<i>In vitro</i> model reveals structural and metabolic insights into the porcine cecal microbiota in response to β-mannan exposure

ABSTRACT The gastrointestinal microbiota plays a pivotal role in shaping host physiology and health. By selectively promoting bacteria associated with improved host health, microbiota-directed fibers offer a strategy to enhance the beneficial functions of the microbiota. In this work, we developed a pH-controlled in vitro fermentation system (InVitSim) as a model to evaluate the effects of such a fiber—acetylated galactoglucomannan from Norway spruce—on the composition and functionality of porcine cecal microbial communities. We validated the experimental outcomes by comparing the response of the in vitro model to a previous in vivo feeding trial utilizing the same β-mannan fibers. Long-read sequencing with Oxford Nanopore, metatranscriptomics, and short-chain fatty acid measurements were undertaken to survey microbial community dynamics and functionality. Microbial communities in pigs and InVitSim responded similarly to β-mannan supplementation, with taxa like Prevotella , Catenibacterium, and Faecalibacterium increasing in abundance. Intriguingly, some taxa were observed to be more affected by β-mannan supplementation in InVitSim than in vivo . These taxa included several bacterial species that were not previously known to utilize β-mannan, yet exhibited upregulated genes encoding carbohydrate-active enzymes involved in the degradation of this substrate. IMPORTANCE In this study, we establish a fermenter system able to preserve more than 70% of over 300 distinct microbial taxa identified in the porcine cecal gut. The in vitro model and the functional omic data generated from it enabled us to identify relevant microbial populations that responded to the presence of AcGGM by upregulating β-mannan-specific polysaccharide utilization loci. Our results highlight the value of in vitro approaches as a complementary tool to in vivo trials for learning about the gastrointestinal microbiome's response to dietary interventions on the host level.