Different fates between extracellular and intracellular antimicrobial resistome in full-scale activated sludge and membrane bioreactor processes

Treated effluent of wastewater treatment plants (WWTPs) are major sources of extracellular antimicrobial resistance genes (eARGs) into aquatic environments. This study aimed to clarify the fate and origins of eARGs from influent to treated effluent at a full-scale WWTP. The compositions of eARG and intracellular ARG (iARG) were acquired via shotgun metagenomic sequencing in influent wastewater, activated sludge, and treated effluent of the target WWTP, where identical wastewater was treated by conventional activated sludge (CAS) and membrane bioreactor (MBR) processes. The proportion of eARGs to iARGs increased from influent to effluent in both processes, reaching almost half of the total ARG. Most eARGs in influent were associated with clinically important antimicrobials, whereas eARGs in sludge and effluent were dominated by aminoglycoside resistance genes of aadA and APH variants. Although the eARGs composition in influent wastewater mirrored that of iARGs, a substantial shift occurred in activated sludge and effluent, highlighting the presence of distinct dissemination and reduction mechanisms between eARGs and iARGs. Notably, the origin of eARGs in treated effluent was mainly iARGs in the effluent rather than the carryover of eARG from activated sludge, which were substantially reduced in MBR, compared to CAS. Consequently, these differences in selective mechanisms led to different fates between eARG and iARG during wastewater treatment.