Objective To identify the pathogenic cause of acute death in yellow pond turtle (Mauremys mutica), a Gram-negative bacterial strain GDDG2513 was isolated from diseased individuals in this study, followed by strain identification and pathogenicity analysis.

Method The strain was identified through morphological analysis, 16S rRNA gene sequencing, and a species-specific SYBR green qPCR assay. Its virulence was evaluated determining the median lethal dose(LD₅₀) based on artificial infection challenges. Pathological damage and tissue tropism induced by the train in yellow pond turtles were analyzed via anatomical observation and histological H&E staining. Additionally, the antibiotic susceptibility profile of the strains was determined against a panel of commonly used antimicrobials.

Result Morphological characterization, 16S rRNA gene sequencing, and qPCR analysis consistently identified strain GDDG2513 as Aeromonas hydrophila. In artificail infection challenges, all infeced turtles died within 24 hours at a dose of 1×10⁸ CFU per turtle and the median lethal dose (LD₅₀) was calculated to be 4.0×10⁵ CFU per turtle over a 15-day overvation period. Necropsy of these turtles revealed typical pathological sign, suchi as ascites, hepatic congestion, and intestinal inflammation, which mirrored the findings in natural cases. A. hydrophila was successfully re-isolated from the lesions, fulfilling Koch's postulates. Tissue tropism analysis indicated that the liver and intestine were the primary target organs. H&E staining further confirmed severe tissue damage and inflammatory cell infiltration in both organs. According to antibiotic susceptibility testing, strain GDDG2513 was sensitive to 16 antibiotics, including thiamphenicol, flumequine, florfenicol and enrofloxacin.

Conclusion In conclusion, A. hydrophila strain GDDG2513 is pathogenic to yellow pond turtles and has been confirmed as the primary causative agent of the observed acute mortality. This study elucidates the pathogenicity, pathological features, and antibiotic susceptibility profile of this strain, thereby providing a scientific basis for the clinical diagnosis, prevention, and control of A. hydrophila infections in yellow pond turtles.