Children with autism spectrum disorders (ASD) often suffer gastrointestinal problems consistent with imbalances in the gut microbial population. Treatment with antibiotics or pro/prebiotics has been postulated to regulate microbiota and improve gut symptoms, but there is a lack of evidence for such approaches, especially for prebiotics. This study assessed the influence of a prebiotic galactooligosaccharide (B-GOS) on gut microbial ecology and metabolic function using faecal samples from autistic and non-autistic children in an in vitro gut model system. Bacteriology was analysed using flow cytometry combined with fluorescence in situ hybridization and metabolic activity by HPLC and 1H-NMR. Consistent with previous studies, the microbiota of children with ASD contained a higher number of Clostridium spp. and a lower number of bifidobacteria compared with non-autistic children. B-GOS administration significantly increased bifidobacterial populations in each compartment of the models, both with autistic and non-autistic-derived samples, and lactobacilli in the final vessel of non-autistic models. In addition, changes in other bacterial population have been seen in particular for Clostridium, Rosburia, Bacteroides, Atopobium, Faecalibacterium prausnitzii, Sutterella spp. and Veillonellaceae. Furthermore, the addition of B-GOS to the models significantly altered short-chain fatty acid production in both groups, and increased ethanol and lactate in autistic children.

At present, there is a huge interest in finding new prebiotics from agrofood industrial waste, such as the soyabean by-product Okara, rich in insoluble dietary fibre. A previous treatment of Okara with high hydrostatic pressure assisted by the food-grade enzyme Ultraflo ® L achieved a 58·2 % increment in its soluble dietary fibre (SDF) contents. Therefore, potential prebiotic effect of both treated and native Okara was assayed using 48 h, pH-controlled, anaerobic batch cultures inoculated with human faecal slurries, which simulate the human gut. Changes in faecal microbiota were evaluated using 16S rRNA-based fluorescence in situ hybridisation, whereas release of SCFA and lactic acid was assessed by HPLC. Both Okara samples exhibited potential prebiotic effects but Okara treated to maximise its SDF content showed higher SCFA plus lactic acid, better growth promotion of beneficial bacteria, including bifidobacteria after 4 and 48 h and lactobacilli after 4 h of fermentation, and a greater inhibition of potentially harmful bacterial groups such as clostridia and Bacteroides. Differences found between fructo-oligosaccharides and Okara substrates could be attributed to the great complexity of Okara's cell wall, which would need longer times to be fermented than other easily digested molecules, thus allowing an extended potential prebiotic effect. These results support an in vitro potential prebiotic effect of Okara.Key words: Okara: Hydrostatic pressure: Ultraflo ® L enzyme: Fermentation: Microbiota: Prebiotics: Food by-productsIt is generally accepted that non-digestible dietary carbohydratesresistant to digestion in the small intestine -are the main substrates available for fermentation by bacteria in the human colon (1) . When this fermentation is carried out by selective bacteria, causing a beneficial effect on the gut microbiota and consequently on the host, they are considered prebiotics (2)(3)(4) . Many of the beneficial health effects are related to soluble dietary fibre (SDF) and non-digestible oligosaccharides, such as the regulation of metabolic disorders related to obesity and reduction of cancer risk (2,3,5) . The most important healthpromoting bacteria of the gut microbiota are bifidobacteria and lactobacilli. Both are common targets for dietary intervention that improves health (1,(6)(7)(8) . Other bacteria such as streptococci, enterococci, eubacteria and Bacteroides can be classified as potentially beneficial to health or as potentially harmful depending on the species (7) . Healthy bacteria are beneficial to the host through their metabolisms such as SCFA formation (principally acetate, propionate and butyrate), absence of toxin production and synthesis of defensins or vitamins (9)(10)(11) .Typical prebiotics include SDF, inulin-derived fructans (fructo-oligosaccharides; FOS) and galacto-oligosaccharides (GOS) (3,7,9,12) , but nowadays there is great interest in finding novel prebiotics from waste biomass or by-products from food industry (9,(13)(14)(15) . New candidates for prebiotics incl...