Biosynthesis, characterisation, and design of bacterial exopolysaccharides from lactic acid bacteria

Lactic acid bacteria (LAB) are characterised by their conversion of a large proportion of their carbon feed, fermentable sugars, to lactic acid. However, in addition to lactic acid production, the LAB are able to divert a small proportion of fermentable sugars towards the biosynthesis of exopolysaccharides (EPSs) that are independent of the cell surface and cell wall material. These microbial EPSs when suspended or dissolved in aqueous solution provide thickening and gelling properties, and, as such, there is great interest in using EPSs from food grade microorganisms (such as the LAB that are traditionally used for food fermentations) for use as thickening agents. The current review includes a brief summary of the recent literature describing features of the biosynthetic pathways leading to EPS production. Many aspects of EPS biosynthesis in LAB are still not fully understood and a number of inferences are made regarding the similarity of the pathway to those involved in the synthesis of other cell polysaccharides, e.g., cell wall components. The main body of the review will cover practical aspects concerned with the isolation and characterisation of EPS structures. In the last couple of years, a substantial number of structures have been published and a summary of the common elements of these structures is included as is a suggestion for a system for representing structures. A brief highlight of the attempts that are being made to design ‘tailor’-made polysaccharides using genetic modification and control of metabolic flux is presented.     

Recent advances in extracellular biopolymer flocculants

Extracellular biopolymer flocculants (EBFs) are flocculating substances, consisting of polysaccharides, proteins, and lipids, which are secreted in the culture broth by many microorganisms. Some of EBFs have attracted much attention as biodegradable and nontoxic substitutes for conventional chemical flocculants. This paper reviews the recent development of EBFs. Aspects discussed include an introduction to conventional chemical flocculants and EBFs, isolation of novel bioflocculant-producing microorganisms, culture conditions, chemical structure and molecular weight of EBFs, the physico-chemical factors affecting flocculating activity, fermentation process design and recent and emerging application fields of EBFs.     

The role of cyanobacterial exopolysaccharides in structuring desert microbial crusts

Abstract: Microbial crusts are present on surfaces of soils throughout the world. A key feature of these crusts in arid zones is the abundance of filamentous sheath-forming and polysaccharide-excreting cyanobacteria. Several isolates of cyanobacteria were prepared from crust samples (Nizzana sand dunes, north-western Negev Desert, Israel). Optimal growth conditions for two such isolates of Microcoleus sp. were defined, and the role of the excreted polysaccharides in affecting the hydrological properties of crust-covered sand dunes was studied. Experiments with the native crust microbial population demonstrated the possibility of net primary productivity at both high relative air humidities and low moisture content.     

Purification,partial structural characterization and health benefits of exopolysaccharides from potential probiotic Pediococcus acidilactici NCDC 252

Probioceuticals are probiotic-derived biologically active compounds that positively influence human health. Exopolysaccharides (EPS) are long chain polymers of sugars and their derivatives with diverse biological functions. The objective of this work was to study EPS from Pediococcus acidilactici (PA) NCDC 252, a potential probiotic. In silico analysis of PA NCDC 252 genome revealed an EPS gene cluster (10 genes) and genes were further analysed for their functional domains and products. EPS from PA NCDC 252 were purified by ethanol precipitation and gel filtration chromatography on Sephadex G-100. Molecular mass of purified EPS was 89.1 KDa as determined by gel filtration chromatography. Structural and component analysis by FTIR, NMR and UPLC revealed purified EPS to be linear homopolysaccharides (α-glucans) with few α-(1→3) branches. in vitro studies showed anti-oxidant, reduction potential and anti-cancer activity in dose dependent manner. Total antioxidant potential of EPS was 11.9 %. In-vitro cell viability assay revealed anti-proliferative effect of EPS on human colon cancer cell line (HCT116). At 10 and 100 μg/mL, EPS inhibited HCT116 cells up to 67.1 % and 87.3 % respectively. These results suggest that PA EPS can be therapeutically used as anti-oxidant and anticancer agent after in vivo studies.     

