Sammendrag
After hatching, all available mucosal surfaces of fish fry are rapidly colonized by microbes, and this is known to induce host responses that are essential for development and health. To which extent the composition of the fry microbiota is important for these responses, and whether it is possible to steer the colonization process toward a healthy fish microbiome, is poorly understood.
We used germfree yolk sac fry in two studies to investigate the early bacterial colonization of Atlantic salmon. First, we applied a two-factorial experimental design to examine the influence of host genetics and source microbiota in the rearing water on the gut microbiota. A wild and an aquaculture strain of Atlantic salmon was hatch under germfree conditions and exposed to two types of rearing water; either containing communities subjected to either r-selection (presumably containing opportunistic bacteria) or K-selection. We found large interindividual variations in the early stage gut microbiota, and no effect of host genetics. The microbial communities associated with the rearing water, however, had a significant influence on the gut microbiota.
In the second study, we exposed germfree salmon yolk sac fry to five bacterial strains previously isolated from salmon fry, in mono-associations and in combination. The colonization success for the strains in gut and on skin was evaluated by CFU counts. Furthermore, we measured the adhesion strength of the same strains to the fish skin mucosa by atomic force microscopy. The initial colonization success, registered after 3 days of exposure to bacteria, showed no apparent correlation with the adhesion strength. The most efficiently colonizer of the fish was a strain classified as Janthinobacterium lividum. Generally, bacterial counts for mono-associations were very low for the gut, even weeks after mouth opening. However, exposure to all five strains simultaneously, hugely increased the bacterial density in the gut.
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