Isolation, identification and virulence gene detection of Streptococcus iniae in silver scat (Selenotoca multifasciata)
YANG Lindi，JIA Xinlei，HUANG Zengchao，Lü Jing，LIANG Huafang，HUANG Yucong*
1.College of Fisheries，Guangdong Ocean University，Zhanjiang 524088，China；2.Guangdong Provincial Key Laboratory of Aquatic Animal Disease Control and Healthy Culture，Zhanjiang 524088，China；3.Key Laboratory of Control for Diseases of Aquatic Economic Animals of Guangdong Higher Education Institute，Zhanjiang 524088，China
Abstract: In order to explore the cause of an outbreak of unknown disease in cultured silver scat (Selenotocamultifasciata) 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 Streptococcusiniae. 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.
2023, Vol. 38 
