Biopolymers are polymers that can be produced or biodegraded by a living thing. Exopolysaccharides, as the name suggests, are polysaccharide (carbohydrate) biopolymers secreted out of the cell. Only some living things have the ability to produce exopolysaccharides.
Exopolysaccharides (EPS) can be imagined as “a sticky ball of wool” produced inside the microorganism cell and secreted outside.
This secreted structure gives the microorganism different abilities such as protection against drying, resistance to high pressure, protection from the attacks of other microorganisms, protection from antibiotics or toxins and recognition of each other.
Generally, these biopolymers are not used as an energy source by the producer microorganism, but they provide effective protection of microbial cell integrity in an ecosystem with harsh environmental conditions.
In this respect, the presence of a microorganism’s ability to secrete EPS gives that microorganism quite superior virtues.
(This picture is of the growth in a broth of a strain of Lactobacillus rhamnosus, which I isolated and identified from traditionally produced butter samples as part of my doctoral study. In general, a colony of a strain that does not produce EPS appears as sediment on the bottom with turbidity. EPS-producing ones may be expected to look like mild thrush on the bottom. However, the EPS production of this strain was so high that it formed a seriously cloudy colony in the broth.)
Two reasons make exopolysaccharides very attractive to humans. The first of these is the realization that EPS secreted by some microorganisms can be highly beneficial for human health.
The second is that EPSs increase the structural properties of the food they are in.
Today, exopolysaccharides are used in the food, pharmaceutical and cosmetic industries and their use is becoming more common day by day. It has been discovered that exopolysaccharides have antioxidant, antibacterial, cholesterol-lowering, immunoregulatory functions, anticarcinogen, antitumor, anticoagulant and antiviral effects in terms of human health.
Exopolysaccharides also play a role in biofilm formation and the ability of bacteria to attach to a surface. In this way, it is easier for bacteria to colonize an environment. This effect of EPS may contribute to the probiotic characteristic of the bacteria.
In other words, probiotic bacteria, which can produce EPS, can pass through the intestines more easily, adhere more easily and develop more easily. It is reported that EPS can also show a prebiotic effect.
In terms of food technology, they have the characteristics of increasing the different characteristics of the food they are in, such as gelling, thickening, emulsifying and water binding.
Therefore, the structural properties of fermented food containing an EPS-producing microorganism can be increased without adding any food additives thanks to EPS. This is very evident in yogurt.
If there is a microorganism producing EPS in the yogurt, that yogurt will be the consistency of cream and the release of water will decrease. In this respect, the presence of EPS in yogurt can be diagnosed observational.
In the presence of EPS, the consistency of yogurt is like cream and elongation is observed under the spoon. Considering as it is, in the presence of EPS, yogurt both has a better appearance and increases its health benefits.
Of course, the EPS-producing microorganism species that can be used in fermented food production must be safe for use in food. Lactic acid bacteria and bifidobacteria stand out in terms of microorganisms that can be used in foods.
Most LABs, especially Fructilactobacillus, Lacticaseibacillus, Lactiplantibacillus, Lactobacillus, Lactococcus, Latilactobacillus, Lentilactobacillus, Leuconostoc, Limosilactobacillus, Pediococcus, Streptococcus and Weissella genus are capable of synthesizing EPS.
However, this does not mean that all species of a genus can produce EPS. Even based on species, the ability to produce EPS varies. For example, one Streptococcus thermophilus strain can produce EPS, while another Streptococcus thermophilus strain cannot produce EPS.
On the other hand, to date, no Weissella strains have been recognized as “generally recognized as safe” (GRAS) by the US Food and Drug Administration. Likewise, bacteria of the genus Weissella have not been given Qualified Safety Scorecard (QPS) status by the European Food Safety Authority, even though they are common in Europe.
In this context, a group of scientists, including myself, is looking for lactic acid bacteria that can produce EPS in local fermented foods.
By investigating the properties of these bacteria and the EPS they produce, possibilities for use in foods are sought.
In this way, it is aimed to discover strains with superior properties and use them as starter cultures, improve the properties of fermented foods without adding food additives and increase their health benefits.
Structure and Classification of Exopolysaccharides
It was previously stated that exopolysaccharides, as their name suggests, are in the form of carbohydrates. However, a standard uniform structure should not come to mind when exopolysaccharide is mentioned.
The structure of EPS varies according to which bacterial strain it is produced by. The structure of EPS to be secreted by a bacterium is encoded in its DNA.
