The objectives of the present studies were to evaluate: 1) thein vivoimpact of the supplementation with a precision biotic (PB) on the growth performance and microbiome modulation of broiler chickens; 2) the role of PB on the modulation of functional pathways of the microbiome collected from animals with low and high body weight gain, and 3) to develop a Microbiome Protein Metabolism Index (MPMI) derived from gut metagenomic data to link microbial protein metabolism with performance. Thein vivowork consisted of 2 experiments with 2 treatments: Control vs. PB at 1.1 kg/MT of PB with 21 or 14 replicates of 40 birds per replicate, in experiments 1 and 2, respectively. Growth performance was evaluated in both experiments, and from experiment 1, cecal samples from one bird/replicate was collected on d 21 and 42 (n= 21/treatment) to evaluate the microbiome through whole genome sequencing. In theex vivoassay, 6 cecal samples were collected from low body weight (BW) birds (at 10% below average), and 6 samples from high BW birds (at least 10% above average). The samples were incubated in the presence or absence of PB. After incubation, DNA was isolated to develop a functional genomic assay and the supernatant was separated to measure short-chain fatty acid (SCFA) production. The MPMI is the sum of beneficial genes in the pathways related to protein metabolism. In thein vivogrow out experiments, it was observed that the supplementation improved the BW gain by 3% in both studies, and the corrected feed conversion ratio (cFCR) by 3.7 and 3.4% in studies 1 and 2, respectively (P< 0.05). The functional microbiome analysis revealed that the PB shifted the microbiome pathways toward a beneficial increase in protein utilization, as shown by higher MPMI. In theex vivoexperiment, the PB increased the abundance of genes related to the beneficial metabolism of protein (quantitative MPMI), and the concentration of SCFA, regardless of the underline BW of the birds. Taken together, the microbiome metabolic shift observed in thein vivostudy and higher MPMI, plus the observations from theex vivoassay with higher SFCA production, may explain the improvement in growth performance obtained with the supplementation of PB.