The transfer dynamics of Pseudomonas sp. plasmid pQBR11 in biofilms

Plasmid pQBR11 (294 kb) is one of a group of genetically similar conjugative plasmids that are common to and persist in pseudomonad populations colonising leaves and roots of crops and wild plants. This plasmid was marked by insertion of a green fluorescent protein reporter cassette to facilitate epifluorescence microscopy tracking and enumeration of transconjugant formation in biofilms. The intrinsic transfer activity of the plasmid, the conditions which affect this activity and the patterns of infectious spread of the plasmid in simple biofilms were evaluated. Transfer was observed over a range of nutritional and other conditions but no transitory de-repression of plasmid transfer was observed from new transconjugants and re-transfer of the plasmid by transconjugant cells played only a secondary role in the establishment of the plasmid in biofilm populations. As the plasmid was poorly invasive the parasitic persistence of pQBR11 by transfer alone is highly unlikely. The key factors for establishment of this plasmid in these biofilm populations were the size and activity of the donor population and the effect of plasmid carriage on host fitness.