneity
Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
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.
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.
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
A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A p ...
have been issued for processes involving this material.
. Bacterial cellulose pellicles were proposed as a temporary skin substitute in case of human burns and other dermal injuries 4. 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
nata de piña
''Nata de piña'' ("cream of pineapple" in Philippine Spanish, Spanish), also marketed as pineapple gel or pineapple gelatin, is a chewy, translucent, Fruit preserves, jelly-like food produced by the fermentation (food), fermentation of pineapple j ...
, a traditional sweet candy dessert of the
Philippines
The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no),
* bik, Republika kan Filipinas
* ceb, Republika sa Pilipinas
* cbk, República de Filipinas
* hil, Republ ...
. Several natural colored pigments (oxycarotenoids, anthocyanins and related antioxidants and free radical scavengers) were incorporated in to bacterial cellulose cubes in order to render the dessert more attractive
5. Fontana, J.D. et al (2017) Handbook of Food Bioengineering, Elsevier / Academic Press, chapter 7 : New Insights on Bacterial Cellulose, pages 213-249 Bacterial cellulose has also been used as a thickener to maintain the
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inte ...
in food and as a stabilizing agent. Due to its texture and fiber content, it has been added to many food products as a
dietary fiber
Dietary fiber (in British English fibre) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fibers are diverse in chemical composition, and can be grouped generally by the ...
. A specific example is Cellulon ®, which is a
bulking agent
Food additives are substances added to food to preserve flavor or enhance taste, appearance, or other sensory qualities. Some additives have been used for centuries as part of an effort to preserve food, for example vinegar (pickling), salt (salt ...
used as a food ingredient to act as a thickener, texturizer, and/or calorie reducer.
[Okiyama, A., Motoki, M. and Yamanaka, S., ''Food Hydeocoll.'', 1992, 6, 479.] Microbial cellulose has also been used as an additive in diet beverages in
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
since 1992, specifically
kombucha
Kombucha (also tea mushroom, tea fungus, or Manchurian mushroom when referring to the culture; Latin name ''Medusomyces gisevii'') is a fermented, lightly effervescent, sweetened black tea drink commonly consumed for its purported health ben ...
, a fermented tea drink.
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.
Due to its high
sonic velocity
The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as ...
and low dynamic loss, bacterial cellulose has been used as an acoustic or filter membrane in hi-fidelity loudspeakers and headphones as marketed by the
Sony Corporation
, commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professional ...
.
Bacterial cellulose is also used as an additive in the
cosmetic
Cosmetic may refer to:
*Cosmetics, or make-up, substances to enhance the beauty of the human body, apart from simple cleaning
*Cosmetic, an adjective describing beauty, aesthetics, or appearance, especially concerning the human body
*Cosmetic, a t ...
industry. Furthermore, it is being tested in the textile industry, with the possibility of manufacturing cellulose based clothing.
Medical
In more modern applications, microbial cellulose has become relevant in the
medical
Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care practic ...
sector. It has been tested and successfully used as a wound
dressing, especially in burn cases. Studies have shown that burns treated with microbial cellulose coverings healed faster than traditional treatments and had less scarring. The microbial cellulose topical applications are effective due to the cellulose's water holding ability and water vapor permeability. The high water holding ability provides a moist atmosphere at the injury site, which is critical in healing, while the wicking ability allows seepage from the wound to be removed from the site. Also, the microbial cellulose molds very well to the surface of the
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
, providing a conformal covering even in usually difficult places to dress wounds, such as areas on the face. This technique has been so successful that commercial microbial cellulose products, such as Biofill ®, have been developed.
Another microbial cellulose commercial treatment product is XCell produced by the Xylos Corporation, which is mainly used to treat wounds from venous
ulcers
An ulcer is a discontinuity or break in a bodily membrane that impedes normal function of the affected organ. According to Robbins's pathology, "ulcer is the breach of the continuity of skin, epithelium or mucous membrane caused by sloughing o ...
.
Studies have also been performed where traditional
gauze
Gauze is a thin, translucent fabric with a loose open weave. In technical terms "gauze" is a weave structure in which the weft yarns are arranged in pairs and are crossed before and after each warp yarn keeping the weft firmly in place. ...
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.
