Host-Microbe Coadaptation Enables Cellulose Digestion in Bamboo-Specialized Giant Pandas

Giant pandas (Ailuropoda melanoleuca) retain a phylogenetically carnivorous digestive system (e.g., short intestine, rapid transit time) yet exclusively consume bamboo - a fiber - rich diet requiring specialized cellulose digestion. Their gut microbiome lacks signature cellulolytic taxa typical of herbivores and demonstrates functional profiles aligning with carnivores. This paradox remains unresolved: how does the panda gut ecosystem achieve fiber decomposition without typically cellulolytic microbes? In this study, Integrated culturomics and PacBio HiFi metagenomics were applied to generate the expanded genome repository Pbac v2 (466 MAGs/genomes). Species - level enterotyping revealed three distinct clusters dominated by keystone taxa: Escherichia coli (ET - Ecoli), Streptococcus alactolyticus (ET - StrepA), and Clostridium SGBP116 (ET - Clos). Functional validation involved: (1) In vitro fiber degradation assays confirming E. coli - driven cellulose decomposition; (2) Transcriptomics and untargeted metabolomics revealing O2 (original O2) - dependent upregulation of E. coli cellulases; (3) Microbiome single - cell Transcriptomics (in situ) demonstrating Escherichia dominance in cellulase expression within the panda's gut. We demonstrate the contribution of Escherichia coli to fiber degradation within the giant panda gut microbiota, while establishing invaluable genomic resources - including high - quality genomes and microbial single - cell transcriptomes - for elucidating functional adaptations in this specialized symbiosis.