EPS structure can also change according to the nutrient medium in which the microorganism is located. A bacterium can produce EPS of different structures in different nutrient media. Likewise, the amount of EPS secreted may vary greatly depending on the species and the environmental conditions.
Naturally, EPSs of different structures have different benefits both to the structure of the food and to health.
Exopolysaccharides may contain varying proportions of sugars and sugar derivatives such as galactose, glucose, mannose, rhamnose, ribose, xylose, fructose, fucose, arabinose, galacturonic acid, glucuronic acid, glucosamine and galactosamine.
Exopolysaccharides with only one type of monosaccharide in their structure are called homopolysaccharides (HoPS) and those consisting of different monosaccharides are called heteropolysaccharides (HePS).
However, HoPS and HePS differ not only according to the monomers that make up their structures but also according to the chain lengths, degree of branching and sugar linkages.
HoPS is usually composed of glucose or fructose units that form glucans or fructans. The most common of these are α-glucans. The two general subgroups of fructans are the levan-type and inulin-type polysaccharides.
Most EPS produced by LAB are HePS. The repeating units are usually D-glucose, D-galactose and L-rhamnose and in some cases N-acetylglucosamine, N-acetylgalactosamine or glucuronic acid.
In general, exopolysaccharides are classified as follows;
1.) Homopolysaccharides
1.1.) Glucans
a)α Glucans
a.1.) Dextran
a.2.) Mutant
a.3.)Reuteran
a.4.) Alternan
b) β Glucans
1.2.) Fructans
a) Levans
b) Inulin-derivate polysaccharides
2.) Heteropolysaccharides
2.1.) According to the type of monosaccharide it contains
2.2.) According to repeating units
2.3) According to the chain structure; branched or straight chain
2.4.) According to the modification
Biochemical Characteristics and Biosynthesis of Exopolysaccharides
The enzymes involved in EPS production by lactic acid bacteria can be divided into two main groups.
The first group includes proteins necessary to synthesize essential sugar nucleotides used by other cellular pathways. These proteins are not specific to EPS.
The other group includes EPS-specific enzymes such as glycosyl- and acetyl-transferases, which use monomeric molecules as glycosyl donors during synthesis or enzymes responsible for polymerization and secretion and regulate the whole process. However, the biological function of some EPS-specific proteins is currently unknown.
EPS-specific enzymes are regulated by certain genes. The genes encoding EPS biosynthesis in LAB are typically arranged in a cluster and are usually chromosomal (genophore) in thermophilic strains, but can also be located on plasmids in mesophilic LAB. In the latter case, the ability to produce EPS may have high instability and bacteria may lose their EPS production potential without selection pressure.
From a biochemical point of view, HoPS synthesis is a relatively simple process and since there are no active transport steps in the synthetic pathway, there is no energy expenditure other than the biosynthesis of essential extracellular enzymes.
The mechanism of HePS synthesis is a more complex and energy-intensive process.
Biosynthesis of exopolysaccharides (Welman and Maddox 2003)
EPS production of different lactic acid bacteria strains varies between 10 mg/L and 400 mg/L under different conditions and much more can be produced if ideal environmental conditions are provided for growth and production.
References
Akgül, H.I. (2020). Isolation of Lactic Acid Bacteria Producing Exopolysaccharide (EPS) from Butters and Their Usability as Starter Culture in Butter Production, Doctoral thesis, Atatürk University, Erzurum.
Welman, A.D., & Maddox, I.S., (2003). Exopolysaccharides from lactic acid bacteria: perspectives and challenges. Trends in biotechnology, 21(6), 269-274.
Ruas-Madiedo, P., & De Los Reyes-Gavilán, C. G. (2005). Invited review: methods for the screening, isolation, and characterization of exopolysaccharides produced by lactic acid bacteria. Journal of dairy science, 88(3), 843-856.
Korcz, E., & Varga, L. (2021). Exopolysaccharides from lactic acid bacteria: Techno-functional application in the food industry. Trends in Food Science & Technology, 110, 375-384.
Zhou, Y., Cui, Y., Qu, X., (2019). Exopolysaccharides of lactic acid bacteria: Structure, bioactivity and associations: A review. Carbohydrate polymers, 207, 317-332.
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Lactic Acid Bacteria; Definition, Classification and Characteristics
Starter Cultures; History, Definition and Classification
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