Some factors affecting the potency of Yersinia ruckeri bacterins

Abstract. Several factors affecting the potency of Yersinia ruckeri bacterins were evaluated by vaccinating rainbow trout, Salmo gairdneri Richardson, with various bacterins using the immersion method and determining the level of protective immunity after a virulent challenge. The potency of bacterins prepared with tryptic soya broth at room temperature was not affected by growth at pH from 6.5 to 7.7 or by a culture age from 9 to 96 h. Chloroform and formalin inactivated (0.3%) bacterins gave comparable results and no enhancement of potency occurred by prior extraction of bacterial cells with either butanol or phenol. Cell lysis, as measured by reduced optical density, occurred when cells were held at pH 9–8 for 60 to 120 min. Bacterins prepared from pH-lysed cells resulted in a significant increase in protective immunity. Bacterins prepared at pH 7.2 for 48 h, pH-lysed and inactivated with 0.3 % formalin could be diluted up to 1:100 without loss of efficacy when applied to rainbow trout by a single 20 s immersion. However, with bacterin diluted 1:10 loss of potency occurred after 20 consecutive immersions (100 kg of fish) in the same bacterin at a rate of 0.5 kg/1 of diluted bacterin for each immersion. Factors affecting optimum duration of immunity are discussed.