In vitro and in vivo investigation of the potential of dietary natural bioactives to improve parameters of the gastrointestinal health in weaned pigs and broiler chickens

The main research objective of this thesis was to evaluate the potential of candidate dietary supplements, namely casein hydrolysate (CH) and its 5 kDa retentate (5kDaR), yeast ß-glucan and crude fucoidan- and laminarin-rich seaweed extracts, to promote growth and gastrointestinal health in weaned pigs with a focus on modulating the composition of the gastrointestinal microbiota. The first three experimental chapters (Chapter 2-4) involved an initial assessment of the different dietary candidates for their antibacterial and prebiotic effects using two in vitro GIT microbiota models: 1) a batch fermentation assay inoculated with faeces from weaned pigs, and; 2) a panel of pure culture growth assays with selected beneficial (Lactobacillus plantarum, L. reuteri, Bifidobacterium thermophilum) and pathogenic (Salmonella Typhimurium, enterotoxigenic Escherichia coli) bacterial strains. Chapter 2 focused on CH, 5kDaR and yeast ß-glucan and whether CH could substitute 5kDaR, whereas Chapters 3 and 4 investigated how seaweed species, harvest season and extraction methodologies/conditions influenced the antibacterial and prebiotic potential of the whole biomass samples and extracts of Ascophyllum nodosum and Laminaria spp., respectively. During the in vitro assessment of the dietary candidates, an A. nodosum extract (ANE1) produced using the E1 extraction conditions of a novel hydrothermal-assisted extraction (HAE) methodology exhibited a strong antibacterial activity against S. Typhimurium and ETEC and, thus, was selected for further in vivo investigation. In Chapter 5, a new fucoidan-rich A. nodosum extract (ANE) produced using the above-mentioned extraction protocol, was evaluated at two inclusion levels (high and low) as a dietary intervention to control Salmonella infection in naturally infected weaned pigs and improve parameters of gastrointestinal health. ANE exhibited similar in vitro antibacterial activity to the ANE1. High ANE supplementation reduced Salmonella shedding during the post-weaning period; however, no benefits were observed in Salmonella shedding and intestinal colonisation following movement to the grower houses, regrouping and experimental re-infection with S. Typhimurium. High ANE supplementation led to a more beneficial colonic microbiota, characterised by reduced Enterobacteriaceae counts and increased Bifidobacterium spp. counts, and reduced expression of inflammatory markers that facilitate Salmonella colonisation and persistence. Fucoidan was considered the bioactive component, as it was the major constituent of ANE. The second research objective of this thesis was to evaluate the potential of a laminarin-rich extract as a dietary supplement in broiler chickens based on its previous beneficial effects on growth and gastrointestinal health in pigs, as these two animal species share similarities in terms of gastrointestinal ecosystem and production and intestinal challenges. In Chapter 6, dietary supplementation with 300 ppm laminarin improved performance, increased the caecal Bifidobacterium spp. population and enhanced nutrient absorption, intestinal integrity and protective immune responses indicating improved resistance to intestinal pathogens without compromising growth. Overall, seaweed extracts are promising dietary supplements to enhance performance and gastrointestinal health in broiler chickens and, potentially under challenging conditions in pigs, thus, meriting further exploration. Nevertheless, prior in vitro evaluation and purification of the bioactive components are required to improve their efficacy in in vivo conditions.