New protocol for the rapid quantification of exopolysaccharides in continuous culture systems of acidophilic bioleaching bacteria

In this study, we investigate exopolysaccharide production by a bacterial consortium during the bioleaching of a cobaltiferrous pyrite. Whereas comparable studies have looked at exopolysaccharide production in batch systems, this study focuses on a continuous system comprising a series of four stirred bioreactors and reveals the difficulties in quantifying biomolecules in complex media such as bioleached samples. We also adapted the phenol/sulphuric acid method to take into account iron interference, thus establishing a new protocol for sugar quantification in bioleached samples characterised by low pH (1.4) and high iron concentration (2 g l⁻¹). This allows sugar analysis without any prior sample preparation step; only a small amount of sample is needed (0.5 ml) and sample preparation is limited to a single filtration step. We found that free exopolysaccharides represented more than 80% of the total sugars in the bioreactors, probably because stirring creates abrasive conditions and detaches sugars bound to pyrite or bacteria and that they were produced mainly in the first two reactors where bioleaching activity was greatest. However, we could not establish any direct link between the measured exopolysaccharide concentration and bioleaching activity. Exopolysaccharides could have another role (protection against stress) in addition to that in bacterial attachment.     

Protein phosphorylation on tyrosine in bacteria

Protein phosphorylation on tyrosine has been demonstrated to occur in a wide array of bacterial species and appears to be ubiquitous among prokaryotes. This covalent modification is catalyzed by autophosphorylating ATP-dependent protein-tyrosine kinases that exhibit structural and functional features similar, but not identical, to those of their eukaryotic counterparts. The reversibility of the reaction is effected by two main classes of protein-tyrosine phosphatases: one includes conventional eukaryotic-like phosphatases and dual-specific phosphatases, and the other comprises acidic phosphatases of low molecular weight. Less frequently, a third class concerns enzymes of the polymerase-histidinol phosphatase type. In terms of genomic organization, the genes encoding a protein-tyrosine phosphatase and a protein-tyrosine kinase in a bacterial species are most often located next to each other on the chromosome. In addition, these genes are generally part of large operons that direct the coordinate synthesis of proteins involved in the production or regulation of exopolysaccharides and capsular polysaccharides. Recent data provide evidence that there exists a direct relationship between the reversible phosphorylation of proteins on tyrosine and the production of these polysaccharidic polymers, which are also known to be important virulence factors. Therefore, a new concept has emerged suggesting the existence of a biological link between protein-tyrosine phosphorylation and bacterial pathogenicity.     

灵芝胞外生物活性多糖的pH控制发酵

灵芝深层发酵过程中 ｐＨ值对灵芝胞外多糖形成的影响。结果表明起始ｐＨ为５．５时,有利于胞外多糖的形成,找到了一种有效的ｐＨ控制发酵策略,当发酵过程中控制ｐＨ４．０时,胞外多糖的产量最大,达到２．３２９／Ｌ,较未控制提高了２４％。另外,还对胞外多糖的生物活性进行了检测,结果表明抗肿瘤活性平均达到５１．２％,且能明显提高小鼠的免疫力。     

Extraction of extracellular polymeric substances from extreme acidic microbial biofilms

The efficiency of five extraction methods for extracellular polymeric substances (EPS) was compared on three benthic eukaryotic biofilms isolated from an extreme acidic river, Río Tinto (SW, Spain). Three chemical methods (MilliQ water, NaCl, and ethylenediamine tetraacetic acid [EDTA]) and two physical methods (Dowex 50.8 and Crown Ether cation exchange resins) were tested. The quality and quantity of the EPS extracted from acidic biofilms varied according to which EPS extraction protocol was used. Higher amounts were obtained when NaCl and Crown Ether resins were used as extractant agents, followed by EDTA, Dowex, and MilliQ. EPS amounts varied from approximately 155 to 478 mg g⁻¹ of dry weight depending on the extraction method and biofilm analyzed. EPS were primarily composed of carbohydrate, heavy metals, and humic acid, plus small quantities of proteins and DNA. Neutral hexose concentrations corresponded to more than 90% of the total EPS dry weight. The proportions of each metals in the EPS extracted with EDTA are similar to the proportions present in the water from each locality where the biofilms were collected except for Al, Cu, Zn, and Pb. In this study, the extracellular matrix heavy metal sorption efficiencies of five methods for extracting EPS from eukaryotic acidic biofilms were compared.     

Synthesis of oligosaccharide fragments corresponding to the exopolysaccharide released by Streptococcus macedonicus Sc 136

Hexa-, penta- and tetrasaccharide fragments related to the repeating unit of the exopolysaccharide secreted by Streptococcus macedonicus Sc 136 were synthesized in a very efficient manner involving minimum number of steps. A general glycosylation condition has been applied for all glycosylation steps and yields were excellent. Presented as a poster in the 3rd International Syposium on Current Trends in Drug Discovery Research 2007. CDRI Communication no. 7244.