Cadmium enhances conjugative plasmid transfer to a fresh water microbial community.

Co-selection of antibiotic resistance genes (ARGs) by heavy metals might facilitate the spread of ARGs in the environments. Cadmium contamination is ubiquitous, while, it remains unknown the extent to which cadmium (Cd) impact plasmid-mediated transfer of ARGs in aquatic bacterial communities. In the present study, we found that Cd amendment at sub-inhibitory concentration significantly increased conjugation frequency of RP4 plasmid from Pseudomonas putida KT2442 to a fresh water microbial community by liquid mating method. Cd treatment (1-100 mg/L) significantly increased the cell membrane permeability and antioxidant activities of conjugation mixtures. Amendments of 10 and 100 mg/L Cd significantly enhanced the mRNA expression levels of mating pair formation gene (trbBp) and the DNA transfer and replication gene (trfAp) due to the repression of regulatory genes (korA, korB and trbA). Phylogenetic analysis of transconjugants indicated that Proteobacteria was the dominant recipients and high concentration of Cd treatment resulted in expanded recipient taxa. This study suggested that sub-inhibitory Cd contamination would facilitate plasmid conjugation and contributed to the maintenance and spread of plasmid associated ARGs, and highlighted the urgent need for effective remediation of Cd in aquatic environments.