Investigations on the gut microbiota of salmonids and the applications of probiotics-based feed additives
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A series of investigations were conducted in order to characterise the GIT microbiota of salmonids and to determine the effect of microbial modulating feed additives on the intestinal microbiota, immunity and growth of salmonids. The first experiment, Chapter three, used PCR-DGGE and 16S rRNA gene sequence analysis of cultivable bacteria were used to investigate the GIT microbiota of brown trout. 16S rRNA gene sequence analysis demonstrated that Citrobacter freundii and Carnobacterium maltaromaticum were the predominant culturable viable bacteria and lactic acid bacteria, respectively in all regions of the GIT. DGGE revealed complex communities with a diverse range of microbes from the Firmicutes and Proteobacteria phyla. The latter chapters focused not only identifying the gut microbiota of salmonids, but also on the ability of probiotics and prebiotics to modulate these communities. In Chapter four, rainbow trout were fed a commercial diet supplemented with P. acidilactici for four weeks. P. acidilactici was detected in the GIT of the probiotic group by multiple methods and P. acidilactici was able to persist for at least 24h at the cessation of probiotic feeding. Histological appraisal on the intestine revealed significantly higher microvilli density in the posterior mucosa and a higher density of goblet cells in the anterior mucosa of the probiotic fed fish. RT-PCR results demonstrated that IL-1β, IL-8 and IgT gene expression were up-regulated in the P. acidilactici fed fish at the end of the study. Whilst mRNA of PCNA, HSP70 and casp-3 were down-regulated in the probiotic group at both sampling points. In Chapter five, the efficacy of dietary administration of P. acidilactici and short chain fructooligosaccharide (scFOS) on Atlantic salmon (Salmo salar L.) was evaluated at 63 and 132 days. Compared to the control group, total bacterial cell counts in all regions of the intestine with exception of the anterior digesta were significantly lower in the synbiotic group at the mid sampling point. PCR-DGGE revealed that species richness, diversity and the number of OTUs were significantly higher in the synbiotic group in the anterior digesta at the mid sampling point. Intestinal microvilli and villi length were increased in the anterior intestine of the synbiotic fed group at the end sampling point. IEL levels were increased in the synbiotic group in the posterior intestine at both sampling points. The expression of immunological genes were significantly up-regulated in the synbiotic fed salmon. In Chapter six, rainbow trout were fed three diets fishmeal (FM), soybean meal (SBM) and PlantMix diets supplemented with or without P. acidilactici for 12 weeks. At both sampling points, with exception of fish fed FM, LAB levels were significantly higher in all probiotic groups compared to the control groups. Serum lysozyme activity was significantly higher in fish fed FM and SBM diets containing P. acidilactici than that of fish fed the control diets. This body of research demonstrates that P. acidilactici can modulate immune response via up-regulation of immune genes as well as modulate IEL and goblet cell levels. Despite these benefits, P. acidilactici had no detrimental effects on growth performance.
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