Isolation and characterization of Streptococcus agalactiae from Nile Tilapia Oreochromis niloticus in China

Streptococcosis became an increasingly significant health problem in intensive aquaculture in China. In 2009, eight Gram-positive, chain-forming coccus strains were isolated from the outbreak epidemical diseased tilapias from Hainan Province, China. Characteristics based on morphological, physiological and biochemical tests show that the eight strains were suggested to Streptococcus agalactiae. Furthermore, sequencing of the 16S rRNA genes shows high sequence similarities (99.5 to 99.9%) with S. agalactiae. All results indicated that the eight isolated strains (Genbank no JF423941~JF423948) were S. agalactiae. The median lethal doses (LD50) of HN0101 and HN0301 which were isolated from Wenchang and Xinglong villages were 6.8×106 and 5.3×106 CFU/fish, respectively by intraperitoneal injections. All the isolates were sensitive to cefoxitin, and had quite different sensitiveness to other antibiotics. This research hinted that S. agalactiae had become an important potential bacterial agent of tilapias in the south of China.   Key words: Streptococcus agalactiae, tilapia, identification