Microbial cellulose has also been used for internal treatments, such as
bone grafts
Bone grafting is a surgical procedure that replaces missing bone in order to repair bone fractures that are extremely complex, pose a significant health risk to the patient, or fail to heal properly. Some small or acute fractures can be cured wit ...
and other
tissue engineering
Tissue engineering is a biomedical engineering discipline that uses a combination of Cell (biology), cells, engineering, Materials science, materials methods, and suitable biochemistry, biochemical and physicochemical factors to restore, maintai ...
and regeneration. A key ability of microbial cellulose for medical applications is that it can easily be molded into various shapes while still retaining all of its useful properties. By molding microbial cellulose into long, hollow tubes, they can be used as replacement structures for several different areas, such as the
cardiovascular system
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
, the
digestive tract
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans a ...
,
urinary tract
The urinary system, also known as the urinary tract or renal system, consists of the kidneys, ureters, bladder, and the urethra. The purpose of the urinary system is to eliminate waste from the body, regulate blood volume and blood pressure, con ...
, or the
trachea
The trachea, also known as the windpipe, is a Cartilage, cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends ...
. A recent application of microbial cellulose has been as synthetic
blood vessels
The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away f ...
and
stents
In medicine, a stent is a metal or plastic tube inserted into the lumen of an anatomic vessel or duct to keep the passageway open, and stenting is the placement of a stent. A wide variety of stents are used for different purposes, from expandab ...
. The cellulose can also be modeled into mesh membranes that can be used for internal replacement structures, such as the brain's outer membrane, the
dura mater
In neuroanatomy, dura mater is a thick membrane made of dense irregular connective tissue that surrounds the brain and spinal cord. It is the outermost of the three layers of membrane called the meninges that protect the central nervous system. ...
. In addition to replacement, these structures have also been used as grafts to interact with existing internal biological material. Microbial cellulose has also been used in guided
tissue regeneration
In biology, regeneration is the process of renewal, restoration, and tissue growth that makes genomes, cells, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of rege ...
.
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.
Current research/future applications
An area of active research on microbial cellulose is in the area of
electronic paper
Electronic paper, also sometimes electronic ink, e-ink or electrophoretic display, are display devices that mimic the appearance of ordinary ink on paper. Unlike conventional flat panel displays that emit light, an electronic paper display ref ...
. Currently, plant cellulose is used to produce the bulk of traditional paper, but due to its low purity it must be mixed with other substances such as
lignin
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity ...
. However, due to microbial cellulose's higher purity and microfibril structure, it may prove to be an excellent candidate for an electronic paper substrate. Microbial cellulose can be fashioned into sheets approximately 100 micrometers thick, about the thickness of normal paper, by a wet synthesis process. The microbial cellulose produces a sturdy substrate with a microfibril structure that allows the paper to be implanted with
dopants
A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. When ...
. Through the application of solutions to the microbial cellulose paper, conductive dopants and
electrochromic
Electrochromism is a phenomenon in which a material displays changes in color or opacity in response to an electrical stimulus.
In this way, a smart window made of an electrochromic material can block specific wavelengths of ultraviolet, visib ...
dyes
A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...
can be placed into the microfibril structure. The bistable dyes change from clear to dark upon the application of the appropriate
voltages
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to mo ...
, which when placed in a pixel structure, would allow images to be formed. This technology is still in the research stage and has not yet been scaled to commercial production levels. Further research has been done to apply bacterial cellulose as a substrate in electronic devices with the potential to be used as e-book tablets, e-newspapers, dynamic wall papers, rewritable maps and learning tools.
Another possible example of bacterial cellulose use in the electronics industry includes the manufacture of
organic light-emitting diodes
An organic light-emitting diode (OLED or organic LED), also known as organic electroluminescent (organic EL) diode, is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light i ...
(OLEDs).
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.
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.
See also
*
Materials science
*
Microbiology
Microbiology () is the scientific study of microorganisms, those being unicellular (single cell), multicellular (cell colony), or acellular (lacking cells). Microbiology encompasses numerous sub-disciplines including virology, bacteriology, prot ...
*
Biotechnology
Biotechnology is the integration of natural sciences and engineering sciences in order to achieve the application of organisms, cells, parts thereof and molecular analogues for products and services. The term ''biotechnology'' was first used b ...
References
External links
{{Commons category, Bacterial cellulose
Cellulose
Bacteria
Nanomaterials