Effect of β‐1/3,1/6‐glucan upon immune responses and bacteria in the gut of healthy common carp (Cyprinus carpio)

β‐Glucans are frequently included in the diet of healthy common carp Cyprinus carpio as a pre‐emptive measure for combatting disease. In order to study the effect this has on the relationship between the gut bacteria and host immune response, carp were maintained on either a β‐glucan free diet or feed containing 0.1% MacroGard, a β‐1/3, 1/6‐glucan, for up to 7 weeks and analysis of innate immune gene expression and molecular analysis of the gut bacteria was performed. The data reveals feeding of MacroGard to healthy carp does not induce bactericidal innate immune gene expression in the gut but does appear to alter bacterial species richness that did not have a negative effect on overall health. Analysis of innate immune gene expression within the upper midgut revealed that there were significant changes over time in the expression of Interleukin (il)‐1β, inducible nitric oxide synthase (inos), mucin (muc2 ) and C‐reactive protein (crp2). Diet did not affect the number of copies of the bacterial 16s rDNA gene in the gut, used as a as a measure of total bacteria population size. However, PCR‐denaturing gradient gel electrophoresis (DGGE) analysis revealed a shift in bacterial species richness with MacroGard feeding. Bactericidal immune gene expression of crp2, muc2 and il‐1β was weakly correlated with gut bacteria population size indicating a potentially limited role of these genes in interacting with the gut bacteria in healthy carp in order to maintain gut homeostatic conditions. These findings highlight the importance of considering both host immunity and the microbiome together in order to fully elucidate the effect of immunomodulants, such as β‐glucans, upon gut health