36Despite the documented antibiotic-induced disruption of the gut microbiota, the 37 impact of antibiotic intake on strain-level dynamics, evolution of resistance genes, 38 and factors influencing resistance dissemination potential remains poorly understood. 39To address this gap we analyzed public metagenomic datasets from 24 antibiotic 40 treated subjects and controls, combined with an in-depth prospective functional study 41 with two subjects investigating the bacterial community dynamics based on 42 cultivation-dependent and independent methods. We observed that short-term 43 antibiotic treatment shifted and diversified the resistome composition, increased the 44 average copy number of antibiotic resistance genes, and altered the dominant strain 45 genotypes in an individual-specific manner. More than 30% of the resistance genes 46 underwent strong differentiation at the single nucleotide level during antibiotic 47 treatment. We found that the increased potential for horizontal gene transfer, due to 48 antibiotic administration, was ~3-fold stronger in the differentiated resistance genes 49 than the non-differentiated ones. This study highlights how antibiotic treatment has 50 individualized impacts on the resistome and strain level composition, and drives the 51 adaptive evolution of the gut microbiota. 52 53