Conjugative potential of antibiotic resistance plasmids to activated sludge bacteria from wastewater treatment plants

Wastewater treatment plants (WWTPs) have been considered as hotspots to spread antibiotic resistance genes (ARGs). However, the potential of activated sludge bacteria to disseminating ARGs in WWTPs has not been clearly understood. In this study, we investigated the conjugative potential of bacteria in five activated sludge samples from different WWTPs, by using Escherichia coli as donor of broad-host range IncP-1 plasmid pKJK5. The maximum transfer frequencies were in the range of 1 x 10(-3)-4.3 x 10(-2) per recipient. Conjugative potentials were determined as the ratio of relative abundances in transconjuants to those in recipients. In all transconjugant pools, there were fifteen shared genera, which contained pathogenic bacteria in potential. The dominant shared genera included Aeromonas, Acinetobacter and Enterobacter, accounting for 65%-96% of the total abundance, with the conjugative potential in the range of 0.4-4.9. Gram-positive bacteria, Vagococcus and Kurthia, were also found in the transconjugants. Their relative abundances were in the range of 1.5%-7.2% and conjugative potentials were much higher (ranged from 2.1 to 48), deserving more attention to estimating the risks. Conjugative potential for certain genera were found diverse in different processes, implying possible correlation with process operation. This study can be helpful for ARG risk assessment and control in WWTPs.