Emergence of highly virulent Aeromonas dhakensis in channel catfishaquaculture: genomic insights into pathogenicity and antimicrobial resistance

Aeromonas dhakensis has emerged as a significant pathogen in aquaculture, causing severe disease outbreaks and resulting in substantial economic losses. However, its pathogenic mechanism and virulence factors remain largely unexplored. In this study, we isolated ahighly virulent strain of A. dhakensis, CWH5, from a severe disease outbreak in farmed channel catfish (Ictalurus punctatus). Through comprehensive whole-genome analysis, we elucidated its pathogenicity and the genetic basis for its high virulence and multi-antimicrobial resistance in channel catfish. Experimental infections showed that CWH5 exhibited exceptional virulence, with an LD50 of 7.94×10⁴ CFU/mL and causing 100% mortality within 72 h at a concentration of 10⁷ CFU/mL. Histopathological examinations revealed severe multi-organ damage, including extensive hepatocellular necrosis, gill epithelial destruction, and fin tissue deterioration. Whole-genome sequencing revealed a 4.92 Mb circular chromosome encoding sophisticated virulence mechanisms, such as complete Type III, IV, and VI secretion systems, and a vast arsenal of 60 antibiotic resistance genes across 15 drug classes. Comparative genomic analysis positioned CWH5 within the A. dhakensis clade, sharing the highest sequence similarity with A. dhakensis CIP 107500ᵀ. The co-localization of virulence and resistance determinants within mobile genetic elements suggests the potential for horizontal gene transfer. Our work underscored the importance of A. dhakensis CWH5 as an emerging pathogen in channel catfish aquaculture, providing crucial insights into the molecular mechanisms of its exceptional virulence and implying significant implications for disease management and antimicrobial resistance surveillance in aquaculture settings.达卡气单胞菌（Aeromonas dhakensis）已成为水产养殖中的重要病原菌，可引发严重的病害暴发并造成巨额经济损失，但其致病机制与毒力因子仍有大量未被阐明的内容。本研究从暴发严重病害的养殖斑点叉尾鮰（Ictalurus punctatus）中分离得到一株高毒力达卡气单胞菌菌株CWH5。通过全面的全基因组分析，我们阐明了该菌株的致病性，以及其在斑点叉尾鮰中表现出的高毒力与多重抗菌耐药性的遗传基础。人工感染试验显示，菌株CWH5具有极强毒力，其半数致死量（LD50）为7.94×10⁴ CFU/mL，当浓度为10⁷ CFU/mL时可在72小时内导致100%的受试鱼死亡。组织病理学检查显示其引发了严重的多器官损伤，包括广泛的肝细胞坏死、鳃上皮破坏以及鳍组织变性坏死。全基因组测序结果显示，该菌株拥有一条4.92 Mb的环状染色体，编码了复杂的毒力机制，例如完整的Ⅲ型、Ⅳ型和Ⅵ型分泌系统，以及涵盖15类药物的60种抗生素耐药基因。比较基因组学分析将CWH5归类到达卡气单胞菌进化分支中，其与达卡气单胞菌CIP 107500ᵀ的序列相似性最高。毒力与耐药决定因子在可移动遗传元件中的共定位现象，提示其存在水平基因转移的潜力。本研究凸显了达卡气单胞菌CWH5作为斑点叉尾鮰水产养殖中新兴病原菌的重要性，为解析其极强毒力的分子机制提供了关键见解，并对水产养殖领域的病害防控与抗菌药物耐药性监测具有重要的指导意义。