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，可引发严重的疾病暴发并造成巨额经济损失。然而，其致病机制与毒力因子仍有待深入探索。本研究从养殖斑点叉尾鮰（Ictalurus punctatus）的严重疾病暴发样本中分离得到一株强毒力达卡气单胞菌菌株CWH5。通过全面的全基因组分析，我们阐明了该菌株在斑点叉尾鮰中的致病性，以及其强毒力与多重抗菌药物耐药性的遗传基础。感染实验表明，菌株CWH5具有极强的毒力：其半数致死量（LD50）为7.94×10⁴菌落形成单位（CFU）/mL，当浓度达到10⁷ CFU/mL时，可在72小时内导致100%的受试个体死亡。组织病理学检查显示，该菌株可引发严重的多器官损伤，包括广泛的肝细胞坏死、鳃上皮破坏以及鳍组织退变。全基因组测序结果显示，该菌株拥有一条长度为4.92 Mb的环状染色体，编码多种复杂的毒力机制，包括完整的Ⅲ型、Ⅳ型及Ⅵ型分泌系统，同时携带涵盖15类药物的60个抗生素耐药基因，耐药基因储备丰富。比较基因组分析显示，菌株CWH5隶属于达卡气单胞菌进化枝，与达卡气单胞菌CIP 107500ᵀ的序列相似性最高。毒力与耐药决定元件共同定位于可移动遗传元件中，提示其存在水平基因转移的潜在可能。本研究证实了达卡气单胞菌CWH5作为斑点叉尾鮰水产养殖中新出现病原菌的重要性，为解析其极强毒力的分子机制提供了关键见解，同时也为水产养殖中的疾病防控与抗菌药物耐药性监测工作提供了重要参考价值。