Bifidobacterium castoris strains isolated from wild mice show evidence of frequent host switching and diverse carbohydrate metabolism potential.

Members of the gut microbiota genus Bifidobacterium are widely distributed human and animal symbionts believed to exert beneficial effects on their hosts. However, in-depth genomic analyses of animal-associated species and strains are somewhat lacking, particularly in wild animal populations. Here, to examine patterns of host specificity and carbohydrate metabolism capacity, we sequenced whole genomes of Bifidobacterium isolated from wild-caught small mammals from two European countries (UK and Lithuania). Members of Bifidobacterium castoris, Bifidobacterium animalis and Bifodobacterium pseudolongum were detected in wild mice (Apodemus sylvaticus, Apodemus agrarius and Apodemus flavicollis), but not voles or shrews. B. castoris constituted the most commonly recovered Bifidobacterium (78% of all isolates), with the majority of strains only detected in a single population, although populations frequently harboured multiple co-circulating strains. Phylogenetic analysis revealed that the mouse-associated B. castoris clades were not specific to a particular location or host species, and their distribution across the host phylogeny was consistent with regular host shifts rather than host-microbe codiversification. Functional analysis, including in vitro growth assays, suggested that mouse-derived B. castoris strains encoded an extensive arsenal of carbohydrate-active enzymes, including putative novel glycosyl hydrolases such as chitosanases, along with genes encoding putative exopolysaccharides, some of which may have been acquired via horizontal gene transfer. Overall, these results provide a rare genome-level analysis of host specificity and genomic capacity among important gut symbionts of wild animals, and reveal that Bifidobacterium has a labile relationship with its host over evolutionary time scales.

Psyllium reduces inulin-induced colonic gas production in IBS: MRI and in vitro fermentation studies.

OBJECTIVE: Health-promoting dietary fibre including inulin often triggers gastrointestinal symptoms in patients with IBS, limiting their intake. Our aim was to test if coadministering psyllium with inulin would reduce gas production. DESIGN: A randomised, four-period, four-treatment, placebo-controlled, crossover trial in 19 patients with IBS. Subjects ingested a 500 mL test drink containing either inulin 20 g, psyllium 20 g, inulin 20 g+ psyllium 20 g or dextrose 20 g (placebo). Breath hydrogen was measured every 30 min with MRI scans hourly for 6 hours. Faecal samples from a subset of the patients with IBS were tested using an in vitro fermentation model. Primary endpoint was colonic gas assessed by MRI. RESULTS: Colonic gas rose steadily from 0 to 6 hours, with inulin causing the greatest rise, median (IQR) AUC(0-360 min) 3145 (848-6502) mL·min. This was significantly reduced with inulin and psyllium coadministration to 618 (62-2345) mL·min (p=0.02), not significantly different from placebo. Colonic volumes AUC(0-360 min) were significantly larger than placebo for both inulin (p=0.002) and inulin and psyllium coadministration (p=0.005). Breath hydrogen rose significantly from 120 min after inulin but not psyllium; coadministration of psyllium with inulin delayed and reduced the maximum increase, AUC(0-360 min) from 7230 (3255-17910) ppm·hour to 1035 (360-4320) ppm·hour, p=0.007.Fermentation in vitro produced more gas with inulin than psyllium. Combining psyllium with inulin did not reduce gas production. CONCLUSIONS: Psyllium reduced inulin-related gas production in patients with IBS but does not directly inhibit fermentation. Whether coadministration with psyllium increases the tolerability of prebiotics in IBS warrants further study. TRIAL REGISTRATION NUMBER: NCT03265002.

Contrasting effects of viscous and particulate fibers on colonic fermentation in vitro and in vivo, and their impact on intestinal water studied by MRI in a randomized trial.

BACKGROUND: Wheat bran, nopal, and psyllium are examples of particulate, viscous and particulate, and viscous fibers, respectively, with laxative properties yet contrasting fermentability. OBJECTIVES: We assessed the fermentability of these fibers in vitro and their effects on intestinal function relevant to laxation in vivo using MRI. METHODS: Each fiber was predigested prior to measuring gas production in vitro during 48-h anaerobic incubation with healthy fecal samples. We performed a randomized, 3-way crossover trial in 14 healthy volunteers who ingested 7.5 g fiber twice on the day prior to study initiation and once with the study test meal. Serial MRI scans obtained after fasting and hourly for 4 h following meal ingestion were used to assess small bowel water content (SBWC), colonic volumes, and T1 of the ascending colon (T1AC) as measures of colonic water. Breath samples for hydrogen analysis were obtained while patients were in the fasted state and every 30 min for 4 h following meal ingestion. RESULTS: In vitro, the onset of gas production was significantly delayed with psyllium (mean ± SD: 14 ± 5 h) compared with wheat bran (6 ± 2 h, P = 0.003) and was associated with a smaller total gas volume (P = 0.01). Prefeeding all 3 fibers for 24 h was associated with an increased fasting T1AC (>75% of values >90th centile of the normal range). There was a further rise during the 4 h after psyllium (0.3 ± 0.3 s P = 0.009), a fall with wheat bran (-0.2 ± 0.2 s; P = 0.02), but no change with nopal (0.0 ± 0.1 s, P = 0.2). SBWC increased for all fibers; nopal stimulated more water than wheat bran [AUC mean (95% CI) difference: 7.1 (0.6, 13.8) L/min, P = 0.03].Breath hydrogen rose significantly after wheat bran and nopal but not after psyllium (P < 0.0001). CONCLUSION: Both viscous and particulate fibers are equally effective at increasing colonic T1 over a period of 24 h. Mechanisms include water trapping in the small bowel by viscous fibers and delivery of substrates to the colonic microbiota by more fermentable particulate fiber. This trial was registered at clinicaltrials.gov as NCT03263065.