Identification and Characterizations of Streptococcus agalactiae Isolated from Tilapia Cultured in Guangdong and Hainan Provinces

Abstract:

Bacteria pathogens were isolated from diseased tilapia cultured in several farms of Guangdong and Hainan provinces. Artificial infection experiments showed that these isolates possessed strong virulence. Intraperitoneal injection (≥1 × 106 CFU/mL) of some isolates caused 100% mortality of healthy tilapia. 7 bacteria strains with strong virulence were chosen for antibiotics sensitivity test and identifications. Antibiotic sensitivity assays showed that among 29 antibiotics tested 13 were sensitive, 7 were resist, while 9 items showed various reactions among strains. The isolates were all gram-positive and β-hemolysis and were identified as Streptococcus agalactiae by biochemistry assays of ID 32 STREP test, Lancefield test and other assays. Molecular analysis of 16S rRNA and cfb genes (GBS-specific gene cfb, CAMP factor) were used for further identification. BLAST showed that 16S rRNA and cfb genes of all the strains possessed high similarities with their counterparts registered in GenBank, and with each other. Taken together, these experiment results showed that pathogen caused high mortality of tilapia in both provinces in 2009 was S. agalactiae. The result was helpful for tilapia disease control and prevention, and for development of streptococcus vaccine for tilapia.

Key words:Tilapia, Streptococcus agalactiae, Virulence, Biochemical analysis, 16s rRNA, cfb gene, Antibiotic sensitivity