In the current study we investigated whether reducing the particle size of wheat bran affects the colonizing microbial community using batch fermentations with cecal inoculum from seven different chickens. We also investigated the effect of in-feed administration of regular wheat bran (1690 μm, WB) and wheat bran with reduced particle size (280 μm, WB280) on the cecal microbial community composition of broilers. During batch fermentation, WB280 was colonized by a lactic acid producing community (Bifidobacteriaceae and Lactobacillaceae) and by Lachnospiraceae, that contain lactic acid consuming, butyric acid producing species. The relative abundance of the Enterobacteriaceae decreased in the particle-associated community for both WB and WB280 compared to the control. In addition, the community attached to wheat bran was enriched in xylan degrading bacteria. When administered as a feed additive to broilers, WB280 significantly increased the richness of the cecal microbiota and the abundance of bacteria containing the butyryl-CoA:acetate CoA-transferase gene, a key gene involved in bacterial butyrate production, while decreasing abundances of Enterobacteriaceae family members in the ceca. Particle size reduction of wheat bran thus resulted in colonization of the bran particles by a very specific lactic acid and butyric acid producing community and can be used to steer towards beneficial microbial shifts. This can potentially increase the resilience against pathogens and increase animal performance, when the reduced particle size wheat bran is administered as feed additive to broilers.ImportancePrebiotic dietary fibres are known to improve gastrointestinal health of both humans and animals in many different ways. They can increase the bulking capacity, improve transit times and, depending on the fibre, even stimulate the growth and activity of resident beneficial bacteria. Wheat bran is a readily available byproduct of flour processing and is a highly concentrated source of (in)soluble dietary fibre. The intake of fibre rich diets has been associated with increased Firmicutes and decreased Proteobacteria numbers. Here we show that applying only 1 % of a relatively simple substrate which was technically modified using relatively simple techniques, reduces the concentration of Enterobacteriaceae This could imply that in future intervention studies one should take the particle size of dietary fibres into account.