The causes of stress on intestinal microbiota of Atlantic salmon, Arctic charr, Atlantic cod and rainbow trout

Abstract

The gastrointestinal (GI) tract of fish reacts to stress, sometimes with severe cell damage to intestinal
enterocytes and modulation of the gut bacterial community. The effect of dominance hierarchy
formation, acute - and handling stress on the intestinal bacterial community have been reported in three
salmonids; Arctic charr (Salvelinus alpinus L.), Atlantic salmon (Salmo salar L.) and rainbow trout
(Oncorhynchus mykiss Walbaum), and Atlantic cod (Gadus morhua L.). In Arctic charr, hierarchy
formation reduced cultivable autochthonous (adherent) bacteria level in proximal intestine of
subordinate fish compared to dominant fish, but this effect was not noticed in the distal intestine (DI).
Furthermore, hierarchy formation modulates the gut microbiota. When Atlantic salmon and rainbow
trout were exposed to acute stress the population level of adherent bacteria decrease in DI with
concomitant increase in faeces, but this effect was not observed in Atlantic cod. The reason for the
effect in Atlantic salmon is probably a peel-off effect of mucus and transporting autochthonous gut
bacteria out of the fish. This will in turn allow allochthonous bacteria (present in the lumen or
associated to digesta) to adhere and colonise the mucus layer. The elimination of the existing beneficial
adherent gut microbiota and the lack of protecting mucus during acute stress might have relevance in
pathogenesis. The present study reveals that stress eliminates certain protecting bacteria in the GI tract
making the gut more prone to pathogen infections.