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Prebiotics are a group of nutrients that are degraded by gut microbiota. Their relationship with human overall health has been an area of increasing interest in recent years. They can feed the intestinal microbiota, and their degradation products are short-chain fatty acids that are released into blood circulation, consequently, affecting not only the gastrointestinal tracts but also other distant organs. Fructo-oligosaccharides and galacto-oligosaccharides are the two important groups of prebiotics with beneficial effects on human health. Since low quantities of fructo-oligosaccharides and galacto-oligosaccharides naturally exist in foods, scientists are attempting to produce prebiotics on an industrial scale. Considering the health benefits of prebiotics and their safety, as well as their production and storage advantages compared to probiotics, they seem to be fascinating candidates for promoting human health condition as a replacement or in association with probiotics. This review discusses different aspects of prebiotics, including their crucial role in human well-being.

Cross-feeding is an important metabolic interaction mechanism of bacterial groups inhabiting the human colon and includes features such as the utilization of acetate by butyrate-producing bacteria as may occur between Bifidobacterium and Faecalibacterium genera. In this study, we assessed the utilization of different carbon sources (glucose, starch, inulin and fructooligosaccharides) by strains of both genera and selected the best suited combinations for evidencing this cross-feeding phenomenon. Co-cultures of Bifidobacterium adolescentis L2–32 with Faecalibacterium prausnitzii S3/L3 with fructooligosaccharides as carbon source, as well as with F. prausnitzii A2–165 in starch, were carried out and the production of short-chain fatty acids was determined. In both co-cultures, acetate levels decreased between 8 and 24 h of incubation and were lower than in the corresponding B. adolescentis monocultures. In contrast, butyrate concentrations were higher in co-cultures as compared to the respective F. prausnitzii monocultures, indicating enhanced formation of butyrate by F. prausnitzii in the presence of the bifidobacteria. Variations in the levels of acetate and butyrate were more pronounced in the co-culture with fructooligosaccharides than with starch. Our results provide a clear demonstration of cross-feeding between B. adolescentis and F. prausnitzii. The article provides the first experimental demonstration of enhanced butyrate formation by a cross feeding mechanisms betweeen Faecalibacterium prausnitzii, a comensal bacteria of the human colon, with Bifidobacterium adolescentis, one of the most abundant bifidobacteria from the adult's intestinal microbiota.

Bioactive compounds possess different health benefits. Onion contains various bioactive compounds, such as organosulfur compounds, flavonols, ascorbic acids, and carbohydrate prebiotics, and its by‐products have more content of flavonoids than the bulb. Diallyl monosulfide, diallyl disulfide, diallyl trisulfide, and diallyl tetrasulfide are the major organosulfur compounds, whereas quercetin, kaempferol, anthocyanin, and luteolin are considered as main flavonoids. Ascorbic acid and fructooligosaccharides are also regarded as bioactive compounds. Onion bioactive compounds have the strong antioxidant potential for neutralizing oxidative stress of the cells. These bioactive components are beneficial as anticarcinogenic, antibiotic, anti‐inflammatory, antiplatelet, antidiabetic, and cardioprotective agents along with other nutritional benefits. However, various postharvest practices have an impact on these bioactive compounds, for example, curing mostly enhances the bioactive level and processing temperature generally decreases the concentration of many of them, whereas storage studies suggest an increase of others under optimized conditions. Additionally, conventional extraction techniques showed a negative impact on bioactive compounds of onion, whereas innovative methods yielded a higher amount of bioactive components. There is a need for innovative and integrated procedures in the postharvest sector to maintain or enhance the level of bioactive compounds without compromising the quality of onions. The present review comprehensively describes different bioactive compounds of onion, their chemistry, and their pharmacotherapeutic roles. Moreover, it also explores the effects of various postharvest factors, such as temperature, storage duration, and extraction conditions on the level of the bioactive components. It also suggests industrial applications of onion waste and its bioactive compounds in the food sector.  

