Pantoea stewartii subsp stewartii is the causal agent of Stewart's vascular wilt and leaf blight in sweet corn and maize. The wilting condition results from vascular occlusion due the production of large amounts of bacterial exopolysaccharide (EPS). How P. stewartii colonizes and disseminates within the plant host is a complex process and still needs to be elucidated. This microorganism is described as non-motile and non-flagellated bacterium. However, the biofilm-type colonization pattern and the preliminary genome sequence of the organism support the idea that P. stewartii experience phases of motility. The aim of this study is to examine the second stage in biofilm formation, namely bacterial surface migration, and its role in the Stewart's wilt disease. Different medium conditions were assayed to evaluate motility. P. stewartii displays two types of surface motility, flagella-dependent motility (swarming) and flagella-independent motility (sliding). Swarming motility in P. stewartii is characterized by the formation of a well defined palisade-ring structure that precedes a directional movement on surface. Genes responsible for flagella-dependent motility were identified by transposon and/or gene deletion mutagenesis. The role of flagella on swarming motility was demonstrated by fliC mutant, which was defective in motility. The fliC mutant was also impaired in xylem dissemination and virulence in sweet corn. The presence of flagella structures was confirmed by transmission electron microscopy. No flagella are visible if the organism is planktonically grown. Other genes involved in swarming motility are related to the synthesis of global transcriptional regulators, EPS and LPS biosynthesis, flagella export, membrane transport, and central metabolism. Swarming motility is regulated by EsaI/EsaR QS in an RcsAB dependent manner. Furthermore, this surface motility is related to virulence factor-type III secretion system.
Finally, P. stewartii exhibited sliding motility, which is characterized by a dendritic-like pattern of colonization on agarose containing semisolid medium. Moreover, flagella and QS system are not involved in sliding motility. These findings challenge the taxonomic classification of P. stewartii as a non-motile, non-flagellated microorganism. Further, these studies emphasize the need to evaluate bacterial processes in the context of an overall developmental program and conditions that more closely reflect a natural setting.
