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SUMMARYThirty-nine out of 72 drug-resistant strains of Aeromonas liquefaciens isolated from cultured fish and soft-shelled turtles (Trionyx sinensis japonicus) in various districts of Japan carried transferable drug resistance factors. The fish included eel (Anguilla japonica), carp (Cyprinus carpio), ayu (Plecoglossus altivelis) and goldfish (Carassius auratus). The most common type of resistance factors had the markers of resistance to sulphanilamide and tetracycline and all belonged to the Ji-type. Transferable drug resistance was not found in any of 12 strains of A. liquefaciens isolated from wild eels. The high incidence of resistance factors in A . liquefaciens from cultured fish is assumed to be due to the selective pressure exerted by chemotherapeutics used in fish culturing.

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