Aims:  Moritella viscosa is a Gram‐negative bacterium that causes winter ulcer disease in salmonid fish cultured in sea water below 8°C. The aim of this study was to study the antigen profiles of these bacteria and to reveal the protection which the antigens induce in fish. Methods and Results:  Lipooligosaccharides (LOS) and an approximately 17–19 kDa outer membrane antigen were shown to be the major specific antigens of M. viscosa. The size of the wall antigen differed between strain groups and even between strains reacting positively in the same sera. Four different serotypes of M. viscosa were determined by producing polyclonal sera. Western blot analysis revealed that sera from vaccinated fish groups that had good or fair protection reacted against the LOS and the 17/19 kDa antigen, while no antibody response was observed with sera from groups that showed no efficacy. Conclusions:  The study provides evidence that LOS and an approximately 17–19 kDa outer membrane antigen are the major specific protective antigens of M. viscosa, and that the M. viscosa species consists of many different serotypes. Significance and Impact of the Study:  The results are important for the production of vaccines against winter ulcers and should also lead to better methods of verifying the bacteria and monitoring winter ulcers.

Aims:  The sensitivity to lysis is a profound bottleneck to studies of the fish pathogen Moritella viscosa. The aim of this study was to examine the growth and the lysis process of M. viscosa cells under different physical and chemical conditions. Methods and Results:  Growth and cell lysis were studied under different conditions. The growth rate was highest at 15°C and lowest at 4°C, but the cells reached a higher density at 4° than at 15°C and the cells were more stable. The presence of minerals reduced lysis. Conclusions:  Premature lysis of the cells is dependent on environmental factors. Moritella viscosa should be cultivated and kept in media containing a certain set of minerals and at temperatures as low as 4°C. Formalin favours the stability of cells. The instability of the M. viscosa cells at temperatures above 10°C might be one of the factors responsible for their inability to infect fish at higher temperatures. The presence of DHA in the cell membranes is predicted to be responsible for the susceptibility of the cells to lysis. Significance and Impact of the Study:  The cultivation of M. viscosa cells is a key factor in studying the pathogenicity of the bacteria and in making an effective vaccine to prevent winter ulcers in farmed fish. The study provided recommendations on how to cultivate M. viscosa and how lysis of the cells can be minimized.