Isolation And Identification Of Streptococcus Agalactiae From Tilapia And Protective Immunity Of Sip Protein

In recent years, aquaculture in central-south area of China suffered serious epidemic diseases. In thisstudy, a strain named HNLFYL4was isolated from sickness Tilapia in Hainan province. It was proved thatthis strain was Streptococcus agalactiae through biochemical analysis, physiological tests, andphylogenetic analyses of16S rRNA. Virulent tests showed that HNLFYL4was pathogenic to tilapia andmouse. LD50to mouse (6-week old) and tilapia (body weight:500±20g) was1.0×104CFU/ml and1.729×109CFU/ml, respectively. Drug sensitivity experiment showed that HNLFYL4was sensitive tochloramphenicol, penicillin G, and nitrofurantoin, insensitive to amikacin, streptomycin and kanamycin.For the development of aquaculture, it is important to establish strategies to prevent and curestreptococcosis. It has been demonstrated that the surface immunogenic protein(Sip) antigen ofS.agalaetiae had good immunoprotection against S. agalactiae infection in mice．But it is still not clearwhether Sip can confer protection against S. agalactiae infection in fish．In this study, we investigated theprotection of Sip protein against S. agalactiae infection in tilapias by immuning fish with expressedrecombinant Sip. The result is useful for the genetic engineering of S.agalactiae vaccine.Full Sip gene fragments were obtained by PCR. Using the pET28a(+) expression system, the Sip genewas expressed in E. coli BL21(DE3). The molecular weight of Sip is about49KD, which was confirmedby the sodium dodecyl sulfate polyacrylamide gel electrophoresis. As to immune tilapia, Sip could confersignificant protection to fish against S.agalactiae. The relative percentage survivals (RPS) are59.3%,70.3%and66.7%, respectively. Therefore, the recombinant strain BL21(DE3)(pET28a-Sip) could beused to develop genetic engineering subunit vaccines against the infection of S. agalactiae.