GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

12. METABOLIC ACTIVITY OF AEROBIC HETEROTROPHS

12.6. INCOMPLETE OXIDATION

12.6.5. Production of Secondary Metabolites

12.6.5.3. Microbial Exopolysaccharides

The Practical significance of EPS is due to their ability to significantly alter the rheological characteristics of aqueous systems even at low concentrations. The Diversity of the PHYSICOCHEMICAL PROPERTIES OF microbial Polysaccharides determines their application in the oil-recovery, food, pharmaceutical, and chemical industries, as well as in agriculture and medicine.

According to the Classification proposed by the British scientist I. Sutherland, microbial EPS are divided into five groups.

The first group comprises dextrans and related polysaccharides (levans, mutans). They consist of a single type of monosaccharide, i.e., they are Homopolysaccharides. The synthesis of these EPS occurs on media containing sucrose as a specific substrate. In the absence of such a specific substrate (which, besides sucrose, can include other related CARBOHYDRATES), EPS production is not observed. Producers of the first-group EPS are Representatives of the genera Streptococcus and Leuconostoc. Dextran (α-D-glucan) is produced by Bacteria such as Leuconostoc mesenteroides, Streptococcus bovis, and Streptococcus viridans. Dextran is used as a plasma volume expander, as well as for analytical research in chemistry and biology. Despite the fact that the industrial production of dextran has existed since the 1940s, its market share among microbial EPS remains relatively low.

The formation of second-group EPS also requires the presence of a specific carbon substrate, but the synthesized EPS are Heteropolysaccharides. Currently, The production of such EPS has been established for fluorescent pseudomonads.

The third group includes homopolysaccharides synthesized from various carbon substrates. Some of these homopolysaccharides consist solely of carbohydrates, such as bacterial Cellulose or pullulan (produced by Aureobasidium pullulans)₁ while others contain acetyl groups (for example, EPS synthesized by certain Agrobacterium species).

Curdlan is a β(1→3)-glucan synthesized by the bacteria Alcaligenes faecalis and Agrobacterium radiobacter. When heated to 54 °C, this polysaccharide forms a gel which, unlike Agar gels, retains its Structure across a wide Temperature range (from 18 to 80 °C). Curdlan is exceptionally resistant to acid Treatment. Due to these properties, curdlan is widely used in the food industry and for the preparation of microbiological media.

The fourth group of microbial EPSs is the most numerous. Its representatives are heteropolysaccharides composed of structures with repeating units. This group includes xanthan, the most extensively studied microbial EPS, as well as gellan and emulsan, which are of significant industrial value.

Xanthan (produced by Xanthomonas campestris NRRL B-1459) consists of D-glucose, D-mannose, and D-glucuronic acid residues in a ratio of 2.8:2.0:2.0. In addition, the EPS contains approximately 4.7% O-acetyl groups and about 3% pyruvic acid residues linked to glucose residues in the side chains as a cyclic ketal. Xanthan was first described in the 1960s. At the Institute of Microbiology and Virology of the National Academy of Sciences of Ukraine, a strain of Xanthomonas campestris pv. campestris 8162 was selected that synthesizes a biopolymer analogous to xanthan.

Xanthan solutions exhibit high viscosity at low concentrations, which remains stable over a wide pH range and is independent of temperature and the presence of salts in solution. The key characteristics of xanthan enable its application in enhanced oil recovery and food manufacturing.

Etapolan is a novel polysaccharide with unique properties developed by the Institute of Microbiology and Virology of the National Academy of Sciences of Ukraine.

The complex polysaccharide preparation etapolan, synthesized by the bacterial strain Acinetobacter sp. 12S, consists of one neutral and two acidic EPSs, one of which is acylated. The neutral EPS is a minor component. The acylated and non-acylated polysaccharides are identical in the molar ratio of D-glucose, D-mannose, D-galactose, L-rhamnose, D-glucuronic, and pyruvic acids (3:2:1:1:1:1) and in the repeating unit STRUCTURE OF THE carbohydrate chain. The difference between these EPSs is that the acylated polysaccharide contains Fatty acids (C₁₂-C₁₈). The rheological properties of etapolan solutions (emulsifying capacity, viscosity enhancement in the presence of mono- and divalent cations, at lower pH, at low shear rates, and in the Cu²⁺-Glycine system) are determined by The ratio of its acylated and non-acylated components, as well as the fatty acid content in the acylated polysaccharide.

Etapolan is a multifunctional polysaccharide that can be used in the petroleum, food, and chemical industries as a thickening, stabilizing, emulsifying, and suspending agent. Based on etapolan, a method has been developed for formation Water shutoff, which enables the recovery of up to 240 tons of additional oil per 1 ton of etapolan used and reduces its water cut from 84% to 15%. Utilizing etapolan as a main ingredient, manufacturing technologies have been developed for cosmetic creams under the general name "Ekol" and the industrial detergent "BIMS 2". Due to its ability to adsorb and remove heavy metal salts from the body, etapolan can be incorporated into the formulation of bakery products recommended for preventive Nutrition.

Gellan is a heteropolysaccharide synthesized by Pseudomonas elodea ATCC 31461. It consists of tetrasaccharide repeating units containing glucose, rhamnose, and glucuronic acid residues, along with O-acetyl groups. Gellan exists in three forms: native, low-acylated, and low-acylated clarified forms. Low-acylated gellan is obtained by heating native gellan at pH 10. Upon cooling, gellan forms firm gels whose strength depends on the gellan concentration and the presence of salts in solution. Under the trade name "Gelrite," gellan is used as a gelling agent for the preparation of microbiological media. Compared to agar, Gelrite offers several advantages: stability during repeated autoclaving; inertness to most additives that comprise biological growth media; resistance to enzymatic degradation; higher transparency; and lower toxicity to sensitive microorganisms. The high melting point of gellan gels allows this EPS to be used in heat-processed food products.

Emulsan is a microbial EPS produced on an industrial scale using non-food raw Materials, specifically ethanol. Emulsan consists of N- and O-acylated residues of D-galactosamine, D-glucose, and aminouronic acid. The O-acyl portion of emulsan contains 5–19% fatty acid residues. The protein content in emulsan ranges from 5 to 15%. Emulsan solutions possess pronounced emulsifying properties, which has led to its application in the petroleum industry to enhance oil recovery.

The fifth group of microbial EPSs includes bacterial alginate. This heteropolysaccharide consists of Two Types of monomers: D-mannuronic and L-guluronic acids. Unlike the EPSs of the fourth group, alginate lacks repeating units. Alginate producers include Pseudomonas aeruginosa and Azotobacter vinelandii. Bacterial alginate differs from seaweed-derived alginate by the presence of O-acetyl groups attached to D-mannuronic acid. Microbial alginates are used in the food industry as substitutes for algal alginates.