Understanding how dietary nutrients modulate the gut microbiome is of great interest for the development of food products and eating patterns for combatting the global burden of non-communicable diseases. In this narrative review we assess scientific studies published from 2005 to 2019 that evaluated the effect of micro- and macro-nutrients on the composition of the gut microbiome using in vitro and in vivo models, and human clinical trials. The clinical evidence for micronutrients is less clear and generally lacking. However, preclinical evidence suggests that red wine- and tea-derived polyphenols and vitamin D can modulate potentially beneficial bacteria. Current research shows consistent clinical evidence that dietary fibers, including arabinoxylans, galacto-oligosaccharides, inulin, and oligofructose, promote a range of beneficial bacteria and suppress potentially detrimental species. The preclinical evidence suggests that both the quantity and type of fat modulate both beneficial and potentially detrimental microbes, as well as the Firmicutes/Bacteroides ratio in the gut. Clinical and preclinical studies suggest that the type and amount of proteins in the diet has substantial and differential effects on the gut microbiota. Further clinical investigation of the effect of micronutrients and macronutrients on the microbiome and metabolome is warranted, along with understanding how this influences host health.

Abstrac Fructooligosaccharides (FOS) are promising prebiotics in the relevant and increasing market of functional food. Industrially, these compounds are produced from sucrose by the action of fructosyltransferase or b-fructofuranosidase enzymes. However, this process often yields low conversion rates and results in impure mixtures due to the release of high levels of glucose. Zymomonas mobilis is a well-known ethanol-producing bacterium with native levansucrase enzymes able to convert sucrose into FOS. This study aimed to use synthetic biology tools to eliminate invertase ( sacC ) activity in Z. mobilis , reducing substrate competition and maximizing FOS production. Additionally, we explored the potential use of agro-industrial by-products, such as sugarcane molasses (M) and corn step liquor (CSL), as nutrients for FOS production using Z. mobilis in an in vivo bioprocess strategy. Invertase deletion from the Z. mobilis ZM4 genome was accomplished by homologous recombination of an engineered suicide plasmid. Using Z. mobilis sacC − , we observed a 70% reduction in monosaccharide production and a 9.0-fold increase in levan formation compared to the wild-type strain. Implementation of a fed-batch approach with CSL and molasses (CSLM) medium at flask-scale allowed to produce 41.9 g L −1 of FOS (0.25 g FOS g sucrose −1 ). To our knowledge, this work describes for the first time the production of FOS from agro-waste residues using a genetically modified Z. mobilis strain in a one-step fermentation. Through this innovative approach, we aim to contribute to the advancement of biotechnological strategies for prebiotic production, offering insights into genetic engineering techniques for improving the efficiency and sustainability of FOS synthesis in Z. mobilis .

Dietary fiber is a group of heterogeneous substances that are neither digested nor absorbed in the small intestine. Some fibers can be classified as prebiotics if they are metabolized by beneficial bacteria present in the hindgut microbiota. The aim of this review was to specify the prebiotic properties of different subgroups of dietary fibers (resistant oligosaccharides, non-starch polysaccharides, resistant starches, and associated substances) to classify them by prebiotic categories. Currently, only resistant oligosaccharides (fructans [fructooligosaccharides, oligofructose, and inulin] and galactans) are well documented as prebiotics in the literature. Other fibers are considered candidates to prebiotics or have prebiotic potential, and apparently some have no prebiotic effect on humans. This dietary fiber classification by the prebiotic categories contributes to a better understanding of these concepts in the literature, to the stimulation of the processing and consumption of foods rich in fiber and other products with prebiotic properties, and to the development of protocols and guidelines on food sources of prebiotics. •“Dietary fiber” and “prebiotic” are not synonyms since not all fiber acts as a prebiotic.•Currently, only fructans (fructooligosaccharides and inulin) and galactans are classified as prebiotics.•Fibers can be categorized in prebiotic, candidate to prebiotic, and not recognized as prebiotic.•More clinical studies should be conducted with dietary fibers classified as candidates to prebiotic.

