Effects of cimetidine on vaccine efficacy of orally administered live auxotrophic Edwardsiella tarda mutant in olive flounder (Paralichthys olivaceus)

• Cimetidine on oral vaccine efficacy of auxotrophic E. tarda mutant was investigated.
• Cimetidine increased CFU number of live E. tarda mutant arrived to the intestine.
• Cimetidine induced the highest serum agglutination activity and survival rates.

Protection of olive flounder (Paralichthys olivaceus) from virulent Edwardsiella tarda infection by oral immunization with two auxotrophic genes – alanine racemase gene (alr) and aspartate semialdehyde dehydrogenase (asd) gene – knock-out E. tarda mutant (ΔalrΔasd E. tarda) was previously demonstrated. In the present study, we investigated the effects of cimetidine, a histamine2 (H2) receptor antagonist, on the arrival of the live auxotrophic E. tarda mutant to the intestine of olive flounder and on the protective efficacy of the live ΔalrΔasd E. tarda oral vaccine. In the experiment to determine optimal administration time of cimetidine, fish administered cimetidine at 3 h before the administration of the E. tarda mutant showed the highest bacterial colony number from the intestine. In the experiment to determine optimal dose of cimetidine, the groups of fish administered 100 mg or 200 mg of cimetidine per kg of fish showed significantly higher CFU numbers in the intestine compared to the group intubated with cimetidine 10 mg or 50 mg per kg of fish. However, as there was no significant difference between cimetidine 100 mg and 200 mg groups, a dose of 100 mg per kg of fish was chosen as the optimal dose of cimetidine. In the oral vaccine efficacy experiment, the group of fish administered the live auxotrophic E. tarda mutant in combination with the above optimal regime of cimetidine showed the highest serum agglutination activity and survival rates, suggesting that cimetidine can be used to enhance the protective efficacy of the oral live bacterial vaccines in fish. To our knowledge, this is the first study that demonstrated cimetidine-mediated enhancement of protective efficacy in oral vaccine of fish.