What are exopolysaccharides (EPS)?
They are polysaccharides synthesized by bacteria (especially soil bacteria) and secreted into the external environment. EPS are natural polymers and have distinctive chemical structures and physicochemical properties. Alike to synthetic polymers, they too have potential applications in food, pharmacy, cosmetics, agriculture and civil industry but have advantage of being ecofriendly, non-toxic and biodegradable.
Chemical composition and physicochemical properties:
EPS can be homopolymers or heteropolymers of molecular weight from 10kDa to the values of mDa. Homopolysaccharide is a neutral polymer of single monosaccharide such as glucose, mannose, rhamnose or galactose etc. Heteropolysaccharide type of EPS are structurally very diverse; they contain co-polymerized branched chain monomers of hexose sugars, glucuronic acid, uronic acid or gluconogalactone and non-carbohydrate moieties of pyruvate, succinate, sialic acid, lactate, phosphate or carboxylates like succinoglycans. Due to the presence of non-carbohydrate moieties, heteropolysaccharides are acidic. Unique physico-chemical properties of EPS are the basis of their varied applications. Distinctive physical properties they altogether possess are gelling, flocculation, viscous, thickening, water absorption and retention (hygroscopy), hydrophilicity and oil emulsification. EPS are able to absorb and hold water over 100 times of their weight; they also retard or slow the rate of evaporation when in contact with water. They are stably hydrophilic and in the soil environment tends to be hygroscopic retaining soil moisture.
Soil bacteria are benefitted from their EPS production in variety of ways. They are required to fulfill different functions in a hostile soil environment. Both plant growth promoting rhizobacteria (PGPR) and phytopathogenic bacteria are known to produce EPS for the establishment in soil and root environment, protection from predation and sustenance during nutrient starvation and desiccation. The most important primary role of EPS is in survival under harsh environmental conditions such as drought or desiccation. Soil particles and EPS together form soil aggregates or matrix wherein bacteria can reside ultimately protected from dry weather. EPS associated with aggregates also maintain water balance in confined niche of bacteria thereby avoiding water logging or extra water evaporation in hot air. In other words, bacteria maintain soil water potential which is one of vital soil characteristics. This is achieved by equilibrium between internal and external water potential of every single bacterial cell and its environment. Therefore during dry season, when external water potential decreases, bacteria selectively take in inorganic solutes like K+ from the soil and retain internal water potential. This effect is similar to electral solution which helps to maintain body fluids during dehydration. EPS also play important role in wet or drenched soil; under wet conditions EPS retains water by slowing rate of water evaporation thereby preventing entry of external solutes inside the bacterial cell. EPS thus efficiently provides wet-dry stress endurance to bacteria in the soil environment. Some exquisite functions performed by EPS are stated here (mandatory references given) which were investigated in vitro. EPS from phytopathogen Pantoea agglomerans elicited rapid production of oxygen radical in wheat, rice, tobacco and parsley cell cultures to drought conditions (Ortmann et al, 2006; FEBS Lett., 580(18):4491-4). Phytopathogens like Agrobacterium, Xanthomonas, Pseudomonas and Erwinia enhanced their root colonization and survival by timely secretion of EPS (Denny, T.P. 1995; Ann. Rev. Phytopathol., 33:173-197). Wilting symptom developed by bacterial pathogenesis has also been observed to be induced by EPS (Leigh and Coplin, 1992; Annu. Rev. Microbiol., 46:307-346). Elicitation of induced systemic resistance (ISR) against pathogen of cucumber, Colletotrichum orbiculare by Burkholderia gladioli is the first report of ISR induction by PGPR (Kyungseok et al, 2008; .J. Microbiol. Biotechnol.18 (6):1095-1100). EPS of non-pathogenic strain of Pantoea agglomerans successfully assisted to colonize roots and also promoted growth of sunflower (Alami et al, Appl. Environ. Microbiol.66: 3393-3398). EPS secretions of Sinorhizobium meliloti were found to trigger and control development responses and defense mechanisms in host alfalfa (Mendrygal and Gonzalez, 2000; J. Bacteriol., 82:599-606).
