Cloning and expression of outer membrane proteins from Vibrio alginolyticus, and evaluation of their immune response in Epinephelus coioides.

Vibrio alginolyticus and Vibrio parahaemolyticus pose significant threats to various marine fish species, with mortality rates reaching as high as 90 %. Outer membrane proteins represent promising antigens for creating immune responses. The bacterial strains V. alginolyticus A2 and V. parahaemolyticus N9 were isolated from diseased grouper samples collected in Vietnam. The two bacterial strains, V. alginolyticus A2 and V. parahaemolyticus N9, were identified as highly virulent, causing disease in Epinephelus coioides. This study assesses the protective efficacy of the recombinant proteins OmpK and OmpU, derived from the ompK and ompU genes of V. alginolyticus A2, in E. coioides against the V. parahaemolyticus N9 and V. alginolyticus A2 strains. The ompK and ompU genes were 798 bp and 1023 bp in length, respectively, encoding polypeptide chains of 265 and 340 amino acids. The amino acid sequences of OmpK and OmpU proteins demonstrated high conservation across the other Vibrio species. The relative percent survival (RPS) for the OmpK protein in orange-spotted groupers challenged with V. alginolyticus A2 and V. parahaemolyticus N9 was 89.18 % and 72.22 %, respectively. Similarly, the group of fish injected with OmpU protein exhibited the RPS values of 82.14 % and 61.11 % against V. alginolyticus A2 and V. parahaemolyticus N9, respectively. Both recombinant antigenic proteins, OmpK and OmpU, induced a strong immune response in E. coioides to V. alginolyticus A2. Most interestingly, OmpK could also cross-protect against V. parahaemolyticus. This result suggests that OmpU and OmpK protein could be used as potential candidates for vaccine development targeting V. alginolyticus and V. parahaemolyticus.