e year old and maintained on pelleted food for rabbit (RM-4, Funabashi Farm, Funabashi, Japan) supplemented with fresh Japanese mustard spinach (*Brassica rapa* var. *perviridis*) and Blueberries (*Vaccinium* spp.) purchased from the local market. All birds were in normal condition, but YL5 experienced chronic diarrhea of unknown causes and treated with a probiotic (Miyarisan, MIYARISAN Pharmaceutical Co., Ltd., Tokyo, Japan) 10 days before the sample collection.

In both cases, special care was taken not to contaminate the feces with snow or dirt. Samples were immersed in DNA conservation solution \[[@r11]\], and bacterial DNA was extracted within 24 hr.

*DNA extractions and high-throughput, ultra-deep sequencing of 16S rRNA gene*: To obtain bacterial pellets, samples were centrifuged at 20,000 ×*g* for 10 min with a TOMY MX-307 centrifuge (TOMY SEIKO, Tokyo, Japan), and the pellets were washed with Tris-EDTA several times to remove residual plant metabolites, such as phenolic compounds.

Genomic DNA was extracted from the pellets using a Quick Gene DNA Tissue kit (Kurabo, Tokyo, Japan) as per the manufacturer's instructions (n=3). DNA concentration was determined with a Quant-iT™ dsDNA HS assay kit using a Qubit® fluorometer (Invitrogen, Carlsbad, CA, U.S.A.).

The microbial community structure was analyzed by 16S rRNA gene amplicon sequencing carried out with a MiSeq Desktop Sequencer (Illumina, Hayward, CA, U.S.A.) as per the manufacturer's instructions (<https://support.illumina.com/content/dam/illumina-support/documents/documentation/chemistry_documentation/16s/16s-metagenomic-library-prep-guide-15044223-b.pdf>). Partial 16S rRNA gene sequences including V3 and V4 regions were amplified using primers Bakt_341F and Bakt_805R \[[@r12]\] with Illumina overhang adaptor sequences attached to their 5′ ends. The reaction mixture (25 *µl*) contained 1 × KAPA HiFi HotStart ReadyMix (Kapa Biosystems, Wilmington, MA, U.S.A.), 0.2 *µ*M of each primer and 2 *µl* of template DNA. PCR was performed with Takara'