Emulsification of oils-hydrocarbons, organic solvents and binding to heavy metals are some of the most important functions of EPS producing microbes. They are predominant in soils contaminated with petroleum fuels, solvents and also heavy metals. Irrespective of the source of contamination, EPS producing bacteria not only emulsify pollutant contaminants but also degrade them efficiently until they are converted into non-toxic products. The role of EPS in hydrocarbon degradation is to attach or adhere to individual oil droplet, break it or emulsify it to smaller nuclei to be assimilated to degrade or use as energy source. Other way is to reduce surface/interfacial tension, increase the cell surface hydrophobicity so that hydrocarbon binds to EPS layer present outside the bacterial cell and exposed to bacterial attack. EPS provides shield to bacteria from direct effect of hydrocarbons or heavy metals. Here the structural components of EPS play a crucial role and also offer versatility to degrade broad spectrum of hydrocarbons. Without bacterial emulsification and degradations, soil would have been a bin full of contaminants.
EPS from soil bacteria and their exclusive applications:
Examples of EPS of soil bacteria is enlisted here and some of them have been commercialized for variable applications. Distinctive physico-chemical and functional characteristics of EPS have made them very prospective for applications in different industries. Applications in general are many but exclusive applications of EPS are just pointed here.
Note: these bacteria are not only found in soil but also in other habitats. EPS is formed by a particular strain under suitable conditions irrespective of habitat; for commercial production cultures are maintained appropriately so that their genetic, physicochemical properties remain stable and won’t affect EPS synthesis.
• Cellulose (Acetobacter xylinum): Dietary fiber
• Acetan (Acetobacter xylinum): Gelling agent
• Dextran (Leuconostoc mesenteroides): Anticoagulant, in chromatography matrices
• Xanthan (Xanthomonas campestris): Stabilizer in cosmetics
• Levan (Bacillus subtilis): Raw material for cosmetic formulations and as fragrance carrier
• Alginate (Azotobacter vinelandii): Food thickening agent
• Sphingans (Sphingomonas spp.): Aqueous rheological control agents
• Gellan (Sphingomonas spp.): Anti-settling agent in liquid fluids, starch substitute
• Glucomannan (Rhizobium leguminosarum): Food emulsifier
• Curdlan (Alcaligenes faecalis, Rhizobium spp.): Nanoparticle based drug delivery
• Heteropolysaccharide (Arthrobacter spp.): Emulsifier and oil degradation
• Glucan (Azospirillum): Solubility enhancers
• Succinoglycan (Sinorhizobium meliloti): Gravel packing in drilling industry
• Glucuronan (Sinorhizobium meliloti): Immunostimulant
• Cyclosophorans (Agrobacterium, Xanthomonas, Rhizobium): Encapsulating agent
• Emulsan (Acinetobacter calcoaceticus): Microbially enhanced oil recovery (MEOR)
• Indican (Beijerinckia indica): Paint thickener
• Welan (Alcaligenes spp.): Cement manufacturing
Role in crop growth, health and yield:
As discussed in previous paragraph of functional properties of EPS, crop growth and yield is directly proportional to soil water potential. As we know, EPS plays central role in maintaining water potential and thus directly responsible for healthy growth of crop and subsequent yield. EPS bind soil particles forming aggregates; which not only maintains soil moisture but also ensures the obligate contact between plant roots and rhizobacteria. We already know the plant growth promoting beneficial interactions of rhizobacteria and host plants which sustain for each other under the conditions of stress or pathogenesis. Plants that harbor EPS producing bacteria have better chances of optimum stand and yield even during stress periods such as saline soils, dry weather or water logging. EPS of root nodule bacteria genus Rhizobium play a crucial role in Rhizobium-Legume symbiosis, all rhizobial have been found to be EPS producers.
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