Integrated Pathogen-Host Analysis of Citrobacter braakii SCGY-1L: Genomic Determinants and Host Transcriptional Dynamics During Infection.

Citrobacter braakii is an emerging opportunistic pathogen of escalating clinical significance in animal hosts, though its pathogenic mechanisms remain poorly characterized. This study isolated a C. braakii strain (SCGY-1L) from diseased Siniperca chuatsi and confirmed its identity through integrated morphological, physiological, and molecular analyses. Comprehensive genomic sequencing revealed a 5.75 Mb genome comprising one circular chromosome and two plasmids. A Circos plot was constructed to visualize the genomic architecture of strain SCGY-1L, revealing 5482 protein-coding genes, 25 tRNA genes, and 86 rRNA genes. Additionally, 738 virulence-associated genes and 366 antibiotic resistance determinants were annotated, elucidating multidrug-resistant phenotypes including insensitivity to erythromycin and penicillin. Pathogenicity assessment established an LD50 of 1.28 × 106 CFU/mL in infected hosts, with histopathological analysis showing significant hemorrhage and necrosis in target organs (liver, spleen, kidney). Host transcriptome profiling generated 41.21 Gb of high-quality clean data, identifying 2201 differentially expressed genes post-infection (1568 up-regulated; 633 down-regulated). These were significantly enriched in phagocytosis, cytokine-mediated signaling, and inflammatory regulation pathways. These molecular insights establish C. braakii's mechanistic framework for pathogenesis and host adaptation, providing critical targets for diagnostics and therapeutics against emerging Citrobacter infections.