Plasmid profiling of Flavobacterium psychrophilum isolates from ayu (Plecoglossus altivelis altivelis) and other fish species in Japan.

In order to evaluate the genetic variability of the causative agent of cold water disease (CWD), plasmid profiling was used to characterize Flavobacterium (F.) psychrophilum isolates (n = 169). Size analysis of plasmids in F. psychrophilum isolates (n = 128) from several fish species demonstrated that six kinds of plasmids were harbored, and ayu isolates had different profiles compared to other isolates. Moreover, multiple isolates (n = 41) from CWD outbreaks in 2002 to 2003 at a single ayu farm were examined to determine differences between isolates from successive outbreaks and showed different profiles by the sources of seedlings.