The prevalence of extra- and intra- cellular antibiotic resistance genes and the relationship with bacterial community in different layers of biofilm in the simulated drinking water pipelines

Biofilms are one of the main habitats for microorganisms and promote the antibiotic resistance spread in the drinking water distribution system (DWDS). Nowadays, the research about antibiotic resistance genes (ARGs) in biofilms of DWDS focuses mainly on intracellular ARGs (iARGs) in the whole biofilm, lacking comprehensive knowledge of extracellular ARGs (eARGs), iARGs, and bacterial community in different biofilm layers. Moreover, the effect of residual chlorine on them is unclear. In this study, two identical biofilm reactors were set up to culture and collect biofilm. The eARGs/iARGs abundance and bacterial community composition were analyzed. We found that the eARGs relative abundance was generally higher than iARGs. The total relative abundance of eARGs in 95 % of biofilm samples was higher than that of iARGs. The PCoA results showed a sample-type difference in the iARGs abundance between the surface biofilm and the inner biofilm. Residual chlorine increased the total abundances of eARGs and iARGs. Furthermore, the bacterial community of the inner biofilm was more abundant than that of the surface biofilm in the chlorinated reactor. We also found that residual chlorine reduced the diversity of the surface biofilm. This study was beneficial for the risk assessment and dissemination control of ARGs.