Effects of edible algae on caecal microbiomes of ICR mice fed a high-sucrose and low–dietary fibre diet

To clarify the presence of algal-susceptible indigenous bacteria (SIB) in the gut, ICR mice were fed high-sucrose (50% w/w) diets containing either no fibre (NF), 5% brown alga “arame”, Eisenia bicyclis; 5% red alga “tsunomata”, Chondrus ocellatus; 5% (w/w) green alga “hitoegusa”, Monostroma nitidum; or 5% (w/w) cyanobacterium “blue-green alga”, Aphanizomenon flos-aquae for 14 days. Faecal frequency and weight were the highest in mice fed M. nitidum. Plasma cholesterol was the lowest in the mice fed C. ocellatus. The caecal microbiome was examined by 16S rDNA (V4) amplicon sequencing. Principal component analysis of operational taxonomical units (OTUs) revealed that the edible algae altered the microbiome. An increase in abundance levels of OTUs by E. bicyclis (Bacteroides acidifaciens-, Bacteroides intestinalis-, Bifidobacterium pseudolongum-like), C. ocellatus (Bacteroides vulgatus- and Escherichia coli-like), M. nitidum (Faecalibaculum rodentium- and Muribaculum sp.-like), and A. flos-aquae (Muribaculum sp.) was detected. Abundance of Lactobacillus johnsonii was the lowest in mice fed the algal diets. Bacteria that increased in numbers were identified as algal SIBs. SIBs might have different effects on host health depending on the food material consumed. From the algal SIBs, B. pseudolongum, B. vulgatus, F. rodentium, and L. johnsonii were isolated using blood-liver agar and identified with the 16S rDNA BLAST search. Future studies should be focused on isolation of other SIBs.