Gallibacterium anatis establishes primary pathogenicity through fecal-oral transmission and systemic dissemination in immunocompetent poultry

Gallibacterium anatis represents an emerging bacterial pathogen causing substantial economic losses in global poultry production through respiratory, reproductive, and systemic diseases. Despite mounting clinical evidence of its devastating impact and increasing global prevalence with multidrug-resistant strains, the fundamental pathogenic mechanisms and transmission dynamics of this pathogen remain incompletely defined. This study investigates the pathogenic capacity, microbiome disruption patterns, and molecular mechanisms underlying G. anatis virulence through integrated clinical discovery, controlled experimental infections, and dual transcriptomics. Using comprehensive 16 S rRNA sequencing of clinical samples, we identified G. anatis as a differentially abundant pathogen comprising 1.94% of the diarrhea-exclusive bacterial community. Controlled experimental infections confirmed that G. anatis induces severe intestinal pathology, achieves persistent systemic colonization in spleen and liver tissues, and reduces cumulative egg production by 36.7% (YJ922) and 19.1% (F149) in commercial laying hens over eight weeks. Comprehensive microbiome analysis revealed that G. anatis accounted for 86.25% of the OTUs detected exclusively in infected birds within 24 h, while representing only 0.038% of the total cecal community, and that infection induced marked community restructuring and dysbiosis characterized by Bacteroidota expansion and Firmicutes depletion. Dual RNA-sequencing analysis identified 2,683 differentially expressed host genes and 650 bacterial genes, revealing coordinated upregulation of virulence programs including barrier disruption, iron acquisition, and immune evasion mechanisms. Host–pathogen protein interaction analysis predicted 89 molecular interactions supported by concordant differential expression 44 involving downregulated and 45 involving upregulated genes targeting host cell cycle regulation, transcriptional control, and structural organization systems. These findings definitively establish G. anatis as a primary bacterial pathogen with intrinsic virulence characteristics, demonstrating that pathogenicity represents a fundamental species trait rather than an acquired property of specific field isolates. The remarkable pathogenic efficiency achieved through precise molecular targeting rather than overwhelming colonization, combined with the ability to induce rapid and severe microbiome disruption, provides essential groundwork for developing targeted therapeutic interventions and establishes a methodological framework for characterizing neglected opportunistic pathogens in agricultural systems. Not applicable.