Isolation and identification of pathogenic bacterium associated with ascites in Cynoglossus semilaevisis

Dominant bacteria designed as 8301 was isolated from ascites and kidney of diseased Cynoglossus semilaevisis and proved to be pathogenic strongly to C. semilaevisis by artificial challenge. The biological characteristics including morphological, physiological and bio-chemical characteristics were examined, and the 16 S rDNA gene were partially sequenced and compared with sequences deposited in GenBank databases. The results showed that strain 8301 appeared gram-positive cocci in chains; based on the physiological and biochemical characteristic analysis, strain 8301 was identified as Streptococcus iniae. Phylogeny analysis with 16 S rDNA showed that strain 8301 was branched into the same cluster of S. iniae and the bootstrap was 96%. These results determined that the pathogen of causing ascites in C. semilaevisis was S. iniae. Antimicrobial susceptibility assay showed that strain 8301 was susceptible to nystatin, rifampicin, penicillin and azithromycin, but resistant to roxithromycin, furazolidone, etc.