Effects of a dietary β-(1,3)(1,6)-D-glucan supplementation on intestinal microbial communities and intestinal ultrastructure of mirror carp (Cyprinus carpio L.)

Aim: To assess the effects of dietary Saccharomyces cerevisiae β-(1,3)(1,6)-D-glucan supplementation (MacroGard(®)) on mirror carp (Cyprinus carpio L.) intestinal microbiota and ultrastructure of the enterocyte apical brush border.

Methods and results: Carp were fed either a control diet or diets supplemented with 0.1, 1 or 2% w/w MacroGard(®). Culture-dependent microbiology revealed that aerobic heterotrophic bacterial levels were unaffected by dietary MacroGard(®) after 2 and 4 weeks. No effects were observed on the allochthonous lactic acid bacteria (LAB) populations at either time point; however, reduced autochthonous LAB populations were observed at week 4. PCR-DGGE confirmed these findings through a reduction in the abundance of autochthonous Lactococcus sp. and Vagococcus sp. in MacroGard(®)--fed fish compared with the control-fed fish. Overall, sequence analysis detected microbiota belonging to the phyla Proteobacteria, Firmicutes, Fusobacteria and unidentified uncultured bacteria. DGGE analyses also revealed that dietary MacroGard(®) reduced the number of observed taxonomical units (OTUs) and the species richness of the allochthonous microbiota after 2 weeks, but not after 4 weeks. In contrast, dietary MacroGard(®) reduced the number of OTUs, the species richness and diversity of the autochthonous microbiota after 2 weeks, and those parameters remained reduced after 4 weeks. Transmission electron microscopy revealed that intestinal microvilli length and density were significantly increased after 4 weeks in fish fed diets supplemented with 1% MacroGard(®).

Conclusions: This study indicates that dietary MacroGard(®) supplementation modulates intestinal microbial communities of mirror carp and influences the morphology of the apical brush border.

Significance and impact of the study: To the authors' knowledge, this is the first study to investigate the effects of β-(1,3)(1,6)-D-glucans on fish gut microbial communities, using culture-independent methods, and the ultrastructure of the apical brush border of the enterocytes in fish. This prebiotic-type effect may help to explain the mechanisms in which β-glucans provide benefits when fed to fish.