Role of extracellular polysaccharide (EPS) slime of plant pathogenic bacteria in protecting cells to reactive oxygen species

Pseudomonas syringae pv. phaseolicola, a phytopathogenic bacterium, seemed very sensitive in planta to the adverse action of reactive oxygen species (ROS) produced by two chemical systems. The disease symptoms in host plants were also suppressed by ROS. Several other plant pathogenic bacteria (P. syringae pv. pisi, Erwinia amylovora, Xanthomonas campestris pv. pelargonii) as well as P. fluorescens were also sensitive in vitro to the inhibiting or killing action of ROS. It was shown that O-2(.-) and H2O2 were produced in our two chemical systems and were involved in the killing action. OH however was not involved in the adverse action on bacteria of the ROS. Superoxide dismutase and catalase were able to reverse the killing action of ROS. When the EPS slime around bacteria was removed by washing and centrifuging the cells, bacteria were more sensitive to ROS. However, when the cells of EPS- mutants were washed and centrifuged, their sensitivity to the killing action of ROS did not change because the lack of slime around the mutant cells. The EPS- Tn5 mutants of P. syringae pv. phaseolicola and the natural EPS- mutant of E. amylovora were more sensitive to ROS than the wild type strains. These results support the idea that the EPS slime protects bacteria from ROS (O-2(.-) and H2O2).