Abstract

To elucidate the pathogen and host response mechanisms underlying mass mortality in Quasipaa spinosa , dominant bacteria were isolated from diseased individuals and identified as Citrobacter freundii by morphological, physiological, and biochemical characterization, 16S rRNA sequencing, and phylogenetic analysis. The median lethal dose (LD 50 ) of this bacterium for Q. spinosa was determined to be 4.65 × 10 5 CFU/mL through experimental infection. The histopathological examination revealed that the liver showed significant vacuolar degeneration, necrosis, and thrombosis. Transcriptome sequencing identified 2404 differentially expressed genes (DEGs) (1338 up-regulated and 1066 down-regulated). DEGs were significantly enriched in protein processing in the endoplasmic reticulum, cytochrome P450-mediated xenobiotic metabolism, and the PPAR signaling pathway. Key immune genes ( HSP70 and Bax ) were markedly up-regulated, whereas the negative regulator ( TRIM25 ) was down-regulated. Reverse Transcription Quantitative PCR (RT-qPCR) validation was highly consistent with the RNA-seq data. This study systematically revealed how Q. spinosa resists C. freundii infection by activating innate immunity, apoptosis, and metabolic reprogramming, providing theoretical insights and candidate molecular targets for the prevention and control of bacterial diseases in amphibians.