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Pathogen Biology

Causal organism

Erwinia stewartii is a facultative anaerobic, Gram-negative, nonflagellate, nonspore-forming, rod-shaped bacterium. Culture medium affects colony color and growth (Figure 14). Pathogenicity of E. stewartii is related to the production of extracellular polysaccharides that contribute to occlusion (plugging) of xylem vessels and symptom development. Pathogenicity and the appearance of water-soaked symptoms also require the hypersensitive response and pathogenicity (hrp) and water-soaking (wts) gene cluster which is substantially homologous with the hrp gene cluster found in Erwinia amylovora, the bacterium that causes fire bight.

Figure 14

Taxonomy

Erwinia stewartii is closely related to other bacteria in the Erwinia herbicola-Enterobacter agglomerans complex. The taxonomy of this pathogen was debated throughout the first half of the 20th century until D.W. Dye proposed Erwinia stewartii in 1963. Prior to this, the bacterium was named: Pseudomonas stewartii, Bacterium stewartii, Aplanobacter stewartii, Bacillus stewartii, Phytomonas stewartii, Xanthomonas stewartii, or Pseudobacterium stewartii. Recently, the nomenclature of the genus Erwinia was modified based on chemotaxonomic and molecular methods, however the taxonomic complexity of this group has not been completely resolved, and a dual system is used presently. The genus Pantoea was proposed for some strains of the Erwinia herbicola-Enterobacter agglomerans complex, including E. stewartii, however separation of this group from other Erwinia species is not fully supported by some methods, such as with 16S RNA sequence analysis. Thus, the pathogen is to be referred to as Pantoea stewartii and Erwinia stewartii.

In comparison to other bacteria in this group, E. stewartii appears to be a relatively homogeneous organism that is adapted to surviving primarily in two hosts, corn and the corn flea beetle, Chaetocnema pulicaria. By occupying these two specialized niches, populations of E. stewartii may have been selected for much greater similarity than non-specialist Erwinias, such as E. herbicola and E. carotovora, which are ubiquitous in nature and much more phenotypically diverse than E. stewartii.