Degradation characteristics of intracellular and extracellular ARGs during aerobic composting of swine manure under enrofloxacin stress

Abstract

Composting has been widely recognized as an effective method for removing antibiotic resistance genes (ARGs) from livestock manure. This study investigated the variation in intracellular ARGs (iARGs) and extracellular ARGs (eARGs) during swine manure composting under low (31.44 ± 1.11 mg/kg DM) and high (176.76 ± 1.90 mg/kg DM) enrofloxacin concentration stress. After 25 days of composting, the rates of total iARG and eARG degradation reached 67.30% ∼ 96.13% and 54.70% ∼ 80.30%, respectively. The fluoroquinolone and multidrug ARGs that were degraded included intracellular and extracellular aac(6′)-ib-cr, qnrS, qnrD, parC, oqxB, acrA, acrB and tolC, but i-parE and e-parE were not eliminated. The main reason for degradation were the alteration of bacterial communities and mobile genetic elements (MGEs). The total absolute abundances of iARGs were higher than those of eARGs during composting. iARG degradation was driven by the death and succession of potential hosts, primarily including gram-negative bacteria. Composting time advancement played a greater role in facilitating the microbial succession. High temperatures promoted bacterial death, which caused the release of iARGs and generation of eARGs, but continuous high temperature led to more eARG degradation. The dynamics of longer-term eARGs were more strongly affected by extracellular MGEs (eMGEs), and it is more likely that eARGs enter the environment synchronously with eMGEs and persist in the environment at lower abundances via eMGEs. Once eARGs re-enter bacteria, they may become more mobile and dangerous. This study provides insights for facilitating the removal of ARGs in fertilizer utilization.