Genomic Insights into Edwardsiella ictaluri : Molecular Epidemiology and Antimicrobial Resistance in Striped Catfish ( Pangasianodon hypophthalmus ) Aquaculture in Vietnam.

Edwardsiella ictaluri is responsible for causing bacillary necrosis (BNP) in striped catfish ( Pangasianodon hypophthalmus ) in Vietnam. This study offers a comprehensive genomic characterization of E. ictaluri to enhance understanding of the molecular epidemiology, virulence, and antimicrobial resistance. E. ictaluri isolates were collected from diseased striped catfish in the Mekong Delta. The species was confirmed through PCR. Antimicrobial susceptibility testing was conducted using minimum inhibitory concentrations for commonly used antimicrobials. Thirty representative isolates were selected for whole genome sequencing to delineate their genomic profiles and phylogeny. All strains belonged to ST-26 and exhibited genetic relatedness, differing by a maximum of 90 single nucleotide polymorphisms. Most isolates carried multiple antimicrobial resistance genes, with the tet ( A ) gene present in 63% and floR in 77% of the genomes. The ESBL gene, bla CTX-M-15 , was identified in 30% of the genomes. Three plasmid replicon types were identified: IncA, p0111, and IncQ1. The genomes clustered into two clades based on their virulence gene profile, one group with the T3SS genes and one without. The genetic similarity among Vietnamese isolates suggests that disease spread occurs within the Mekong region, underscoring the importance of source tracking, reservoir identification, and implementation of necessary biosecurity measures to mitigate spread of BNP.