Fates of intracellular and extracellular antibiotic resistance genes during a pilot-scale aerobic granular sludge cultivation process

Antibiotic resistance genes (ARGs) were considered as emerging genetic contaminants, which were threatening public health worldwide. ARGs had two forms of existence: intracellular ARGs (i-ARGs) and extracellular ARGs (e-ARGs), and their emergence and spread patterns were rarely revealed in aerobic granular sludge (AGS) system. The results showed that both i-ARGs and e-ARGs were enriched but had different shift patterns during AGS cultivation process, and e-ARGs had higher growth rate. Most functional bacteria enriched in AGS might play important roles in accumulating of i/e-ARGs, and their oligotypes might be the key reason for the different correlations between taxa and i/e-ARGs. E-ARGs might be transformed from i-ARGs and mainly carried by eintegrons, and three major modules of co-occurrence among e-ARGs were observed. The combined action of environmental factors and bacterial community contributed most (35.43%) to the shift pattern of e-ARGs. This study deciphered the migrations and accumulations of i/e-ARGs during aerobic granulation process, and provided several meaningful comprehensions on i/e-ARGs dissemination during practical AGS application.