Plant cell wall polysaccharides are important dietary fibres in human diets. They are recalcitrant to digestion in the stomach and small intestine and are fermented by the colonic microbiota. The fermentation of dietary fibres provides ca. 10% of daily caloric intake, and is critical in maintaining colonic health. However, the mechanisms by which dietary fibres are degraded by the colonic microbiota are poorly understood. Arabinoxylans (AXs) are important dietary fibres in cereal grains. The complete degradation of AX into monosaccharides requires the concerted action of at least three types of enzyme: endo-β-1,4-d-xylanase, exo-α-l-arabinofuranosidase and exo-β-1,4-d-xylosidase. Under in vitro fermentation conditions, this study compared the enzymatic metabolism of water-soluble wheat AX between a porcine and a human faecal inoculum. Using both colorimetric substrates and AX solution, cellular localisation of the enzymes was investigated by assaying the enzyme activities before and after cell lysis. Results showed that activities of all the three enzymes were detected on the microbial cell-surface for both inocula. In addition, intracellular exo-acting enzymes were detected in the cell-wall and in the cytoplasm for fermentation with the porcine and human faecal inoculum, respectively. However, intracellular (in the cell-wall and/or cytoplasm) endo-β-1,4-d-xylanase was only detected in the human faecal fermentation. Metagenomic sequencing revealed that the microbial community produced from the porcine faecal inoculum promoted the growth of Gram-negative bacteria whereas Gram-positive bacteria dominated in those fermentations initiated with the human faecal inoculum. The different cell-wall structures between Gram-negative and Gram-positive bacteria explained the different enzyme locations by fermentation using different faecal sources. In addition, fermentation using the porcine faecal inoculum produced higher propionate whereas higher n-butyrate was produced from the human faecal inoculum. This study therefore demonstrated that bacterial communities with different cell wall structures exhibited distinct polysaccharide degradative processes, and favoured the production of different short chain fatty acids.