Mass mortality has occurred among cultured Nile tilapia, Oreochromis niloticus, on fish farms in Manzala, Dakahlia province, Egypt, in the summer season, 2019. Moribund fish were reported with deep ulcers, septicaemic lesions and sampled for bacterial isolation. In this study, most isolates were subjected to bacteriological examination, antibiotic sensitivity test, 16S rRNA gene sequencing and histopathological examination. Following isolate identification, intraperitoneal challenge of Nile tilapia with a bacterial suspension 2 × 10⁶ CFU/ml was performed. Samples from liver, spleen and kidney were collected for histological and biochemical analysis. The results showed a high similarity (99%) to Photobacterium damselae strains using phylogenetic analysis of 16S rRNA. P. damselae exhibited resistance to amoxicillin and erythromycin, as well it was highly sensitive to chloramphenicol and doxycycline. Moreover, haemorrhage, oedema, hemosiderosis and melanomacrophage activation in the liver and head kidney of infected fish were detected by light and electron microscopy. Also, significant higher levels of CAT and SOD in the spleen and head kidney, as well as the serum levels of NO were observed in experimentally challenged O. niloticus, compared to the control fish. Our data identified P. damselae for the first time from infected Nile tilapia, describing its sensitivity to a variety of antibiotics, histopathological alterations and oxidative stress impact, and it could be useful indicators for understanding P. damselae pathogenesis, which might provide a preventive efficacy for P. damselae.

中文翻译：

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自然和实验感染的尼罗罗非鱼（Oreochromis niloticus）中的damselae细菌的分子和组织病理学表征

大众死亡率中培养的尼罗罗非鱼，发生罗非鱼，在曼扎拉，Dakahlia省，埃及养鱼场，在盛夏季节，2019年垂死的鱼被报道与深溃疡，败血症病变并取样进行细菌分离。在这项研究中，大多数分离株都经过了细菌学检查，抗生素敏感性测试，16S rRNA基因测序和组织病理学检查。分离株鉴定后，用2×10 ⁶  CFU / ml细菌悬液进行尼罗罗非鱼的腹膜内攻击。收集肝，脾和肾的样品进行组织学和生化分析。结果显示与达姆发光细菌有高度相似性（99％）菌株使用16S rRNA的系统发育分析。damselae（P. damselae）对阿莫西林和红霉素表现出抗性，并且对氯霉素和强力霉素高度敏感。此外，通过光学和电子显微镜检测到感染鱼的肝脏和头肾脏中的出血，水肿，铁血黄素沉着和黑色素巨噬细胞活化。此外，与对照鱼相比，在实验挑战的O. niloticu中观察到脾脏和头肾中CAT和SOD的水平明显较高，以及血清中的NO水平。我们的数据确定了P. damselae首次从受感染的尼罗罗非鱼中描述了其对多种抗生素的敏感性，组织病理学改变和氧化应激影响，并且它可能是了解丹参假单胞菌发病机理的有用指标，可能为丹参假单胞菌提供预防功效。