Pathogens 2021 , 10 , 235
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stimulating trophic interactions with other bacteria resulting in the production of secondary metabolites such as butyric acid [30]. Due to their saccharolytic metabolism, Lactobacil- lus and Bifidobacterium species mainly thrive in the proximal regions of the colon [31]. Therefore, the effects of prebiotic compounds on these bacterial groups are often more difficult to observe in in vivo trials. However, during the current study, the stimulation of Lactobacillus spp. by OFO was also observed in the faecal samples of human subjects with mild hypercholesterolemia during the randomised, single-blind, placebo-controlled, cross-over study. This is a striking observation as in vivo studies are inherently confounded by large variability due to interindividual differences among individuals and a strong influence of external factors such as diet [32]. Moreover, the investigated in vivo samples considered faecal samples that are only to a limited extent representative for the microbial changes in the PC. For Bifidobacterium levels, no significant differences were observed be- tween intervention with OFO and the control test product, although in the population that received OFO during the first intervention period a trend towards increased Bifidobacterium levels was observed upon intervention with OFO. Overall, existing scientific evidence and the preliminary results in this study show the significant stimulatory effects of OFO on lactobacilli in human subjects with mild hypercholesterolemia, which points to the prebiotic potential of OFO. Because it has been reported extensively that lactobacilli and bifidobacteria exert hypocholesterolaemic effects both in both animals and humans [33–36], further in vivo studies are warranted to explore this effect for OFO. Furthermore, as stimu- lationof Lactobacillus spp. by OFO in vivo correlated with the obtained in vitro data, the current study shows that in vitro gut models might be an interesting tool in predicting in vivo response of the microbial community to dietary modulation. The in vitro component of the current study also aimed to assess the prebiotic potential of a novel oat product, i.e., POF, relative to OFO. It was revealed that both exerted a highly similar prebiotic activity. First, both test products resulted in significantly higher levels of acetate and lactate, which correlated with the strongly enhanced levels of Lactobacillaceae and Bifidobacteriacea species [31], indicating the involvement of these bacterial groups in primary substrate degradation upon supplementation of the different oat products. At the OTU level, the main changes were attributed to a significant increase in an OTU related to Bifidobacterium adolescentis . It has been previously reported that fermentation of oat bran stimulates the growth of Bifidobacterium adolescentis in vitro [37], while also fermentation of β -glucans has been associated with this bacterial species [38]. The signif- icant stimulation of Lactobacillaceae upon treatment with the different test products was linked with increased abundance of an OTU related to Pediococcus acidilactici ,whichhas been associated with immune-enhancing effects [39]. Furthermore, both test products enhanced Akkermansiaceae levels in the DC. The only representative of Akkermansiaceae in the gut is the mucin-degrading Akkermansia muciniphila , which has been correlated with several health benefits, such as the inverse relationship between colonisation of Akkerman- sia muciniphila and inflammatory conditions [40]. Additionally, significantly enhanced levels of propionate and butyrate were observed upon administration of both test ingre- dients. Administration of oat products has previously been linked with increased levels of butyrate [41,42]. Indeed, Knudsen et al. [42] reported that addition of oat bran to the diet of pigs increased butyrate concentrations in the luminal environment of the porcine colon. However, administration of β -glucan enriched oat fractions to the porcine diet did not result in butyrate enrichment, indicating that other dietary components (e.g., arabi- noxylans) are responsible for the observed increase in butyrate concentration upon oat supplementation. β -glucans on the other hand have been shown to selectively stimulate propionate levels in the colonic environment [7,43,44]. It was proposed that the hypoc- holesterolaemic effect of oat fibres [12–14] might be associated with this propionogenic response. Indeed, upon its production, propionate is transported to the liver, where it impacts cholesterol and fatty acid synthesis [19,20]. Moreover, next to the reduction of cholesterol levels, health-beneficial effects of propionate include weight management by stimulation of satiety [45], regulation of immune function in adipose tissue [46,47], and
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