Aeromonas jandaei is an emerging fish pathogen associated with massive mortalities in cultured freshwater fish. This study investigated the morphological, biochemical, molecular, virulence, pathogenicity, and antimicrobial susceptibility characteristics of four strains of A. jandaei involved in the occurrence of mass mortalities of cultured Nile tilapia in an earthen pond farm and laboratory fibreglass tanks in Brazil. Isolates were identified by morphological, biochemical, and molecular analyses. Bacteria morphology was assessed under light and scanning electron microscopes, biochemical profile by conventional biochemical tests, and molecular identification by nucleotide sequencing and phylogenetic analysis of the highly conserved 16S rRNA gene, as well as the housekeeping genes, gyrB, and rpoB. The virulence and pathogenicity were confirmed by screening for 12 virulence genes and induced experimental infection. Broth microdilution method was used to assess susceptibility to enrofloxacin, florfenicol, oxytetracycline, and thiamphenicol. All isolates were confirmed as A. jandaei. Haemolysin, temperature-sensitive protease, haemolysin-aerolysin, and nuclease were the most predominant virulence genes. Lateral flagella B and cytolytic enterotoxin/cytotonic enterotoxin were only detected in strains isolated from fish from fibreglass tanks. In the experimental challenge, doses above 4.3 × 10⁷ CFU mL⁻¹ resulted in mass mortality (100%) in a short period (less than 12 h) without remarkable external clinical signs. Typical clinical signs of disease, including lethargy, inappetence, surface swimming, exophthalmia, cloudy eyes, haemorrhagic patches, and redness of the skin, below the opercula, and at the base of all fins, fin rot, and pale body surface were observed in fish challenged with doses below 4.0 × 10⁷ CFU mL⁻¹ (10–100% mortality). The most predominant histopathological changes in the internal organs of diseased fish were melano-macrophage centres, vascular congestion with thrombus, haemorrhage, and necrosis. Strains isolated from fish from the earthen pond farm were resistant to oxytetracycline, while the strains from fibreglass tanks were sensitive to all antimicrobials. This study provides pertinent data on A. jandaei host-pathogen interactions, susceptibility to antimicrobials, morphological, biochemical, and molecular characteristics, and geographical distribution, which may serve as a guideline for A. jandaei isolation and identification in aquaculture.

詹氏气单胞菌是一种新兴的鱼病原体，与养殖淡水鱼的大量死亡有关。这项研究调查了在巴西的一个土池养殖场和实验室玻璃纤维水箱中，养殖的尼罗罗非鱼大量死亡的发生情况，涉及四种詹氏拟青霉菌株的形态，生化，分子，毒力，致病性和抗菌药敏特性。分离物通过形态，生化和分子分析鉴定。在光学显微镜和扫描电子显微镜下对细菌的形态进行了评估，通过常规的生化测试对生化特性进行了评估，并通过对高度保守的16S rRNA基因以及管家基因进行了核苷酸测序和系统发育分析来鉴定分子，gyrB和rpoB。通过筛选12个毒力基因并诱导实验性感染，确认了毒力和致病性。肉汤微量稀释法用于评估对恩诺沙星，氟苯尼考，土霉素和甲砜霉素的敏感性。所有分离株均被确认为詹氏拟南芥。溶血素，温度敏感性蛋白酶，溶血素-aerolysin和核酸酶是最主要的毒力基因。仅在从玻璃纤维水箱的鱼中分离出的菌株中检测到了鞭毛B外侧和溶细胞性肠毒素/胞质性肠毒素。在实验挑战中，剂量应高于4.3×10 ⁷  CFU mL ^-1在短时间内（少于12小时）导致大量死亡（100％），而没有明显的外部临床体征。观察到典型的疾病临床征象，包括嗜睡，食欲不振，表面游泳，眼球突出，浑浊的眼睛，出血斑块和眼睑下方及所有鳍的皮肤发红，鳍腐烂，体表苍白。剂量低于4.0×10 ⁷  CFU mL ^-1的鱼（10-100％的死亡率）。患病鱼内脏中最主要的组织病理学变化是黑素巨噬细胞中心，血管充血伴血栓，出血和坏死。从土池养殖场的鱼类中分离出的菌株对土霉素具有抗性，而玻璃纤维水箱中的菌株对所有抗菌药物均敏感。这项研究提供了有关詹氏拟南芥宿主-病原体相互作用，对抗生素的敏感性，形态，生化和分子特性以及地理分布的相关数据，这些数据可作为水产养殖中詹氏拟南芥分离和鉴定的指南。