Impact of pollutant as selection pressure on conjugative transfer of dioxin-catabolic plasmids harbored by Rhodococcus sp. strain p52

Plasmid-mediated bioaugmentation has potential application in the cleanup of recalcitrant environmental pollutants. In this study, we examined the influence of various contaminants (in different categories or different amounts) as a selection pressure on the transfer of catabolic plasmids within an activated sludge bacteria community bioaugmented with Rhodococcus sp. strain p52 harboring pDF01 and pDF02. The distinguishable genera of transconjugants were isolated under the stresses of phenanthrene, dibenzothiophene, and dibenzo-p-dioxin. The difference in genomic G + C content (5.0 ~ 27.5%) between transconjugants and strain p52 indicated the transfer of the catabolic plasmids crossing phylogenetic boundaries. The specific removal rates in activated sludge reactors for phenanthrene, dibenzothiophene, and dibenzo-p-dioxin were elevated in turn. The three contaminants exerted different degrees of influence on the activated sludge bacteria bearing the catabolic plasmids. The highest proportion of transconjugants was detected in the reactor fed with dibenzo-p-dioxin. Additionally, as dibenzo-p-dioxin from 10 to 80 mg/L was fed into the reactors, the proportion of transconjugants increased. Film mating tests demonstrated that the plasmid transfer frequency varied among recipients under the contaminant stresses of phenanthrene, dibenzothiophene, and dibenzo-p-dioxin. Our study provides a characterization of the recalcitrant contaminants as a selection pressure that can modulate catabolic plasmid transfer during genetic bioaugmentation for the removal of contaminants.