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Edwardsiella  bacteria are leading fish pathogens causing huge losses to aquaculture industries worldwide.  E. tarda  is a broad-host range pathogen that infects more than 20 species of fish and other animals including humans while  E. ictaluri  is host-adapted to channel catfish causing enteric septicemia of catfish (ESC). Thus, these two species consist of a useful comparative system for studying the intricacies of pathogen evolution. Here we present for the first time the phylogenomic comparisons of 8 genomes of  E. tarda  and  E. ictaluri  isolates. Genome-based phylogenetic analysis revealed that  E. tarda  could be separate into two kinds of genotypes (genotype I, EdwGI and genotype II, EdwGII) based on the sequence similarity.  E. tarda  strains of EdwGI were clustered together with the  E. ictaluri  lineage and showed low sequence conservation to  E. tarda  strains of EdwGII. Multilocus sequence analysis (MLSA) of 48 distinct  Edwardsiella  strains also supports the new taxonomic relationship of the lineages. We identified the type III and VI secretion systems (T3SS and T6SS) as well as iron scavenging related genes that fulfilled the criteria of a key evolutionary factor likely facilitating the virulence evolution and adaptation to a broad range of hosts in EdwGI  E. tarda . The surface structure-related genes may underlie the adaptive evolution of  E. ictaluri  in the host specification processes. Virulence and competition assays of the null mutants of the representative genes experimentally confirmed their contributive roles in the evolution/niche adaptive processes. We also reconstructed the hypothetical evolutionary pathway to highlight the virulence evolution and niche adaptation mechanisms of  Edwardsiella . This study may facilitate the development of diagnostics, vaccines, and therapeutics for this under-studied pathogen.

Edwardsiella Comparative Phylogenomics Reveal the New Intra/Inter-Species Taxonomic Relationships, Virulence Evolution and Niche Adaptation Mechanisms