Plasmid conjugation in an activated sludge microbial community
Horizontal genetic transfer (HGT) events are thought to play an important role in the development of microbial species capable of degrading toxic contaminants. The goal of the present study was to determine the effect of relative cell density between the donor and recipient cells for HGT conjugation events in a mixed microbial community. Specifically, the transfer of the TOL plasmid pWWO from Pseudomonas putida BBC443 to micro-organisms present in the activated sludge obtained from a wastewater treatment plant in Durham, NC was investigated. Inocula with cell concentrations ranging from 1:1 to 20:1 (activated sludge to P. putida BBC443) were investigated. The TOL plasmid pWWO contains a gene encoding a green fluorescent protein (gfp), which is only expressed following a conjugation event. Thus, fluorescence measurements can be used to estimate plasmid proliferation. HGT rates were monitored by flow cytometry while toluene degradation profiles were obtained by gas chromatography. Conjugation rates ranged from 0 to 14% and peaked after 3 days (p<0.05). An increase in P. putida BBC443 cells in the inoculum did not result in an increase in conjugation rate. However, a higher initial concentration of activated sludge cells was associated with an increase in conjugation rate. The 20:1 (activated sludge to P. putida BBC443) ratio was associated with the highest number of conjugation events comparing all inoculation treatments. It is possible that the higher conjugation rate is linked to either the formation of bacterial aggregates under certain inoculum conditions or the recipient cell's genetic makeup. Toluene degradation rates were found to be similar for all inocula regardless of fluorescence level variations. This result may be linked to the high number of toluene degraders originally present in the activated sludge inoculum. More research is needed to identify all the parameters that control conjugation rates. © 2009 Mary Ann Liebert, Inc.
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