Development of immersion vaccine for bacterial cold-water disease in ayu <i>Plecoglossus altivelis</i>

<p><i>Flavobacterium psychrophilum</i> (<i>F. psychrophilum</i>) is the causative agent of bacterial cold-water disease (BCWD) that occurs in ayu <i>Plecoglossus altivelis</i>. Formalin-killed cell of <i>F. psychrophilum</i> has long been studied as an immersion vaccine for BCWD. In this study, we explored the possibility of <i>F. psychrophilum</i> collagenase (fpcol) for use as the immersion vaccine. BCWD convalescent ayu sera contained specific IgM antibodies against somatic <i>F. psychrophilum</i> and fpcol, meaning that fpcol is a promising antigen for the vaccine development. The recombinant fpcol was successfully expressed in <i>Escherichia coli</i> and <i>Brevibacillus chosinensis</i> (<i>B. chosinensis</i>). The culture supernatant of the <i>B. chosinensis</i> was used as an immersion vaccine solution. The vaccinated ayu were then challenged by soaking into <i>F. psychrophilum</i> culture. In two experimental groups, the relative percentages of survivals were 63 and 38%, respectively, suggesting that fpcol is promising as the immersion vaccine for ayu-BCWD.</p>