Comparative assessment of Vibrio virulence in marine fish larvae

Abstract

Vibrionaceae infections are a major obstacle for marine larviculture; however, little is known about virulence differences of Vibrio strains. The virulence of Vibrio strains, mostly isolated from vibriosis outbreaks in farmed fish, was tested in larval challenge trials with cod (Gadus morhua), turbot (Scophthalmus maximus) and halibut (Hippoglossus hippoglossus) using a multiwell dish assays with single-egg/larvae cultures. The strains differed significantly in virulence as some caused a high mortality of larva reaching 100% mortality after a few days, while others had no or only marginal effects on survival. Some Vibrio strains were pathogenic in all of the larva species, while some caused disease only in one of the species. Twenty-nine of the Vibrio anguillarum strains increased the mortality of larvae from at least one fish species; however, pathogenicity of the strains differed markedly. Other Vibrio species had no or less pronounced effects on larval mortalities. Iron uptake has been related to V. anguillarum virulence; however, the presence or absence of the plasmid pJM1 encoding anguibactin did not correlate with virulence. The genomes of V. anguillarum were compared (D. Castillo, P.W. D'Alvise, M. Middelboe & L. Gram, unpublished data) and most of the high-virulent strains had acquired virulence genes from other pathogenic Vibrio.

Effect of TDA‐producing Phaeobacter inhibens on the fish pathogen Vibrio anguillarum in non‐axenic algae and copepod systems

“…Rønneseth et al . (2017) found a significant decrease in virulence for several V. anguillarum strains tested in turbot, halibut and cod larvae challenge trials when added at low density (10 4  CFU ml −1 ) as compared to high density (10 6  CFU ml −1 ). Hence, although V. anguillarum was not reduced to below inoculum levels in the presence of P. inhibens , our findings support the use of Phaeobacter spp.…”

“…It is also possible that the V. anguillarum strain 90‐11‐286 is avirulent against A. tonsa . The virulence of strains within the same species can vary immensely as seen for thirty V. anguillarum strains tested in three different hosts (turbot, halibut and cod larvae), causing mortality from 100% to 9.1% with the same infection dose (Rønneseth et al ., 2017). Thus, if other V. anguillarum strains had been tested in the Artemia and A. tonsa systems, the outcome might have been different.…”

“…The two non‐target strains were the closely related V. anguillarum PF7 and PF4 (Table 2). All three strains are highly virulent and cluster closely when compared by 163 concatenated virulence factors and by core genome phylogeny (Castillo et al ., 2017; Rønneseth et al ., 2017). However, the semi‐strain‐specific primers developed provide an effective method for detecting few specific vibrios in non‐axenic systems.…”

“…Strain NB10, used for the adhesion/invasion experiment, was isolated from the Gulf of Bothnia and is pathogenic towards turbot and halibut larvae (Rønneseth et al ., 2017). The strain has been tagged by insertion of plasmid pNQFlaC4‐gfp27 ( cat , gfp ) into the chromosome (Croxatto et al ., 2007) and was provided by Prof. Debra L. Milton (Department of Molecular Biology, Umeå University).…”

“…The strain has been tagged by insertion of plasmid pNQFlaC4‐gfp27 ( cat , gfp ) into the chromosome (Croxatto et al ., 2007) and was provided by Prof. Debra L. Milton (Department of Molecular Biology, Umeå University). The highly virulent V. anguillarum strain 90‐11‐286 (Rønneseth et al ., 2017) was used in the co‐culture experiments and was isolated from diseased rainbow trout ( Oncorhynchus mykiss ) from a Danish aquaculture (Pedersen and Larsen, 1993; Skov et al ., 1995). P. inhibens DSM 17395 was used for the co‐culture experiments (Martens et al ., 2006; Vandecandelaere et al ., 2008; Buddruhs et al ., 2013).…”