Isolation, identification and virulence gene detection of Streptococcus iniae in silver scat (Selenotoca multifasciata)
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摘要: 为探究2021年北部湾地区养殖银鼓鱼(Selenotoca multifasciata)疾病暴发的病原,从广东省湛江市南三镇、廉江市和广西北海市的3个养殖场患病银鼓鱼的脑、肝脏和脾脏中分离细菌,综合形态学观察、生理生化特征和16S rRNA序列分析结果,对分离菌株进行种属鉴定,通过人工感染试验确定分离菌株的致病性, 并进行组织病理学观察、毒力基因检测及药物敏感性试验。结果表明:从3个养殖场的患病银鼓鱼中, 各分离出1株优势菌,分别命名为BBW S1、BBW S2和BBW S3,均具有较强的致病性,是引起银鼓鱼链球菌病的病原菌;根据形态学观察、生化特征和分子鉴定结果,确定3株分离菌株均为海豚链球菌(Streptococcus iniae);病理学观察发现,银鼓鱼感染海豚链球菌后,可造成脑、肝和脾等多脏器不同程度损伤,表现为变性、坏死及炎性细胞浸润等;3株分离菌株均携带scpI、simA、pdi、sagA、cpsD、pgmA和cfi 7种毒力基因,均对阿莫西林、盐酸多西环素、恩诺沙星等8种抗菌药物敏感。研究表明,海豚链球菌是2021年北部湾地区银鼓鱼疾病暴发的病原,本研究结果可为银鼓鱼海豚链球菌病的防控提供科学依据。Abstract: In order to explore the cause of an outbreak of unknown disease in cultured silver scat (Selenotoca multifasciata) in the Beibu Gulf area in 2021, the pathogen was isolated from the brain, liver and spleen tissues of the diseased silver scat with body length of (18±4)cm, and was identified by morphological characteristics, physicochemical and biochemical characteristics, and 16S rRNA sequences analysis. The pathogenicity of the isolated strains was confirmed by artificial infection experiment. Furthermore, virulence gene detection, drug sensitivity and histopathological examination were also carried out. The results showed that the three dominant strains of BBW S1, BBW S2 and BBW S3 were isolated from the diseased silver scat and had strong pathogenicity to healthy silver scat, which were the causative strain of streptococcal disease in silver scat. Based on the morphological characteristics, biochemical characteristics and molecular identification results, the three isolated strains were identified as Streptococcus iniae. Histopathological examination found that S. iniae caused significant damages to brain, liver, spleen and other organs of silver scat. The main pathological lesions were degeneration, necrosis and infiltration of the inflammation cells. Seven virulence genes including scpI, simA, pdi, sagA, cpsD, pgmA and cfi were detected by PCR from the three isolated strains. The antibiotic susceptibility test showed that the three isolated strains were sensitive to amoxicillin, doxycycline hyclate, and enrofloxacin. The findings indicated that S. iniae was responsible for the outbreak of disease occurred in silver scat in Beibu Gulf area, and that provided an effective scientific basis for disease control of S. iniae disease in silver scat.
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