Antigen uptake and immune responses after oral vaccination.

Oral delivery of vaccine, with antigen simply mixed with feed, is the easiest method of mass vaccination of fish of all sizes. It is time and labour-saving, and avoids any manipulation stress. However, large quantities of antigen are necessary and it is not possible to know the exact dose absorbed by each fish. Moreover the resulting protection is generally weak and of short duration. To improve the effectiveness of oral vaccination a considerable amount of work has been done in the last 15 years. It is now well established that the second segment of the hindgut of fish has the ability to absorb soluble or particulate antigens. These antigens reach the intraepithelial macrophages which show antigenic determinants on their cell membrane, suggesting an antigen-presenting function. Moreover, a gut-associated lymphoid tissue composed of several lymphoid cell types, macrophages, granulocytes and plasma cells has been described in this region of the gut and recent studies indicate that it is involved in an intestinal immune response. Both mucosal and systemic immune responses seem to develop as indicated by the presence of antibodies in gut mucus, bile and serum. However, these responses are higher following anal delivery of antigen than oral delivery. It is generally considered that during oral delivery antigens are digested in the foregut and/or in the stomach. Thus studies are being carried out now on ways to protect antigens, for instance through the encapsulation of antigen, the neutralisation of gastric secretions and the use of oral adjuvants.

Application of a modified disc diffusion technique to antimicrobial susceptibility testing of Vibrio anguillarum and Aeromonas salmonicida clinical isolates.

Two techniques for antimicrobial susceptibility testing of Vibrio anguillarum and Aeromonas salmonicida strains were compared. The first method was the reference test that determines Minimal Inhibitory Concentrations (MIC); the second was a modified diffusion test that measures the Inhibitory Concentrations in Diffusion (ICD) by carrying out the diffusion test with five discs of differing contents. ICD measurement was not applicable for the susceptibility testing of oxytetracycline and sulfadimethoxine. On the other hand, a good correlation between the MICs and the ICDs was observed for oxolinic acid, sarafloxacin, chloramphenicol and trimethoprim. Moreover, the ICD values were close to those obtained for the MIC values. A. salmonicida resistant strains were detected by ICD determination. Thus, ICD could be used instead of MIC for oxolinic acid, sarafloxacin, trimethoprim and chloramphenicol susceptibility testings. The ICD technique is easy to carry out and is not dependent on the growth characteristics of bacteria.

The effect of incubation temperature and sodium chloride concentration on the growth kinetics of Vibrio anguillarum and Vibrio anguillarum-related organisms.

The effect of temperature and NaCl concentration on the growth kinetics of Vibrio anguillarum and V. anguillarum-related (VAR) strains was studied. For one wild VAR strain, NaCl concentration interfered with growth temperature parameters, in particular, with the maximum growth temperature but also with the optimum temperature (defined as the temperature at which mumax equals its maximal value muopt), and with muopt itself. For the same strain, optimal growth required the adding of NaCl to the medium to a final concentration of 1.5%. These results were not confirmed by tests on a V. anguillarum collection strain. When the NaCl concentration in the culture media was 1.5%, the optimum temperature for the nine strains studied ranged from 29.7 degrees C to 34 degrees C whereas the maximum temperature ranged between 35.3 degrees C and 38.5 degrees C. Hence, antibiotic susceptibility testing as well as biochemical identification might be carried out at 30 degrees C in the presence of 1.5% NaCl, which corresponded to a suboptimal growth.