| Summary: | <i>Laminaria</i> spp. and their extracts have preventative potential as dietary supplements during weaning in pigs. The first objective of this study was to evaluate increasing concentrations of four whole seaweed biomass samples from two different <i>Laminaria</i> species harvested in two different months in a weaned pig faecal batch fermentation assay. Particularly, February and November whole seaweed biomass samples of <i>L. hyperborea</i> (LHWB-F and LHWB-N) and <i>L. digitata</i> (LDWB-F and LDWB-N) were used. In the next part of the study, the increasing concentrations of four extracts produced from <i>L. hyperborea</i> (LHE1–4) and <i>L. digitata</i> (LDE1–4) were evaluated in individual pure-culture growth assays using a panel of beneficial and pathogenic bacterial strains (second objective). The LHE1–4 and LDE1–4 were obtained using different combinations of temperature, incubation time and volume of solvent within a hydrothermal-assisted extraction methodology (E1–4). In the batch fermentation assay, the <i>L. hyperborea</i> biomass samples, LHWB-F and LHWB-N, lowered <i>Bifidobacterium</i> spp. counts compared to the <i>L. digitata</i> biomass samples, LDWB-F and LDWB-N (<i>p</i> < 0.05). LHWB-F and LDWB-N reduced <i>Enterobacteriaceae</i> counts (<i>p</i> < 0.05). LHWB-F and LDWB-F were selected as the most and least promising sources of antibacterial extracts from which to produce LHE1–4 and LDE1–4. In the pure-culture growth assays, E1- and E4-produced extracts were predominantly associated with antibacterial and bifidogenic activities, respectively. LHE1 reduced both <i>Salmonella</i> Typhimurium and Enterotoxigenic <i>Escherichia coli</i> with LDE1 having a similar effect on both of these pathogenic strains, albeit to a lesser extent (<i>p</i> < 0.05). Both LHE1 and LDE1 reduced <i>B. thermophilum</i> counts (<i>p</i> < 0.05). LDE4 exhibited strong bifidogenic activity (<i>p</i> < 0.05), whereas LHE4 increased <i>Bifidobacterium thermophilum</i> and <i>Lactiplantibacillus plantarum</i> counts (<i>p</i> < 0.05). In conclusion, antibacterial and bifidogenic extracts of <i>Laminaria</i> spp. were identified in vitro with the potential to alleviate gastrointestinal dysbiosis in newly weaned pigs.
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