BackgroundInulin, consisting of repetitive fructosyl units linked by β(2,1) bonds, is a readily fermentable fiber by intestinal bacteria that generates large quantities of short-chain fatty acids (SCFA). In individuals with constipation, it was reported that inulin ingestion was associated with a significant increase in stool frequency, suggesting a potential impact of inulin on human gut microbiota composition. Progress in high-throughput technologies allow assessment of human-associated microbiomes in terms of diversity and taxonomic or functional composition, and can identify changes in response to a specific supplementation. Hence, to understand the effects of inulin on the human gut microbiome is pivotal to gain insight into their mechanisms of action.MethodsHere, we conducted a systematic review of human studies in adult individuals showing the effects of inulin on the gut microbiome. We searched in MEDLINE, EMBASE, Web of Science, and Scopus databases for articles in English published in peer-reviewed journals and indexed up until March 2019. We used multiple search terms capturing gut microbiome, gut microflora, intestinal microbiota, intestinal flora, gut microbiota, gut flora, microbial gut community, gut microbial composition, and inulin.ResultsOverall, nine original articles reported the effects of inulin on microbiome composition in adult humans, most of them being randomized, double-blind, placebo-controlled trials (n = 7). Studies varied significantly in design (3 studies associated inulin and oligofructose), supplementation protocols (from 5 to 20 gr per day of inulin consumed) and in microbiome assessment methods (16S sequencing, n = 7). The most consistent change was an increase in Bifidobacterium. Other concordant results included an increase in relative abundance of Anaerostipes, Faecalibacterium, and Lactobacillus, and a decrease in relative abundance of Bacteroides after inulin supplementation.ConclusionsOur systematic review assessed the evidence for the effects of inulin supplementation on the human gut microbiome. However, these in vivo studies did not confirm in vitro experiments as the taxonomic alterations were not associated with increase in short-chain fatty acids levels.

The gut microbiota of individuals are dominated by different fiber-utilizing bacteria, which ferment dietary fiber into short chain fatty acids (SCFAs) known to be important for human health. Here, we show that the dominance of Prevotella versus Bacteroides in fecal innocula, identified into two different enterotypes, differentially impacts in vitro fermentation profiles of SCFAs from fibers with different chemical structures. In a microbiome of the Prevotella enterotype, fructooligosaccharides, and sorghum and corn arabinoxylans significantly promoted one single Prevotella OTU with equally high production of total SCFAs with propionate as the major product. Conversely, in the Bacteroides -dominated microbiota, the three fibers enriched different OTUs leading to different levels and ratios of SCFAs. This is the first report showing how individual differences in two enterotypes cause distinctly different responses to dietary fiber. Microbiota dominated by different fiber-utilizing bacteria may impact host health by way of producing different amounts and profiles of SCFAs from the same carbohydrate substrates.

Zymomonas mobilis ZM4 is an attractive host for the development of microbial cell factories to synthesize high-value compounds, including prebiotics. In this study, a straightforward process to produce fructooligosaccharides (FOS) from sucrose was established. To control the relative FOS composition, recombinant Z. mobilis strains secreting a native levansucrase (encoded by sacB) or a mutated β-fructofuranosidase (Ffase-Leu196) from Schwanniomyces occidentalis were constructed. Both strains were able to produce a FOS mixture with high concentration of 6-kestose. The best results were obtained with Z. mobilis ZM4 pB1- sacB that was able to produce 73.4 ± 1.6 g L −1 of FOS, with a productivity of 1.53 ± 0.03 g L −1  h −1 and a yield of 0.31 ± 0.03 g FOS g sucrose −1 . This is the first report on the FOS production using a mutant Z. mobilis ZM4 strain in a one-step process. Key points • Zymomonas mobilis was engineered to produce FOS in a one-step fermentation process. • Mutant strains produced FOS mixtures with high concentration of 6-kestose. • A new route to produce tailor-made FOS mixtures was presented. Graphical abstract

Short-chain fructooligosaccharides (ScFOS) are a group of linear fructose oligomers that include 1-kestose, 1-nystose and 1- β -fructofuranosylnystose. ScFOS, which naturally occur at low levels in different plant products, are of high interest as food ingredients because of their prebiotic character, organoleptic characteristics and technological properties. Two different industrial processes are used to achieve large-scale ScFOS production: inulin hydrolysis (enzymatic or chemical hydrolysis) or sucrose biotransformation by transfructosylation (enzymatic synthesis) using specific enzymes like fructosyltransferases and fructofuranosidases. Enzymatic ScFOS synthesis seems to be more advantageous than inulin hydrolysis since it is less expensive, and leads to lower molecular weight FOS. The biotechnological process described to carry out this catalysis includes the production of transfructosylation enzymes, separation, enzyme immobilisation and finally the ScFOS production and purification. Such ScFOS production processes may be conducted under submerged or solid-state fermentation under discontinuous or continuous conditions. Several methodologies with different economic/environmental costs and production yields have been described to carry out these ScFOS production stages, although industrial scale-up needs to be optimised. This review tries to address a revision about enzymatic ScFOS production methods and its scale-up to industrial levels.