MOLECULAR CHARACTERIZATION OF PLASMID-MEDIATED OXYTETRACYCLINE RESISTANCE IN AEROMONAS-SALMONICIDA

Using broth conjugation, we found that 19 of 29 (66%) oxytetracycline (OT)-resistant isolates of Aeromonas salmonicida transferred the OT re sistance phenotype to Escherichia coli. The OT resistance phenotype wa s encoded by high-molecular-weight R-plasmids that were capable of tra nsferring OT resistance to both environmental and clinical isolates of Aeromonas spp. The molecular basis for antibiotic resistance in OT-re sistant isolates of A. salmonicida was determined. The OT resistance d eterminant from one plasmid (pASOT) of A. salmonicida was cloned and u sed in Southern blotting and hybridization experiments as a probe. The determinant was identified on a 5.4-kb EcoRI fragment on R-plasmids f rom the 19 OT-resistant isolates of A. salmonicida. Hybridization with plasmids encoding the five classes (classes A to E) of OT resistance determinants demonstrated that the OT resistance plasmids of the 19 A. salmonicida isolates carried the class A resistance determinant. Anal ysis of data generated from restriction enzyme digests showed that the OT resistance plasmids mere not identical; three profiles were charac terized, two of which showed a high degree of homology.