The 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 spread 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 three contaminants exerted different degrees of influence on the activated sludge bacteria bearing the catabolic plasmids. The relatively high ratios of transconjugant-bearing catabolic plasmids were detected in the reactor fed with dibenzo-p-dioxin. As dibenzo-p-dioxin from 10 to 80 mg/L was fed into the reactors, the ratios of transconjugant-bearing catabolic plasmids increased. Additionally, levels of ROS and extracellular LDH of activated sludge bacteria in the contaminants-fed reactors increased, comparing with that in the control reactor, indicating that the contaminants exerted toxicity which promoted the cell membrane permeability of the activated sludge bacteria. 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.