BIOLOGICAL CHARACTERISTICS AND GENOMIC SEQUENCE ANALYSIS OF PATHOGENIC AEROMONAS SCHUBERTII ISOLATED FROM MANDARIN FISH (SINIPERCA CHUATSI)

To investigate the biological characteristics and molecular genetic features of Aeromonas schubertii isolated from mandarin fish, this study aimed to isolate pathogenic bacteria from diseased mandarin fish fed with artificial compound diet, identify the strain ZQ210705, and analyze its biological characteristics, whole-genome sequencing, and genetic evolutionary relationships. The strain ZQ210705 was identified based on 16S rRNA, gyrB, and whole genome sequence analysis as well as physiological and biochemical characteristics. The biological characteristics were analyzed by motility, hemolytic activity, extracellular enzyme activity, pathogenicity, and antimicrobial resistance. The MLST (Multilocus Sequence Typing) type, virulence genes, drug resistance genes, and genetic evolution of the strain were analyzed by whole genome sequencing. Strain ZQ210705 was identified as A. schubertii. This strain exhibited motility and β-hemolytic activity, and produced extracellular protease, amylase, and lipase. It was susceptible to several commonly used aquatic antimicrobial agents, including enrofloxacin, florfenicol, neomycin, doxycycline, and trimethoprim-sulfamethoxazole. It showed high pathogenicity to mandarin fish, with LD₅₀ values of 4.75×10⁷ CFU/mL. Whole-genome sequencing revealed the presence of type II (T2SS), type III (T3SS), and type VI (T6SS) secretion systems, along with multiple virulence-associated genes. Additionally, resistance genes including rsmA, adeF, fosG, EF-Tu, and qnrS2 were found in the genome of strain ZQ210705. The strain represents a novel sequence type (ST=3589) and shares a closer genetic relationship with the tilapia-derived A. schubertii strain LF1708 than with other known isolates. In conclusion, strain ZQ210705 was identified as A. schubertii. It exhibited strong motility and high pathogenicity, and carried a variety of virulence genes and secretion systems. Phylogenomic analysis confirmed its close genetic relatedness to the tilapia-derived strain LF1708, supporting potential interspecific transmission within aquaculture ecosystems. This study provides valuable insights into the pathogenic mechanisms of A. schubertii and provides evidence for its adaptability across farmed fish hosts.