Shift in antibiotic resistance gene profiles associated with nanosilver during wastewater treatment.

This study investigated the response of antibiotic resistance genes (ARGs) to nanosilver (Ag) in lab-scale nitrifying sequencing batch reactors (SBRs), compared to Ag(+)-dosed and undosed controls. Quantitative polymerase chain reaction (q-PCR) targeting sul1, tet(O), ermB and the class I integron gene intI1 and corresponding RNA expression did not indicate measureable effects of nanoAg or Ag(+) on abundance or expression of these genes. However, high-throughput sequencing based metagenomic analysis provided a much broader profile of gene responses and revealed a greater abundance of aminoglycoside resistance genes (mainly strA) in reactors dosed with nanoAg. In contrast, bacitracin and macrolide-lincosamide-streptogramin (MLS) resistance genes were more abundant in the SBRs dosed with Ag(+). The distinct ARG profiles associated with nanoAg and Ag(+) were correlated with the taxonomic composition of the microbial communities. This study indicates that nanoAg may interact with bacteria differently from Ag(+) during biological wastewater treatment. Therefore, it cannot necessarily be assumed that nanosilver behaves identically as Ag(+) when conducting a risk assessment for release into the environment.