Isolation, identification and pathogenicity of Tilapia Streptococcus agalactiae from Guangzhou

Four pathogenic strains were isolated from sick tilapia (Oreochromis niloticus) in Zhongxin town, Guangzhou city, and were identified as Streptococcus agalactiae by bacterial morphological observation, biochemical test, amplification and sequence analysis of specific gene cfb. Antibiotic sensitivity test showed that isolated strains were sensitive to Cefoperazone, Gentamycin, Cephaloridnum吁, Cephaloridnum, Azitromvcin, Clindamycin, and Cefamandole. One of the isolated strains ZX1 was used to infect tilapia, mouse and sucking mouse respectively. The results showed that ZX1 strain caused 100% mortality of healthy tilapia by intraperioneal injection at the concentration of 109 cells/mL, 66.7% mortality of mouse by intraperioneal injection (108 cells/mL) and 100% mortality of sucking mouse by subcutaneous injection (108 cells/mL) in 72 hours. These artificial infection experiments showed that the isolated strain ZX1 had strong virulence to tilapia and mouse. The molecular serotypes of isolated strains resulting from multiple PCR were type a.