Abstract:The cultivation of ornamental fish has been gaining more and more popularity and commercial importance. Consequently ornamental fish diseases have also caught attentions. Aequidens rivulatus is one of the most important ornamental fish in commercial fisheries in Tianjin of China. In recent years mass death of A. rivulatus has caused heavy economic losses. In this study we aimed to identify the biological characteristics of the pathogens which caused diseases in A. rivulatus. A dominant bacterium 071901 was isolated from the kidney of diseased A. rivulatus and was confirmed to be pathogenic to A. rivulatus in artificial challenge test. We examined the morphological, physiological and biochemical characteristics of this bacterium, and analyzed 16S rRNA and cfb gene (GBS-specific gene cfb, CAMP factor) for further identification. The 16S rRNA gene and cfb gene were partially sequenced and compared with data from the GenBank database. BLAST showed that 16S rRNA gene and cfb gene of strain 071901 were highly similar to their counterparts registered in GenBank. Based on the morphological observation and physiological and biochemical characterization, strain 071901 was identified as Streptococcus agalactiae. Phylogenetic trees of S. agalactiae based on 16S rRNA gene indicated that strain 071901 was branched into the same cluster as S. agalactiae and showed the highest similarity to S. agalactiae. The results of molecular analysis of cfb gene were consistent with the previous results from the biochemical assays. We then performed antimicrobial susceptibility assay for 30 antimicrobial agents and found that strain 071901 was susceptible to 19 agents including erythromycin, azithromycin and florfenicol, and was highly resistant to 11 agents including enrofloxacin, bacitracin and furazolidone. In conclusion our results demonstrated that S. agalactiae was the pathogen that caused high mortality in A. rivulatus.
