Plasmid-mediated romet resistance of Edwardsiella ictaluri

Romet®, a potentiated sulfa drug composed of five parts sulfadimethoxine and one part ormetoprim, is used to treat channel catfish Ictalurus punctatus infected with Edwardsiella ictaluri, the causal agent of enteric septicemia of catfish (ESC). Recently, several Romet-resistant isolates of E. ictaluri were isolated from channel catfish that had died from ESC in Virginia and Mississippi. This antimicrobial resistance was determined to be plasmid-encoded, as shown by agarose gel electrophoresis of plasmid DNA, and by the fact that the plasmid and associated antimicrobial resistance could be transferred to a recipient by single-step conjugation. Size of the R plasmid was approximately 55 kilobase pairs. Plasmids also conferred resistance to tetracycline, oxytetracycline, streptomycin, trimethoprim, and SXT (another potentiated sulfonamide, composed of trimethoprim and sulfamethoxazole). Each Romet-resistant E. ictaluri isolate was mated with a plasmid recipient, the Romet-sensitive Escherichia coli isolate 1932. Transconjugates were selected by plating on Mueller-Hinton agar that contained antimicrobials, and the mean transfer frequency was 1.483 × 10⁻³ transconjugates per donor cell in the mating mixture. Each E. coli transconjugate was then mated with 20 Romet-sensitive isolates of E. ictaluri to assess the propensity of these E. ictaluri isolates to accept the R plasmid. The average transfer frequency when E. ictaluri isolates of channel catfish origin (N = 14) were used as recipients was 1.197 × 10⁻² transconjugates per donor cell in the mating mixture; mean transfer frequency when E. ictaluri isolates not originating from channel catfish were used was 1.019 × 10-3 transconjugates per donor cell, which was significantly less (P = 0.0002).