Genome comparison of the pathogens Erwinia amylovora and E. pyrifoliae with the fire blight antagonists E. billingiae and E. tasmaniensis

The genus Erwinia comprises mostly plant-associated pathogenic Enterobacteria with Erwinia amylovora, the causative agent of fire blight, and E. pyrifoliae, causing bacterial shoot blight of pear in Asia. The non-pathogenic species E. tasmaniensis and E. billingiae are epiphytic bacteria and may be applied as antagonists for biocontrol of fire blight. After we (and others) have determined the nucleotide sequences of strains from each of the four Erwinia species, genome comparison indicates large inversions resulting from homologous recombination events. Proteins from genomic sequences reveal a relation of E. tasmaniensis strain Et1/99 to E. amylovora strain Ea1/79 and E. pyrifoliae strain Ep1/96. Pathogenicity of E. amylovora and E. pyrifoliae may have evolved by accumulation of virulence factors connected to type III secretion systems. Others are involved in the production of exopolysaccharides, and the utilization of plant metabolites such as sorbitol and sucrose. Some virulence-associated genes of the pathogenic species are present in E. tasmaniensis but mostly absent in E. billingiae. In particular, the hrp/dsp-gene cluster is present in E. amylovora, E. pyrifoliae and also in E. tasmaniensis, but absent in E. billingiae. The four bacterial species carry the gene cluster for synthesis of capsular EPS, but the exopolysaccharide is not produced by E. tasmaniensis. A levansucrase gene is present in E. amylovora and E. tasmaniensis, but absent in E. pyrifoliae and E. billingiae. Nevertheless, regulatory genes such as rlsA,B,C and orf105, associated with levan synthesis in E. amylovora, are still present in E. pyrifoliae, but all absent in E. billingiae. The requirement of E. amylovora to metabolize sorbitol and sucrose in order to be virulent on host plants is not fulfilled for E. billingiae and E. tasmaniensis. The latter species does not carry a sorbitol-operon and E. billingiae no sucrose operon.