To develop an accurate diagnosis of “new ulcer disease” in koi carp, we produced four monoclonal antibodies (mAbs) against a strain of atypical Aeromonas salmonicida isolated from koi carp. These mAbs did not cross-react with an isolate of atypical A. salmonicida from Japanese flounder and other pathogenic bacteria. Re-isolation from artificially infected koi carp was achieved by selecting the blue colonies on agar medium containing Coomassie brilliant blue, and some of the colonies were detected by immunofluorescent staining using the mAbs. These results suggested that the mAbs can distinguish atypical A. salmonicida from koi carp from resident aeromo-nads.
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