Background Diet acidification, dietary calcium ( Ca ) level, and phytase supplementation are known influences on the microbial community in the digestive tract and on phosphorus ( P ) utilization of broiler chickens. Effects of dietary factors and microbiota on P utilization may be linked because microorganisms produce enzymes that release P from phytate ( InsP 6 ), the main source of P in plant feedstuffs. This study aimed to detect linkages between microbiota and InsP 6 degradation by

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... ng diets (i.e., replacing Ca carbonate (CaCO 3 ) by Ca formate or adding formic acid to CaCO 3 -containing diets), varying Ca levels, and supplementing phytase in a three-factorial design. We investigated i) the microbial community and pH in the digestive tract, ii) prececal ( pc ) P and Ca digestibility, and iii) InsP 6 degradation. Results All factors under investigation influenced digesta pH and the microbiota composition. Predicted functionality and relative abundance of microorganisms indicated that diets influenced the contribution of the microbiota on InsP degradation. Values of InsP 6 degradation and relative abundance of the strains Lactobacillus johnsonii and Lactobacillus reuteri were correlated. Phytase supplementation increased pc InsP 6 disappearance, with differences between Ca levels, and influenced concentrations of lower inositol phosphate isomers in the digestive tract. Formic acid supplementation increased pc InsP 6 degradation to myo -inositol. Replacing CaCO 3 by Ca-formate and the high level of these Ca sources reduced pc InsP 6 disappearance, except when the combination of CaCO 3 +formic acid was used. Supplementing phytase to CaCO 3 +formic acid led to the highest InsP 6 disappearance (52%) in the crop and increased myo -inositol concentration in the ileum digesta. Supplementing phytase leveled the effect of high Ca content on pc InsP 6 disappearance. Conclusions The results point towards a contribution of changing microbial community on InsP 6 degradation in the crop and up to the terminal ileum. This is indicated by relationships between InsP 6 degradation and relative abundance of phosphatase-producing strains. Functional predictions supported influences of microbiota on InsP 6 degradation. The extent of such effects still remains to be clarified. InsP 6 degradation can also be influenced by pH effects caused by dietary concentration and solubility of the Ca sources.