Microoxic conditions promote <i>Escherichia</i> -associated cellulase expression in the giant panda gut

Giant pandas possess a carnivore-like gastrointestinal tract yet subsist on bamboo, and their gut communities contain few canonical cellulolytic taxa. We investigated how fiber processing proceeds in this setting by building a species-resolved reference and linking community features to cellular transcriptional profiles and isolate phenotypes. Using culturomics and PacBio HiFi metagenomics, we assembled a species-resolved reference catalog for the panda gut microbiome (Pbac v2; 466 species-level genomes). Community profiling across 142 samples resolved three enterotypes dominated by Escherichia coli (ET-Ecoli), Clostridium SGBP116 (ET-Clos), and Streptococcus alactolyticus (ET-StreA), with ET-Ecoli enriched for tricarboxylic-acid and respiratory-chain modules and showing higher abundance of an endo-β-1,4-glucanase marker. Droplet-based microbial single-cell RNA-seq from four samples (16 659 cells) assigned a substantial share of cellulase-associated transcripts (GH1/GH3/GH5/GH9) in situ to Escherichia and revealed within-species heterogeneity: E. coli subpopulations segregated into respiration-enriched versus three-carbon/anaerobic-like programs, with cellulase/LPMO-linked transcripts concentrated in the former. Guided by these associations, panda-derived E. coli isolates assayed under defined atmospheres showed oxygen-dependent cellulolytic readouts in vitro. Although in vivo oxygen levels were not measured, the convergence of species-resolved community signatures, single-cell attribution and isolate phenotypes indicates that E. coli can contribute to cellulose processing under microoxic conditions in this cohort. The Pbac v2 resource and the integrated workflow (culturomics + HiFi metagenomes, multi-omics, microbial scRNA-seq) provide a template for species-level assignment of microbiome functions in hosts with unconventional diet-physiology combinations.