Intraspecific Variation of Edwardsiella anguillarum From Non-Anguillid Fish From Varied Geographic Origins

Edwardsiella anguillarum is a gram-negative bacterium, synonymous with previously described atypical, fish-pathogenic Edwardsiella tarda. Originally described from eels in 2015, E. anguillarum is an important global fish pathogen, particularly in tilapia. This study describes intraspecific phenotypic and genotypic variability among 17 E. anguillarum isolates from non-anguillid fish hosts and varied geographic origins. Isolates demonstrated similar biochemical characteristics, with slight variation in motility and hydrogen sulfide production. Genomic relatedness among isolates was analysed with repetitive extragenic palindromic sequence-based PCR (rep-PCR) and multilocus sequence analysis (MLSA). Rep-PCR with the ERIC II primer set revealed two distinct genetic clusters, while amplification strategies utilising the GTG₅, BOX or ERIC I&II primer sets yielded more uniform profiles. While rep-PCR deemed the isolates largely clonal, MLSA schemes using reference genes from published Edwardsiella MLSA studies revealed E. anguillarum isolates formed five discrete phylogroups. A unique, ~91.5 kB plasmid was identified in Costa Rican and Colombian isolates, associated with conjugative and transposable elements, plasmid mobilisation, and adhesion; however, no plasmid mediated antibiotic resistance genes were identified. This study provides insight into genetic diversity among E. anguillarum isolates from different hosts and geographic regions, identifying an optimal MLSA scheme from previous reports applicable to E. anguillarum isolates.