Different Formulations and Concentrations of Sweet Chestnut Tannins Differentially Modulate Gut Microbiota in Broiler Chickens

Sweet chestnut tannins (SCT) are used as phytogenic feed additives in poultry, but the effects of different formulations and concentrations on intestinal microbiota remain poorly understood. This study investigated powdered, soy protein-bound, microencapsulated and orthophosphoric acid-bound SCT at different dietary concentrations in sixty Ross 308 broilers, with six birds per group, including an unsupplemented control. Ileal and caecal microbiota were characterized using 16S rRNA gene sequencing. In the ileum, richness, phylogenetic diversity and beta diversity differed among treatments, whereas Shannon diversity and evenness remained unchanged. Lactobacillaceae remained dominant, and no significant family- or genus-level differences relative to the control remained after multiple-testing correction. In the caecum, all assessed alpha- and beta-diversity metrics differed among treatments, with numerous treatment-specific taxonomic changes. Several treatments increased the Firmicutes/Bacteroidota ratio, while Muribaculaceae decreased across most supplemented groups. Soy protein-bound SCT at 1000 mg/kg increased Parabacteroides and Odoribacter, whereas microencapsulated SCT at 1000 mg/kg increased Romboutsia and Peptostreptococcaceae. The genus Fournierella increased with soy protein-bound SCT at 2000 mg/kg. Responses were not consistently dose-dependent. These findings indicate that SCT-associated microbial changes vary with intestinal compartment, formulation and concentration, with broader responses detected in the caecum. Their implications for bird health and performance require further investigation.