Seasonal incidence of autochthonous antagonistic Roseobacter spp. and Vibrionaceae strains in a turbot larva (Scophthalmus maximus) rearing system.

Bacteria inhibitory to fish larval pathogenic bacteria were isolated from two turbot larva rearing farms over a 1-year period. Samples were taken from the rearing site, e.g., tank walls, water, and feed for larvae, and bacteria with antagonistic activity against Vibrio anguillarum were isolated using a replica plating assay. Approximately 19,000 colonies were replica plated from marine agar plates, and 341 strains were isolated from colonies causing clearing zones in a layer of V. anguillarum. When tested in a well diffusion agar assay, 173 strains retained the antibacterial activity against V. anguillarum and Vibrio splendidus. Biochemical tests identified 132 strains as Roseobacter spp. and 31 as Vibrionaceae strains. Partial sequencing of the 16S rRNA gene of three strains confirmed the identification as Roseobacter gallaeciensis. Roseobacter spp. were especially isolated in the spring and early summer months. Subtyping of the 132 Roseobacter spp. strains by randomly amplified polymorphic DNA with two primers revealed that the strains formed a very homogeneous group. Hence, it appears that the same subtype was present at both fish farms and persisted during the 1-year survey. This indicates either a common, regular source of the subtype or the possibility that a particular subtype has established itself in some areas of the fish farm. Thirty-one antagonists were identified as Vibrio spp., and 18 of these were V. anguillarum but not serotype O1 or O2. Roseobacter spp. strains were, in particular, isolated from the larval tank walls, and it may be possible to establish an antagonistic, beneficial microflora in the rearing environment of turbot larvae and thereby limit survival of pathogenic bacteria.