Abstract
Vibriosis, a hemorrhagic septicemic disease caused by the bacterium Vibrio anguillarum, is an important bacterial infection in Danish sea-reared rainbow trout. Despite of vaccination, outbreaks still occur, likely because the vaccine is based on V. anguillarum strains from abroad/other hosts than rainbow trout. Information about the genetic diversity of V. anguillarum specifically in Danish rainbow trout, is required to investigate this claim. Consequently, the aim of the present investigation was to sequence and to characterize a collection of 44 V. anguillarum strains obtained primarily from vibriosis outbreaks in Danish rainbow trout. The strains were sequenced, de novo assembled, and the genomes examined for the presence of plasmids, virulence, and acquired antibiotic resistance genes. To investigate the phylogeny, single nucleotide polymorphisms were identified, and the pan-genome was calculated. All strains carried tet(34) encoding tetracycline resistance, and 36 strains also contained qnrVC6 for increased fluoroquinolone/quinolone resistance. But interestingly, all strains were phenotypic sensitive to both oxytetracycline and oxolinic acid. Almost all serotype O1 strains contained a pJM1-like plasmid and nine serotype O2A strains carried the plasmid p15. The distribution of virulence genes was rather similar across the strains, although evident variance among serotypes was observed. Most significant, almost all serotype O2 and O3 strains, as well as the serotype O1 strain without a pJM1-like plasmid, carried genes encoding piscibactin biosynthesis. Hence supporting the hypothesis, that piscibactin plays a crucial role in virulence for pathogenic strains lacking the anguibactin system. The phylogenetic analysis and pan-genome calculations revealed great diversity within V. anguillarum. Serotype O1 strains were in general very similar, whereas considerable variation was found among serotype O2A strains. The great diversity within the V. anguillarum serotype O2A genomes is most likely the reason why vaccines provide good protection from some strains, but not from others. Hopefully, the new genomic data and knowledge provided in this study might help develop an optimized vaccine against V. anguillarum in the future to reduce the use of antibiotics, minimize economic losses and improve the welfare of the fish.
Highlights
Vibrio (Listonella) anguillarum is a pathogenic bacterium that causes vibriosis, a hemorrhagic septicemic disease, in many species of fish and shellfish (Hickey & Lee, 2017; Larsen, 1990)
The green cubes represent plasmid content; the purple cubes represent content of acquired antibiotic resistance genes and the blue cubes represent content of the putative virulence factors, which were not conserved amongst all the strains
The genome sequences of the 44 strains analyzed in the present study more than doubles the number of publicly available unique V. anguillarum genome sequences
Summary
Vibrio (Listonella) anguillarum is a pathogenic bacterium that causes vibriosis, a hemorrhagic septicemic disease, in many species of fish and shellfish (Hickey & Lee, 2017; Larsen, 1990). Rainbow trout (Oncorhynchus mykiss) comprise 95% of fish production in Danish aquaculture and are reared at both freshwater and marine farms (Danish Fisheries Agency, 2018). V. anguillarum is one of the most important bacterial pathogens in Danish marine farms and has, in recent years, been a problem for freshwater farms (Pedersen et al, 2008). In Danish rainbow trout farming, it is primarily the O1 and O2A serotypes that cause disease (Pedersen, 1999), so fish are vaccinated against these serotypes (Alphaject 3000; Pharmaq, Norway) prior to transfer from freshwater farms to marine net cages. The vaccine does not seem to provide sufficient protection against vibriosis in rainbow trout in Danish fish farms. Further and more detailed information about the specific genetic diversity in Danish V. anguillarum strains from rainbow trout is required to confirm this reason
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