Biofilms in the drinking water distribution system (DWDS) provide shelter for pathogens and antibiotic resistance genes (ARGs). However, how biofilms alter the microbiome and antibiotic resistome in tap water, as well as the precise quantitative evaluation of their health risks, remains unclear. Herein, biofilm reactors supplied with municipal drinking water were operated for 120 days. Metagenomic sequencing identified significant differences in microbial compositions among the biofilms, influent, and effluents. A total of 69-305 ARGs were detected in this DWDS, and ARG abundances increased in the biofilms (0.246-1.576 cpc) and effluents (0.309-0.503 cpc) compared to the influent (0.131 cpc). Metagenomic assembly pinpointed potential pathogenic ARG hosts such as Acinetobacter, Pseudomonas, and Escherichia. The co-occurrence of ARGs and mobile genetic elements indicated potential mobility, which was further supported by transformation assays demonstrating gene transfers at a frequency of 10