Studies on Drug Resistant Plasmids Derived From Cultured Fish

Twenty percent of cultured carp and their pond water in Gunma and Saitama Prefecture were contaminated with Salmonella. The major isolates were S. typhimurium (30%), 82% of which were drug-resistant. Six kinds of R plasmids, five IncH and one IncK, were isolated from five multiply resistant S. typhimurium. No Salmonella could be isolated when the carp was washed thoroughly with clean fresh water.Each of five kinds of fresh water fish, -carp, ayu, eel, rainbow trout, and yamame-, carried drug-resistant bacteria, the majorioy of which were Aeromonas hydrophila, ranging from 70% (Carp) to 29% (Eel) in the isolates. All of these A. hydrophila were drug-resistant, and 44 R plasmids were isolated. They all belonged to the IncC group. As for 17 standard R plasmids belonging to different incompatibilities, seven kinds of R plasmids (IncFII, FV, C, O, P, T, X) were transferable to A. hydrophila, but only IncC R plasmid was stable in the new host. The results paralleled the epidemiologdcal studies. The IncC plasmid was also characteristic in its high transferability at relatively high salt concentration.