Extracellular polysaccharides, or exopolysaccharides (EPS) are polymers synthesised during fermentation by a range of bacterial groups, including lactic acid bacteria (LAB) and bifidobacteria, in relatively substantial amounts when compared to cell size.
EPS from LAB has been utilised in fermented dairy foods to contribute functional attributes such as emulsifying, thickening, stabilizing and gelling properties, syneresis reduction and rheology augmentation in yoghurt as well as enhanced body and mouth-feel in low fat dairy products. Such properties are of importance to consumers who desire low-fat foods but with the superior texture of their full-fat counterparts. Furthermore, the benefits of utilising these polysaccharides in different food applications may not be confined to their techno-functional properties, with many exhibiting potential health benefits upon ingestion.
α-Glucans are found to be most common in LAB HoPS structure, with fewer β-glucans. There are four types of α- glucans – dextrans, mutans, reuterans and alternans – currently recognised. Dextran, synthesised by Ped. pentosaceus CRAG3 Leuconostoc mesenteroides, Leuconostoc amelibiosum and Lb. curvatus, has an α-1,6-glucan repeating unit with occasional α-(1,3) branches. Glucans with mainly α-(1,3) linkages are termed mutans and those with majority α-(1,4) linkages are reuterans.
Immunomodulation: Lopez et al. recently attempted to characterise the immunological and adhesion effects of a range of bifidobacteria EPS. The results of this experiment demonstrated once again that some high molecular weight, uncharged EPS may confer immunosuppressive effects and low molecular weight, charged EPS may elicit a mild inflammatory response in the host.
Antioxidative effect: Although the mechanisms are currently poorly understood, it is theorised that these EPS attenuate oxidative stress through ROS, DPPH free radical, Mn2+ and iron scavenging, superoxide anion and hydrogen peroxide degradation, myeloperoxidase inhibition and erythrocyte hemolysis inhibition.
Prebiotic potential: The bacteria that comprise the human gut microbiota require a carbon source, partially provided by the undigested oligosaccharides and polysaccharides which reach the lower intestine, traditionally of plant origin. These carbohydrates can positively impact on the gut microbiome and indirectly modulate the immune system, by selectively enhancing the bacterial populations associated with beneficial health effects, including bifidobacteria and Lactobacilli. In addition, the metabolism of such carbohydrates by the anaerobic bacteria of the gut microbiome results in the production of CO2, H2 and the synthesis of short chain fatty acids (SCFAs) butyrate, propionate and acetate which are of importance to human colonocytes, liver and peripheral tissues, respectively.
Techno-functional Applications: EPS represent a diverse range of bioactive molecules with varying structures and functionalities, everything from providing additional technol-functionality to products (such as yogurt) to interacting with the mucosal immune system once ingested. This review has addressed the biosynthesis, physiological function, bioactivity and potential food applications of EPS-synthesising microbes, as well as the factors limiting industrial exploitation, including the genetics of HePS biosynthesis. Microbial EPS display encouraging therapeutic potential in terms of immunomodulation, antioxidation, hypocholesterolemia and promotion of a functional digestive tract through prebiotic activity. It is clear that certain EPS structures play central roles in the bioactivity of the microbial producer and thus, may be important in reclaiming an EFSA “probiotic” immunomodulatory health-claim. Indeed, the impact of EPS on human health appears to be multifactorial and structure-function related. In conclusion, microbial EPS have demonstrated exciting therapeutic and techno-functional applications. However, the data available currently stems mainly from in vitro analyses, which lack the complexity of in vivo models and in particular, human clinical data. EPS from LAB have found uses across many industries, including techno-functional applications within the food sector. However, it must be concluded that these natural ingredients remain wholly under-utilised in the functional food sector. Moving towards full exploitation of EPS-producing microbes, it is paramount that the structure-function relationships for both therapeutic and techno-functional activities are deduced. Whether it is the EPS cultures themselves or simply the EPS that they produce, these bacteria hold tremendous potential for the improvement of human health. As such, it is reasonable to expect that they represent a primary pharma-biotic produced by intestinal microbes and with which the host initially interacts. Deciphering the complexities of these interactions, be they nutritional or immunological, may form the mechanistic basis of unlocking some of the black box surrounding the scientific basis for probiotics – at least for those which are ‘sugar-coated’.