The aquaculture industry is expanding due to an increasing demand for food with the world’s
population growing. However, aquaculture often experience disease outbreaks generally caused
by opportunistic pathogens, including Vibrio anguillarum. This bacterium is the causative agent
of the disease vibriosis which affects a lot of different fish species. As in the rest of the world, an
increase in antibiotic resistant bacteria is also observed in the aquaculture industry making
chemotherapeutics futile. Bacteriophage therapy offers a great potential in treatment and control
of Vibrio anguillarum in aquaculture, however, more knowledge is needed regarding the phagehost
interaction in order to accomplish success. The current PhD thesis address the phage-host
interactions in Vibrio anguillarum and the potential of phage therapy in treating vibriosis outbreaks
in eggs and larvae of cod and turbot.
Lysogeny is widespread in the Vibrio community which underscore the lysogenic phages influence
on bacterial evolution and functional properties. Highly genetically similar Vibrio phages, termed
H20-like prophages, were isolated across large geographical scales being present both as freeliving
phages and as prophages in V. anguillarum genomes. The H20-like phages’ widespread
presence suggests a mutualistic interaction which selects for co-existence with V. anguillarum.
In aquaculture, especially the larvae and fry are vulnerable to pathogens, and they are not
susceptible to alternatives to antibiotics, e.g. vaccines. The lytic broad-host-range phage KVP40
was applied to cod and turbot larvae to control different V. anguillarum strains. We observed that
phage KVP40 was able to reduce and/or delay the mortality. Further, phage KVP40 was also able
to reduced the mortality imposed by the natural bacterial background present in the larvae,
emphasizing its potential as a therapeutic agent.
In phage therapy the resistance mechanisms after phage exposure and the impact on the host are
not fully understood. A molecular and mechanistic understanding of resistance mechanisms and
virulence properties of the host is essential for developing properly phage therapy. In this PhD
thesis phage-resistant V. anguillarum strains were whole-genome sequenced to explain different
resistance mechanisms, and their virulence properties were tested on cod larvae. The strains
consisted of both non-mutational and mutational resistance mechanisms, probably depending on
phage exposure and environmental conditions, which affected the pathogenicity.