Abstract：
Citrobacter freundii, a common pathogen of freshwater fish, causes significant commercial losses to the global fish farming industry. In the present study, a highly pathogenic C. freundii strain was isolated and identified from largemouth bass (Micropterus salmoides). The pathogenicity and antibiotic sensitivity of the C. freundii strain were evaluated, and the histopathology and host immune response of largemouth bass infected with C. freundii were investigated. The results showed that C. freundii was the pathogen causing disease outbreaks in largemouth bass, and the infected fish showed typical signs of acute hemorrhages and visceral enlargement. Antimicrobial susceptibility testing showed that the C. freundii strain was resistant to Kanamycin, Medimycin, Clindamycin, Penicillin, Oxacillin, Ampicillin, Cephalexin, Cefazolin, Cefradine and Vancomycin. Histopathological analysis showed different pathological changes in major tissues of diseased fish. In addition, humoral immune factors such as superoxide dismutase (SOD), catalase (CAT) and lysozyme (LZM) were used as serum indicators to evaluate the immune response of largemouth bass after infection. Quantitative real-time PCR (qRT-PCR) was performed to investigate the expression pattern of immune-related genes (CXCR1, IL-8, IRF7, IgM, CD40, IFN-γ, IL-1β, Hep1, and Hep2) in liver, spleen, and head kidney tissues, which demonstrated a strong immune response induced by C. freundii infection in largemouth bass. The present study provides insights into the pathogenic mechanism of C. freundii and immune response in largemouth bass, promoting the prevention and treatment of diseases caused by C. freundii infection.
摘要：
Freundii柠檬酸杆菌,淡水鱼的常见病原体，对全球养鱼业造成重大的商业损失。在本研究中,从大口鲈鱼（Micropterussalmoides）中分离并鉴定出一种高致病性C.freundii菌株。对该菌株的致病性和抗生素敏感性进行了评价,研究了大嘴鲈鱼感染弗氏梭菌的组织病理学和宿主免疫反应。结果表明，弗氏梭菌是导致大口鲈鱼疾病爆发的病原体，被感染的鱼表现出典型的急性出血和内脏肿大。药敏试验表明,该菌株对卡那霉素具有耐药性,Mediamin,克林霉素,青霉素,苯唑西林,氨苄青霉素,头孢氨苄,头孢唑啉,头孢拉定和万古霉素。组织病理学分析显示，患病鱼类的主要组织有不同的病理变化。此外,体液免疫因子如超氧化物歧化酶（SOD），以过氧化氢酶（CAT）和溶菌酶（LZM）为血清指标，评价大嘴鲈鱼感染后的免疫反应。进行定量实时PCR(qRT-PCR)以研究免疫相关基因(CXCR1、IL-8、IRF7、IgM、CD40,IFN-γ,IL-1β,肝脏中的Hep1和Hep2），脾,脾和头部肾脏组织，这表明在大口鲈鱼中由弗氏梭菌感染诱导了强烈的免疫反应。本研究提供了对大嘴鲈鱼的致病机制和免疫反应的见解。促进预防和治疗由Freundii感染引起的疾病。
