Identification Of Streptococcus Agalactiae From Hybrid Tilapia And Preliminary Study On Immunization

The dominant bacterial strain, designated as LFY-08-23, LFY-08-23-2, LFY-08-23-3, LFY-08-23-4, LFY-08-23-5,was isolated from the diseased Hybrid Tilapia (Oreochromis aureus×Oreochromis niloticus) in Hainan. The biochemical and physiological characteristics of the isolates were learnd by using conventional method and Biolog System. The test showed that the isolates were Gram-positive cocci arranging in chains, catalase negabive,β-haemolytic on 5% sheep blood agar and serogroup B. According to their 16S rRNA gene sequence and molecular phylogenetic dendrogram, the LFY-08-23 was most close to Strptococcus agalactiae with a similarity of 99.8%. The virulent test were carrided out by using the pure culture of the bacteria artificially infected Ictalurus punctatus, weight of 100g and 30g, the consult showed that LFY-08-23 have high virulent. The morphological and biochemical characteristics of the re-isolated bacteria from the artificially infected diseased fish were the same as those of the original infected bacteria. The susceptibility test to antibiotics demonstrated that the isolate strain LFY-08-23 was susceptible to most of the tested antibiotics, especially highly susceptible to 13 kinds of antibiotics, such as Cefotaxime, Ciprofloracin, Rifampicin, and resistant to 5 antibiotics, such as Kanamycin, Streptomycin,Gantrisin.Ictalurus punctatus were immunized with the bacterin of Streptococcus agalactiae from LFY-08-23 and ECP'modified bacterin. The immune response of Ictalurus punctatus were compared by examing the agglutinating antibody titers,lysozyme activity, the phagocytic activity of phagocytes in the blood. The experimental results showed that the agglutinating antibody titers of the two group of immunized Ictalurus punctatus were higher than the control group, and the phagocytic activity of phagocytes in the blood was significantly boosted and had a high level immune response. The two vaccine were thought to have a different protection against the homologous isolate of S. agalactiae in Hybrid Tilapia.