A protein toxin named Aeromonas salmonicida exoenzyme T (AcxT), which belongs to the family of ADP-ribosylating toxins, is disclosed as is a Calcium (or other cation concentration) dependent promoter of A. salmonicida. Also disclosed are diagnostic, preventive, and therapeutic techniques, including the preparation of bacterin vaccines based on AexT for inducing immunity against A. salmonicida infections.