Our results indicated that the growth, staining, characteristics of pulsedfield gel electrophoresis (PFGE) genotype, and virulence of YM001 were changed significantly as compared to the parental strain HN016.High doses of YM001 by intraperitoneal (IP) injection (1.0 109 CFU/fish) and oral gavage (1.0 1010 CFU/fish) respectively did not cause any mortality and morbidity in tilapia. The relative percent survivals (RPSs) of fishes immunized with YM001
(1.0 108 CFU/fish, one time) via injection, immersion, and oral administration were 96.88, 67.22, and 71.81%, respectively, at 15 days, and 93.61, 60.56, and 53.16%, respectively, at 30 days. In all tests with 1e3 times of immunization in tilapia, the dosages at 1 108 and 1 109 CFU/fish displayed the similar best results, whereas the immunoprotection of the dosages at 1 106 and 1 107 CFU/fish declined.
significantly (P < 0.01), and 1 105 CFU/fish hardly displayed any protective effect. In addition, the
efficacy of 2e3 times of immunization was significantly higher than that of single immunization
(P < 0.01) while no significant difference in the efficacy between twice and thrice of immunization was
seen (P > 0.05).