Abstract：
Co-infection of Lactococcus garvieae and Aeromonas hydrophila, has been confirmed from diseased Nile Tilapia (Oreochromis niloticus), Chithralada strain cultured in a freshwater rearing pond of Alappuzha district of Kerala, India. The aetiological agents behind the disease outbreak were bacteriologically proven and confirmed by 16SrRNA sequencing and phylogenetic analysis. PCR detection of the virulent genes, showed existence of adhesin and hemolysin in L. garvieae and aerolysin in A. hydrophila strain obtained. To fulfil Koch\'s postulates, challenge experiments were conducted and median lethal dose (LD50) of L. garvieae and A. hydrophila was calculated as 1 × 105.91 CFU per mL and 1 × 105.2 CFU per mL respectively. Histopathologically, eyes, spleen, and kidney were the predominantly infected organs by L. garvieae and A. hydrophila. Out of the 13 antibiotics tested to check antibiotic susceptibility, L. garvieae showed resistance to almost 7 antibiotics tested, with a resistance to Ciprofloxacin while A. hydrophila was found resistant to Streptomycin and Erythromycin. Understanding the complex interaction between Gram-positive and Gram-negative bacteria in the disease process and pathogenesis in fish host will contribute to efficient treatment strategies. As a preliminary investigation into this complex interaction, the present study is aimed at phenotypic and genotypic characterization, pathogenicity evaluation, and antibiotic susceptibility of the co-infecting pathogens in a diseased sample of freshwater-farmed Nile tilapia.
摘要：
链球菌和嗜水气单胞菌的共感染，已从患病的尼罗河罗非鱼（Oreochromisniloticus）中得到证实，Chistralada菌株在喀拉拉邦Alappuzha区的淡水养殖池塘中培养，印度。疾病爆发背后的病原经过细菌学证明，并通过16SrRNA测序和系统发育分析得到证实。毒力基因的PCR检测，在获得的嗜水气单胞菌菌株中显示出粘附素和溶血素的存在。为了实现科赫的假设，进行了攻击实验，计算出L.garvieae和A.hydrophila的中位致死剂量（LD50）分别为1×105.91CFU/mL和1×105.2CFU/mL。组织病理学,眼睛,脾,脾和肾脏是主要被L.garvieae和A.hydrophila感染的器官。在测试抗生素敏感性的13种抗生素中，L.garvieae显示出对测试的近7种抗生素的抗性，对环丙沙星具有抗性，而嗜水杆菌对链霉素和红霉素具有抗性。了解革兰氏阳性和革兰氏阴性细菌在鱼类宿主疾病过程和发病机理中的复杂相互作用将有助于有效的治疗策略。作为对这种复杂互动的初步调查，本研究针对表型和基因型表征，致病性评价,淡水养殖尼罗罗非鱼患病样品中共同感染病原体的抗生素敏感性。
