BIOCHEMICAL AND CONJUGATION STUDIES OF ROMET-RESISTANT STRAINS OF AEROMONAS-SALMONICIDA FROM SALMONID REARING FACILITIES IN THE EASTERN UNITED-STATES

Strains of Aeromonas salmonicida (n = 585) were collected from covertl y infected and diseased salmonid hosts from 12 hatcheries in the easte rn United States. Strains and sites were selected because of their pot ential for harboring antimicrobial resistance, in particular, to Romet (TM). Resistance to Romet was displayed by 315 strains (53.8%), which were isolated from all six host species sampled at 10 of 12 sites. Thi rty of the resistant strains (9.5%) from five sites had no zone of inh ibition, whereas the other strains had either confluent growth or resi stant colonies within a zone of inhibition. Fifty-one resistant strain s, representing each of the three resistance phenotypes, were selected for biochemical and antimicrobial comparisons with Romet-sensitive st rains. All were confirmed to be A. salmonicida, and no characteristic biochemical phenotypes were found to be associated with resistance to Romet. Differential resistances between resistant and sensitive strain s were detected to the antimicrobials oxytetracycline, tetracycline, s ulfadiazine, sulfamethizole, trimethoprim, and SXT, a potentiated sulf onamide composed of trimethoprim and sulfamethoxazole. Plasmid DNA iso lation and agarose gel electrophoresis were done for 25 Romet-resistan t strains, and R-plasmids, not present in sensitive strains, were dete cted in 23 of these. Two different sizes of R-plasmids were detected, one about 55 kilobase pairs long and another about 50 kilobase pairs. Two strains isolated from New York brook trout Salvelinus fontinalis h ad reduced confluent growth within a zone of inhibition but contained no large plasmids. This may indicate chromosomally mediated resistance . Conjugational mating studies evaluated transfer of the R-plasmid DNA using eight Escherichia coli recipients. Successful R-plasmid transfe r was accomplished with two donor strains (MI1 and MI2 from New Hampsh ire brook trout). Our results, in addition to those of other workers, illustrate the widespread resistance in A. salmonicida to approved ant imicrobials and the capacity of this bacterium to become resistant in the fish culture environment.