Conjugative Transfer Potential of Antibiotic Resistance from Wastewater-derived Coliforms to riverine Escherichia coli.

Abstract

Antibiotic-resistant bacteria (ARB) discharged into rivers from wastewater treatment plants disseminate antibiotic resistance genes (ARGs) through conjugative transfer, suggesting that aquatic environments serve as reservoirs for spreading antibiotic resistance. However, the actual extent of such dissemination of antibiotic resistance is unclear. This study investigated changes in the microbiota and antibiotic resistance profiles of other coliforms during the wastewater treatment process and evaluated whether other antibiotic-resistant coliforms persisting in treated wastewater could potentially transfer antibiotic resistance to Escherichia coli in receiving waters. The treatment process substantially reduced, but did not eliminate, E. coli and other coliforms (most probable number [MPN]/100 mL = 5 for E. coli and 4.3 × 10² for other coliforms), which were subsequently discharged into rivers. The resistance rates of other coliforms tended to increase throughout the treatment process, and the proportion of multidrug-resistant bacteria increased (influent: 4.9%, biologically treated water: 4.3%, effluent: 15.8%). Conjugation experiments using the filter mating method revealed the transfer of ARGs from Klebsiella species to E. coli (conjugation frequency: 10^- 6-10^- 3), suggesting the potential for antibiotic resistance transfer within biofilms. Experiments under simplified riverbed biofilm conditions confirmed the transfer of rifampicin resistance genes introduced into E. coli to antibiotic-resistant other coliforms (conjugation frequency: 9.7 × 10^- 5-5.3 × 10^- 4). The abundance of antibiotic-resistant other coliforms in the biofilm was 5.08 × 10² MPN/mL, which was lower than the observed conjugation frequencies, suggesting that the discharge area of treated wastewater is unlikely to function as a significant hotspot for ARB dissemination.