EFFECTS OF SYNBIOTIC DIETS ON RE-ESTABLISHMENT OF THE ATLANTIC SALMON GUT MICROBIOTA AFTER DYSBIOSIS TRIGGERED BY MEDICATED FEED

Introduction

Salmon aquaculture industry has rapidly expanded over the last decades with Norway and Chile being the main contributors. With this rapid growth, it  has encountered alarms over environmental impacts and sustainability.  Among  the sustainability issues are  factors affecting fish welfare and health including sea lice and disease outbreaks leading to  chemical and  antibiotic usage in some regions of global salmon farming. A ntibiotic usage could  lead to  issues and concerns over the development of antibiotic resistance and undesirable effects on the host.  Several studies  have reported the presence of antibiotic resistant bacteria and/or  antibiotic  resistant genes in intestine of medicated fish  and healthy fish, farm ing  and non-farming sites  indicting the significance of the  antibiotic resistance  related to salmon  aquaculture industry. Antibiotics can trigger perturbations in host gut microbiome resulting in dysbiosis, leading to detrimental health effects. Even though, a few studies  have reported the p erturbations in gut microbiota  of Atlantic salmon caused by some antibiotics, in depth understanding of antibiotic-induced dysbiosis and related health and physiological effects are lacking.

Synbotic , a mixture of probiotic and prebiotic agents, exerts  beneficial affects to the host by increasing the survival  and activity of probiotics  and  indigenous health promoting bacteria in the gut. Several previous  studies described  the  beneficial effects of synbiotics on salmonoids . Mammalian studies showed concomitant use of probiotics , prebiotics  and synbiotics with antibiotics  could re-establish gut  microbiota and  prevent antibiotic resistance and  antibiotic associated gastrointestinal disorders. However, the use of  pre- pro- or synbiotics to assess similar  effects in fish are scanty.

 The present study attempted to  address  those research gaps by evaluating the effects of synbiotic diets on re-establishment of the Atlantic salmon gut microbiota after dysbiosis triggered by medicated feeds.

Materials and methods

Systematic representation of experimental design , experimental procedure and expected outcome  is illustrate d in the Figure 1.  Post-smolt Atlantic salmon were fed with a control commercial diet and a synbiotic diet in which  the  commercial diet was supplemented with  0.04% Pediococcus acidilactici and 0.1% scFOS , for a period of five weeks. Each of the  feed group had five pens. After the acclimatization period, fish in the three out of five pens in each of the groups were fed with medicated feeds containing either control diet + florfenicol (3.5ppm) or synbiotic diet + florfenicol (3.5ppm) for a period of two weeks. Then those fish were re- fed with their respective non-medicated diets i.e control or synbiotic diets, for another six weeks. The fish in the remaining two pens from each group were continuously fed with control or synbiotic diets until the end of the feeding trial.

Fish used for microbiota analysis were pit-tagged and d igesta samples  were collected at the  start  and end of the medicated feeding and end of the re-feeding period.  Microbiota analysis was performed with 16S rRNA  sequencing  and subsequent bioinformatics  analysis  were carried out using QIIME2. Growth performance,  histomorphology  and transcriptomic changes were also evaluated .

 Results and Discussion

 Treatment with  antibiotics negatively affected the thermal growth coefficient  in both  the  control and synbiotic diet fed groups. However ,  the reduction in growth wa s less in  the synbiotic group compared to the control group.  Analysis of  microbiota  data is  currently in progress and will be presented in the conference .  We are expected to see  that a ntibiotic treatment related perturbations in digesta-associated microbiota in both  the groups.  Moreover,  we hypothesized that t he  fish fed  with synbiotic fee d  would  indicate  rapid re-establishment of intestinal microbiota within a period of six weeks or earlier.  Further, it was al so hypothesized that t his rapid maintenance of intestinal microbiota may positively impact in  the  reduction of the growth loss observed in those fishes.  The  knowledge generated in this study could be further explored to  improve antibiotic related dysbiosis in  gut microbiota  and related physiological impacts in Atlantic salmon aquaculture industry.