The influence of beta-glucans on the relationship between carp (Cyprinus carpio) and its associated microbiome

Fish are in contact with microbiota from the moment of hatching. Exterior organs, i.e. skin, gills and intestinal system, are colonised by commensal bacteria populations and a symbiotic relationship is formed. The fish provides a niche and nutrients for the bacteria which stimulate development of the immune response, act as an additional barrier against invading pathogens and, within the gut, aid in digestion. β-glucans are used within aquaculture as a means of improving fish health and can be applied in various forms, e.g. via diet or injection. Whilst the application of β glucan is performed to modulate a fish’s immune system, it has also been shown to affect the gut microbiota population at concentrations above 1% w/w within the diet which is particularly important to consider when applied orally. The effect of the commercially available β-glucan MacroGard® upon the gut of common carp (Cyprinus carpio) is studied after oral application and injection. Whilst feeding with MacroGard® at 0.1% w/w within the diet does not influence the gut bacteria nor expression of bactericidal innate immune genes, injection (2mg kg-1 and 5mg kg-1) resulted in a 90% reduction in bacteria numbers in the gut after 24 hours. Injection of MacroGard® did not significantly alter the expression of CRP, iNOS, bf/C2, IL-1β, ApoA1, HAMP1, LEAP2 and Muc2 within the gut however MSS1, a synthesised β-glucan, significantly increased the gene expression of iNOS, CRP and Muc2. 0.1% MacroGard® in the diet was, however, capable of influencing bacterial species diversity when injection was also performed. This revealed a high proportion of Alphaproteobacteria, which are typically associated with plants rather than gut systems, and corresponded with a reduction in potential pathogenic bacteria. This showed combining injection and oral application of MacroGard® together is capable of influencing the gut microbiota population within a 2 week period