Isolation, characterization and complete genome sequencing of a Streptococcus dysgalactiae associated with cultured channel catfish mortalities in China

Streptococcus dysgalactiae has emerged as an important pathogen that affects fish and mammals. Recently, channel catfish cultured in Sichuan, suffered an infectious disease characterized by neurological symptoms and many partial ulcers throughout the body, disease reaching 30 % cumulative mortality. Thus, the purpose of this study is to describe the characteristics of the disease, analyze disease factors, and clarify its etiology. The histopathologic analysis revealed that granuloma was found in liver, spleen, head and posterior kidney tissues. A Gram-positive, chain-forming coccus was isolated from the diseased fish and was identified as S. dysgalactiae by biochemical analysis, 16 S rDNA, immunogenic secreted protein (isp) gene sequence, MALDI-TOF-MS analysis, and Average Nucleotide Identity (ANI) analysis. The pathogenicity of the isolated strain was confirmed by an experimental analysis, and its LD50 was 7.63 x 10(6) CFU/ml. The whole genome size of the isolated strain was 2.22 Mb and the G+C content was 39.7 % (GenBank accession number: CP_095081). There were 2315 predicted protein coding sequences (CDSs), including 18 rRNA, 67 tRNA, and 41 sRNA genes. A total of 2649 Gene Ontology (GO) terms were assigned to 1414 (61.08 %) genes, and a total of 157 virulence genes and 120 antibiotic resistance genes were predicted. The drug sensitivity results showed that the isolated strain was resistant to fluoroquinolone antibiotic, tetracycline antibiotic, macrolide antibiotic, and aminoglycoside antibiotic. To our knowledge, this is the first report on S. dysgalactiae as a virulent pathogen in the channel catfish in China, and the findings of this study can increase the current literature on the range of hosts affected by S. dysgalactiae.