Proliferation of Multidrug-Resistant New Delhi Metallo-β-lactamase Genes in Municipal Wastewater Treatment Plants in Northern China

The New Delhi metallo-β-lactamase (NDM-1) increases bacterial resistance to a broad range of antibiotics, and bacteria that produce it can cause infections that are very difficult to treat, thus posing great risks to human health. This paper addresses the occurrence of NDM-1 genes through different processes in wastewater treatment plants (WWTPs). NDM-1 genes prevailed through several treatment units (including disinfection by chlorination) in two WWTPs in northern China. Significant NDM-1 gene levels were present in the effluent discharged from both WWTPs (from 1316 ± 232 to 1431 ± 247 copies/mL, representing from 4.4 to 93.2%, respectively, of influent levels). NDM-1 genes were present at much higher concentrations in dewatered waste sludge that is applied to soils [(4.06 ± 0.98) × 10 7 to (6.21 ± 2.23) × 10 7 copies/g of dry weight], raising the possibility of propagation to indigenous bacteria. This concern was validated by a conjugation experiment with Haihe River sediment not harboring NDM-1 genes at detectable levels, where an NDM-1-positive Achromobacter sp. isolated from a WWTP transferred the NDM-1 gene to an indigenous Comamonas sp. The discharge of NDM-1 genes in the effluent and dewatered waste sludge from WWTPs (even at rates higher than influent values) underscores the need to better understand and mitigate their proliferation and propagation from WWTPs.