Phenotype, Virulence and Immunogenicity of Aeromonas salmonicida Cyclic Adenosine 3',5'-Monophosphate Receptor Protein (Crp) Mutants in Fish Host

Aeromonas salmonicida is a Vibrionaceae family member that causes a lethal disease called furunculosis in marine and freshwater fish. Being a mucosal facultative intracellular pathogen, this bacterium is an excellent candidate for developing immersion-oral live attenuated vaccines for the salmon-trout aquaculture industry. Deletion of the cyclic 3’,5’-adenosine monophosphate (cAMP) receptor protein (crp) gene has been utilized in live attenuated vaccines for mammals and birds. Here we characterize the crp gene and report the effect of a crp deletion in A. salmonicida. The A. salmonicida crp gene and encoded protein are similar to other Enterobacteriaceae and Vibrionaceae family members, complementing Salmonella enterica and Edwardsiella Δcrp mutants in a cAMP-dependent fashion. The A. salmonicida Δcrp-12 in frame deletion mutant demonstrated slight growth defects, loss of maltose utilization among other sugars, and lack of brown pigment synthesis. We found that the A. salmonicida Δcrp-12 mutant was attenuated and conferred immune protection against A. salmonicida infection to the fish. We propose that deletion of the crp gene in A. salmonicida is an effective strategy for developing immersion live attenuated antibiotic-sensitive vaccines for the catfish aquaculture industry.