Characterization of atypical Aeromonas salmonicida isolated from an epizootic ulcerative disease in carp (Cyprinus carpio)

Since 1996, an epizootic ulcerative disease has affected both coloured-and common carp in Japan. We isolated atypical Aeromonas salmonicida from ulcerative lesions in the skin of diseased fish. It took more than 5 days for the isolates to make visible colonies on heart infusion agar at 18deg;C. All isolates were Gram-negative, facultatively anaerobic, non-motile, short rods, which reacted positively to rabbit anti-A. salmonicida subsp. salmonicida ATCC14174 serum by fluorescent antibody technique. However, the isolates differed from typical A. salmonicida in the following characters : coloured carp isolates were negative in the catalase test, and common carp isolates were negative in the oxidase test. Sequence analysis of 16S rRNA genes indicated that they could be identified as atypical A. salmonicida, and that coloured carp isolates differed in only one base of the sequences from common carp isolates. Subcutaneous injections of common carp with a representative strain of each type confirmed its virulence in producing ulcerative lesions on the skin.