Conjugal transfer at natural population densities in a microcosm simulating an estuarine environment

Estuarine microcosms were used to follow conjugal transfer of a broad host range IncP1 plasmid from a Pseudomonas putida donor to indigenous bacteria. Donor cells were added at a concentration similar to the natural abundance of bacteria in the water column (10⁶ cells ml^-1). Transfer was not detected in any of the test microcosms (calculated limit of detection of 10^-7 and 10^-4 transconjugants donor^-1 in water column and sediment, respectively), with the exception of transfer to an isogenic recipient (added at 10⁵ cells ml^-1) in sediments of controls that had been inoculated with both donors and recipients. The same plasmid was transferred with high efficiencies (10^-1 to 10^-3) to a variety of recipients in filter and broth matings. These results suggest that if conjugal gene transfer occurred, it was at efficiencies that were not detectable in estuarine microcosms simulating natural population densities.