Abstract
Vibrio anguillarum causes high mortality in European sea bass (Dicentrarchus labrax) larviculture and is a hindering factor for successful sustainable aquaculture of this commercially valuable species. Priming of the innate immune system through administration of immunostimulants has become an important approach to control disease outbreaks in marine fish larviculture. This study was conducted to evaluate immunostimulation by Escherichia coli HSP70 (DnaK) in axenic European sea bass larvae in order to protect the larvae against vibriosis. DnaK stimulates the immune response in crustaceans and juvenile fish against bacterial infections. The use of axenic fish larvae allows to study immunostimulation in the absence of an interfering microbial community. At 7 days post-hatching, larvae received a single dose of alginate encapsulated recombinant DnaK. Two non-treated control groups in which animals either received empty alginate microparticles (C1) or no alginante microparticles (C2 and C3) were included in the study. Eighteen hours later, all larvae, except the ones from group C3 (non-infected control) were challenged with V. anguillarum (105 CFU, bath infection). Mortality was daily recorded until 120 h post infection and at 18, 24, and 36 h post infection, larvae were sampled for expression of immune related genes. Results showed that V. anguillarum induced an immune response in axenic sea bass larvae but that the innate immune response was incapable to protect the larvae against deadly septicaemic disease. In addition, we showed that administration of alginate encapsulated DnaK to axenic European sea bass larvae at DAH7 resulted in a significant, DnaK dose dependent, upreglation of immune sensor, regulatory and effector genes. Significant upregulation of cxcr4, cas1 and especially of hep and dic was correlated with significant higher survival rates in V. anguillarum infected larvae. In the future recombinant DnaK might perhaps be used as a novel immunostimulant in sea bass larviculture.
Highlights
The production of farmed fish has increases 12 times in the past three decades at an average annual growth of 8%, making making it the fastest growing food production sector [1]
At 48 h p.i., significant (P < 0.05) mortality occurred in all V. anguillarum infected groups, except for the group receiving the high dose (1.5 μg) of DnaK (Figure 3)
The current study focuses on oral administration of a candidate immunostimulant, being E. coli HSP70, known as DnaK, to induce a protective innate immune response in sea bass larvae against mortality caused by V. anguillarum
Summary
The production of farmed fish has increases 12 times in the past three decades at an average annual growth of 8%, making making it the fastest growing food production sector [1]. In order to further support the growth of output, production of fish juveniles for stocking on growing farms has to be increased. The latter is hampered by high and unpredictable mortality of fish larvae after hatching [2]. Compared to the other cultured fish species, highly sensitive to disease outbreaks. The increased sensitivity of sea bass is related to its strong and prolonged stress-induced cortisol increase and increased biosynthetic capacity of interrenal tissue involving a high turnover in the corticosteroid production and release [3]. Sea bass larvae are highly susceptible to the opportunistic pathogen Vibrio anguillarum [reviewed in [4]]. Many Vibrio strains have siderophore-dependent and/or siderophore-independent ironacquisition mechanisms [reviewed in [8, 9]], which enables them to use iron for growth even if iron-binding, fish proteins such as ferritin and transferrin are present
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