Comparative genomic analysis of and from diseased striped catfish fingerlings cultured in Vietnam.

Introduction: Motile septicemia (MAS) is a burden for striped catfish () farmers in Vietnam. MAS can be caused by several species of but is seen as the leading cause of MAS in aquaculture, but recent reports suggest that is also causing MAS.

Methods: Here we investigated the bacterial etiology of MAS and compared the genomic features of and . We collected 86 isolates from diseased striped catfish fingerlings over 5 years from eight provinces in Vietnam. Species identification was done using PCR, MALDI-TOF and whole genome sequence (WGS). The MICs of commonly used antimicrobials was established. Thirty presumed isolates were sequenced for species confirmation and genomic comparison. A phylogenetic analysis was conducted using publicly available sequences and sequences from this study.

Results: A total of 25/30 isolates were sequence type (ST) 656 and 5/30 isolates were ST 251. Our isolates and all publicly available isolates from Vietnam belonged to ST 251 and differed with <200 single nucleotide polymorphisms (SNP). Similarly, all isolates from Vietnam belonged to ST 656 and differed with <100 SNPs. The gene was found in 1/5 and 19/25 . All had an MIC ≤2 mg/L while 19/25 had MIC ≥8 mg/L for oxytetracycline. The gene was only found in (14/25) which showed a MIC ≥8 mg/L for florfenicol. Key virulence genes, i.e., /, and were present in all genomes, while was only present in .

Discussion: This study confirms previous findings where was the dominating pathogen causing MAS and that the importance of has likely been overestimated. The differences in antimicrobial susceptibility between the two species could indicate a need for targeted antimicrobial treatment plans. The lipopolysaccharide regions and outer membrane proteins did not significantly differ in their immunogenic potentials, but it remains to be determined with experiments whether there is a difference in the efficacy of available vaccines against and .