Rhizosphere bacterial exopolysaccharides: composition, biosynthesis, and their potential applications.

Bacterial exopolysaccharides (EPS) are biopolymers of carbohydrates, often released from cells into the extracellular environment. Due to their distinctive physicochemical properties, biocompatibility, biodegradability, and non-toxicity, EPS finds applications in various industrial sectors. However, the need for alternative EPS has grown over the past few decades as lactic acid bacteria's (LAB) low-yield EPS is unable to meet the demand. In this case, rhizosphere bacteria with the diverse communities in soil leading to variations in composition and structure, are recognized as a potential source of EPS applicable in various industries. In addition, media components and cultivation conditions have an impact on EPS production, which ultimately affects the quantity, structure, and biological functions of the EPS. Therefore, scientists are currently working on manipulating bacterial EPS by developing cultures and applying abiotic and biotic stresses, so that better production of exopolysaccharides can be attained. This review highlights the composition, biosynthesis, and effects of environmental factors on EPS production along with the potential applications in different fields of industry. Ultimately, an overview of potential future paths and tactics for improving EPS implementation and commercialization is pointed out.

Halophilic lactic acid bacteria - Play a vital role in the fermented food industry.

Halophilic lactic acid bacteria have been widely found in various high-salt fermented foods. The distribution of these species in salt-fermented foods contributes significantly to the development of the product's flavor. Besides, these bacteria also have the ability to biosynthesize bioactive components which potentially apply to different areas. In this review, insights into the metabolic properties, salt stress responses, and potential applications of these bacteria have been have been elucidated. The purpose of this review highlights the important role of halophilic lactic acid bacteria in improving the quality and safety of salt-fermented products and explores the potential application of these bacteria.

Synbiotics: a New Route of Self-production and Applications to Human and Animal Health.

Synbiotics are preparations in which prebiotics are added to probiotics to achieve superior performance and benefits on the host. A new route of their formation is to induce the prebiotic biosynthesis within the probiotic for synbiotic self-production or autologous synbiotics. The aim of this review paper is first to overview the basic concept and (updated) definitions of synergistic synbiotics, and then to focus particularly on the prebiotic properties of probiotic wall components while describing the environmental factors/stresses that stimulate autologous synbiotics, that is, the biosynthesis of prebiotic-forming microcapsule by probiotic bacteria, and finally to present some of their applications to human and animal health.

Correlation Between the Amount of Extracellular Polymeric Substances and the Survival Rate to Freeze-Drying of Probiotics.

To demonstrate that the amount of extracellular polymeric substances (EPS) and the freeze-dried viability of probiotics are correlated. Three strains of probiotics including Lactiplantibacillus plantarum, Lactobacillus acidophilus, and Bifidobacterium bifidum were subjected to environmental challenges, such as temperature, pH, and carbon dioxide. The results indicated that the challenges could stimulate the EPS synthesis of the probiotics. The experimental correlation between the amount of synthesized EPS and the freeze-dried survival rate was also analyzed, and the viability of each of the three strains was represented by the following functions in which the equation of L. plantarum is y = - 0.0336x2 + 2.7059x - 14.849 with R2 = 0.9699, the B. bifidum's equation is y = - 0.0554x2 + 2.6243x - 13.654 with R2 = 0.9554, and the L. acidophilus's one was y = 0.0346x2 + 0.5862x - 9.1339 with R2 = 0.9733. This could be a new approach to determining the freeze-dried viability of probiotic strains based on the measured EPS content.

Efficacy of the incorporation between self-encapsulation and cryoprotectants on improving the freeze-dried survival of probiotic bacteria.

AIMS: This study aimed to improve the viability of probiotic bacteria during freeze-drying by the combination of self-encapsulation and cryoprotectants. METHODS AND RESULTS: Lactiplantibacillus plantarum VAL6 and Lactobacillus acidophilus VAR1 were exposed to environmental stresses including temperature, pH and increased CO2 concentration before performing freeze-drying with the addition of cryoprotectants. The results proved that tested stresses can stimulate the bacteria to synthesize more extracellular polymeric substances to form self-encapsulation that increases their freeze-dried viability. In combination with cryoprotectants to form double-layered microencapsulation, L. plantarum VAL6 stressed at pH 3.5 in combination with whey protein isolate could achieve the highest Improving Cell Viability of 4361-fold, while L. acidophilus VAR1 stressed at 25o C in combination with alginate gave a maximum Improving Cell Viability of 73.33-fold. CONCLUSIONS: The combination of self-encapsulation and cryoprotectants significantly improves the freeze-dried viability of probiotics. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first report that uses environmental stress to stimulate extracellular polymeric substance synthesis for self-encapsulation formation combined with the addition of cryoprotectants to enhance the freeze-dried survival of probiotics. This could be a novel approach in improving the viability of probiotic strains for various applications.

