Effects of Chemotherapeutics on Bacterial Ecology in the Water of Ponds and the Intestinal Tracts of Cultured Fish, Ayu ( Plecoglossus altivelis )

Drug‐resistant gram‐negative bacilli conferred with R factors were isolated with high frequencies from the intestinal tracts of ayu ( Plecoglossus altivelis ) cultured in ponds, in which chemotherapeutics had often been used, and with relatively low frequencies from ayu which received no administration of chemotherapeutics. Drug‐resistant bacteria were also isolated at low frequencies from the intestinal tracts of wild ayu in rivers, as well as from the water of ayu‐culturing ponds and some of them carried R factors. The drug‐resistant bacteria carrying R factors were Aeromonas liquefaciens, Citrobacter, Enterobacter cloacae, Escherichia coli, Hafnia and unidentified strains. All the R factors were classified as the Fi – (F) type, except the two R factors detected in an E. coli strain and in an unidentified strain.