Transmission mechanisms of antibiotic resistance genes in arsenic-contaminated soil under sulfamethoxazole stress

Numerous studies have revealed the spread mechanism of antibiotic resistance genes (ARGs) in single antibiotic-contaminated soils. However, the comprehensive impacts of heavy metals and antibiotics on ARGs and the underlying mechanisms are still unknown. Here, high-throughput quantitative PCR and high-throughput sequencing were used to investigate changes in ARGs and bacterial communities under various sulfamethoxazole (SMX) regimes (0, 1, 10, 50 mg kg-1) in arsenic (As) contaminated soils. The study found that the abundances of ARGs, mobile genetic elements (MGEs), and heavy metal resistance genes (HMRGs) significantly increased in the soil fortified at 10 and 50 mg kg-1 SMX concentrations. The ARGs abundance increased with the increase in the MGEs abundance. Many significant positive correlations between various ARGs subtypes and HMRGs subtypes were found. These results indicate that the HMRGs and MGEs positively contributed to the enrichment of ARGs in As-contaminated soils under SMX stress. Meanwhile, the abundance of copiotrophic (Actinobacteriota) reduced and oligotrophic (Gemmatimonadota) increased, indicating that the life history strategy of the community changed. In addition, Gemmatimonadota was positively correlated to ARGs, HMRGs, and MGEs, suggesting that Gemmatimonadota, which can cope with As and SMX stress, was the host for resistance genes in the soil. Finally, the study found that MGEs play a determinant role in ARGs proliferation due to the direct utilization of HGT, and the indirect effect for ARGs spread under a co-selection mechanism of ARGs and HMRGs, while the bacterial community showed indirect influences by altering environmental factors to act on MGEs. Collectively, this study revealed new insights into the mechanisms of resistance gene transmission under combined SMX and As contamination in soil ecosystems.