Changes of resistome, mobilome and potential hosts of antibiotic resistance genes during the transformation of anaerobic digestion from mesophilic to thermophilic

This study aimed to reveal how antibiotic resistance genes (ARGs) and their horizontal and vertical transfer-related items (mobilome and bacterial hosts) respond to the transformation of anaerobic digestion (AD) from mesophilic to thermophilic using the strategy of one-step temperature increase. The resistomes and mobilomes of mesophilic and thermophilic sludge were investigated using metagenome sequencing, and the changes in twenty-four representative ARGs belonging to three categories, class 1 integron and bacterial genera during the transition period were further followed using qPCR and 454-pyrosequencing. After temperature increase, the abundance of the resistome of digested sludge decreased from 125.97 ppm (day 0, mesophilic) to 50.65 ppm (day 57, thermophilic) with the reduction of most ARG types except for the aminoglycoside resistance genes. Thermophilic sludge also had a smaller mobilome, including plasmids, insertion sequences and integrons, than that of mesophilic sludge, suggesting the lower horizontal transfer potential of ARGs under the thermophilic condition. On the other hand, the total abundance of 18 bacterial genera, which were suggested as the possible hosts for 13 ARGs through network analysis, was decreased from 23.27% in mesophilic sludge to 11.92% in thermophilic sludge, indicating less hosts for the vertical expansion of ARGs after the hike of temperature. The results highlighted that the better reduction of resistome abundance by thermophilic AD might be associated with the decrease of both the horizontal and vertical transferabilities of ARGs.