Expression of genes involved in exopolysaccharide synthesis in Lactiplantibacillus plantarum VAL6 under environmental stresses.

Environmental factors can alter exopolysaccharide biosynthesis in lactic acid bacteria (LAB). To further clarify this potential relationship, the mRNA expression of genes involved in exopolysaccharide synthesis such as glmU, pgmB1, cps4E, cps4F, cps4J, and cps4H in Lactiplantibacillus plantarum VAL6 under different conditions including temperature, pH, sodium chloride (NaCl), and carbon dioxide (CO2) intensification culture was studied. The transcriptomic data revealed that the exposure of L. plantarum VAL6 at pH 3 increased the expression level of cps4H but decreased the expression levels of pgmB1 and cps4E. Under pH 8, cps4F and cps4E were significantly upregulated, whereas pgmB1 was downregulated. Similarly, the expression levels of cps4F, cps4E, and cps4J increased sharply under stresses at 42 or 47 °C. In the case of NaCl stress, glmU, pgmB1, cps4J, and cps4H were downregulated in exposure to NaCl at 7 and 10% concentrations while cps4E and cps4F were upregulated at 1 h of 10%-NaCl treatment and at 5 h of 4%-NaCl treatment. Remarkably, CO2 intensification culture stimulated the expression of all tested genes. In addition, simultaneous changes in expression of cps4E and cps4F under environmental challenges may elicit the possibility of an association between the two genes. These findings indicated that the expression level of eps genes is responsible for changes in the yield and monosaccharide composition of exopolysaccharides under environmental stresses.

Response of Lactobacillus plantarum VAL6 to challenges of pH and sodium chloride stresses.

To investigate the effect of environmental stresses on the exopolysaccharide biosynthesis, after 24 h of culture at 37 °C with pH 6.8 and without sodium chloride, Lactobacillus plantarum VAL6 was exposed to different stress conditions, including pH (pHs of 3 and 8) and high sodium chloride concentration treatments. The results found that Lactobacillus plantarum VAL6 exposed to stress at pH 3 for 3 h gives the highest exopolysaccharide yield (50.44 g/L) which is 6.4 fold higher than non-stress. Under pH and sodium chloride stresses, the mannose content in exopolysaccharides decreased while the glucose increased in comparison with non-stress condition. The galactose content was highest under stress condition of pH 8 meantime rhamnose content increased sharply when Lactobacillus plantarum VAL6 was stressed at pH 3. The arabinose content in exopolysaccharides was not detected under non-stress condition but it was recorded in great amounts after 3 h of stress at pH 3. In addition, stress of pH 8 triggered the mRNA expression of epsF gene resulting in galactose-rich EPS synthesis. According to our results, the stresses of pH and sodium chloride enhance the production and change the mRNA expression of epsF gene, leading to differences in the monosaccharide composition of exopolysaccharides.

Exopolysaccharide production by lactic acid bacteria: the manipulation of environmental stresses for industrial applications.

Exopolysaccharides (EPSs) are biological polymers secreted by microorganisms including Lactic acid bacteria (LAB) to cope with harsh environmental conditions. EPSs are one of the main components involved in the formation of extracellular biofilm matrix to protect microorganisms from adverse factors such as temperature, pH, antibiotics, host immune defenses, etc.. In this review, we discuss EPS biosynthesis; the role of EPSs in LAB stress tolerance; the impact of environmental stresses on EPS production and on the expression of genes involved in EPS synthesis. The evaluation results indicated that environmental stresses can alter EPS biosynthesis in LAB. For further studies, environmental stresses may be used to generate a new EPS type with high biological activity for industrial applications.