Epidemiology of Danish subsp. in Fish Farms Using Whole Genome Sequencing.

Furunculosis, a serious infection caused by the bacterium subsp. is common in sea-reared rainbow trout production in Denmark. Developing an effective control strategy requires knowledge of the epidemiology, as well as the genomic and virulent variability of the Danish subsp. isolates. To obtain this, the genomes of 101 subsp. , including 99 Danish isolates, one Scottish strain and the type strain NCIMB 1102, were sequenced using the Illumina HiSeq platform. Isolates were assembled, examined for presence of plasmids, virulence and iron acquisition proteins, genomic islands, and antibiotic resistance genes. Single Nucleotide Polymorphisms were aligned and subjected to Bayesian temporal phylogenetic and maximum likelihood tree reconstruction using the published genome of subsp. A449 as reference. Bayesian temporal phylogenetic reconstruction suggests that four major introductions of subsp. into Denmark have occurred. The introductions correlate with the freshwater and subsequent seawater expansion of rainbow trout production. Initial transmission of the bacterium could have been from seawater to freshwater or vice versa, and most minor clades include a mixture of strains from different fresh- and seawater farms. Genomic variation of subsp. mostly appeared to be associated with their plasmids and plasmid encoded virulence factors. Nine subsp. isolates harbored worldwide known antibiotic resistance genes against several antibiotics and there is an indication that 33% of the isolates contained the genomic island AsaGEI1b. These findings not only support the usefulness of whole genome sequencing for genetic studies of homogeneous bacteria in general, but provide novel information about the Danish subsp. population, with implications for vaccine development in efforts to better protect Danish rainbow trout in the future.