Relative fitness in vitro and in planta of Pseudomonas syringae strains containing copper and streptomycin resistance plasmids

The effect of resistance plasmids encoding copper resistance (Cu r ), streptomycin resistance (Sm r ), and both Cu r and Sm r on competitive fitness of Pseudomonas syringae pv. syringae was studied in vitro and in planta. The Cu r Sm r plasmid pPSR1 provided a selective advantage to its bacterial host (Pseudomonas syringae pv. syringae FF5.1) on Pyrus calleryana leaves that were treated weekly with copper and (or) streptomycin bactericides. However, populations of the plasmid-free Cu s Sm s FF5.1 were reduced 10- to 1000-fold over a 12-week period on trees treated with bactericides. The resistance plasmids pPSR4 (Cu r ), pPSR5 (Sm r ), and pPSR4::Tn 5393 (Cu r Sm r ) were highly stable for over 200 generations of growth in glucose-limited batch culture. Results of competition experiments in vitro indicated that Pseudomonas syringae pv. syringae FF5 containing pPSR4, pPSR5, or pPSR4::Tn5393 was reduced to less than 5% of the total culture in competition with wild-type FF5. In growth chamber studies, the resistance plasmids studied did not have an impact on epiphytic fitness of Pseudomonas syringae pv. syringae. Our data suggest that resistance plasmids will persist in populations of Pseudomonas syringae pv. syringae following their initial selection regardless of the bactericidal spray regime.