Effect of heavy metal-sulfamethoxazole complexes on the sulfanilamide resistance gene Sul1

Both heavy metals and antibiotics can influence the environmental behaviour of antibiotic resistance gene (ARGs), yet the effects of heavy metal-antibiotic complexes remain poorly understood. This study investigated the effects of heavy metal-antibiotic co-contamination, particularly heavy metal-sulfamethoxazole (SMX) complexes, on the enrichment of the sulfonamide resistance gene sul1. Four heavy metals (Cu, Zn, Pb and Cd) were examined. Pb inhibited sul1 amplification, whereas Zn stimulated it Cu and Cd stimulated the amplification of sul1 at low concentrations but inhibited it at high concentrations (>1.0 mg/L). SMX also promoted sul1 amplification at 0.0125 mg/L. Notably, SMX complexs with Pb, Zn, Cu, and Cd enhanced sul1 amplification at ratios of 1:1, 1:1, 2:1, and 1:2, respectively, with stronger effects than SMX or metals alone. These findings highlight the important role of heavy metal-SMX complexes in shaping sul1 distribution.