Detection of R Plasmid Encoded with Resistance to Florfenicol in Pasteurella piscicida.

Pasteurella piscicida strains resistant to florfenicol (FF), (Minimal inhibitory concentration 6.25 μg/ml) were isolated for the first time from diseased cultured yellowtail (Seriola quinqueradiata) in 1992. A transferable R plasmid encoded with resistance to FF was identified in FF resistant strains co-cultured with Escherichia coli RC85 (nal). The R plasmid also had resistance to chloramphenicol (CP), kanamycin (KM), sulfamonomethoxine (SA), and tetracycline (TC). The frequency of R plasmid transfer ranged from 10-4 to 10-6 per input donor cell for 2 h of co-cultivation with E. coli at 37°C. The R plasmid was highly homologous in DNA profiles with the R plasmids, encoding resistance to CP, KM, SA, and TC, that were detected in 1989 and 1991. The R plasmid detected in 1992 is thought to be a FF resistant determinant inserted into the R plasmid encoding resistance to CP, KM, SA, and TC.