A recombinant protein for the S-layer protein of Aeromonas hydrophila was produced and its ability to
protect common carp Cyprinus carpio L. against six virulent isolates of A. hydrophila was assessed. A
group of 120 carp (30–40 g) were vaccinated intra-peritoneally with 0.1 ml of adjuvanted vaccine (30g
protein per fish). Another group of 120 carp were injected with 0.1 ml of PBS-adjuvant mixture to serve
as controls. Twenty fish from each group were challenged with each one of six virulent isolates of A.
hydrophila 35 days post-vaccination. The fish were maintained in 12 separate tanks before terminating
the experiment at 16 days post-challenge. The relative percentage survival (RPS) for the six isolates of
A. hydrophila ranged from 56 to 87%. The difference in survival rate of fish challenged with four of the
isolates was statistically significant in vaccinated fish compared to control fish, when analysed using a
Chi-square test. The results of the study suggest that the recombinant S-layer protein of A. hydrophila
could be useful as a vaccine antigen to protect fish against different isolates of this pathogenic bacterium.