Antibiotic resistance and plasmid profiles of vibrio isolates from cultured silver sea bream, Sparus sarba

A total of 51 potential pathogenic vibrios were isolated from moribund silv er sea bream (Sparus sarba) collected from fish farms in Hong Kong, Using t he API 20E system and the scheme of Alsina and Blanch (1994), 7 species mer e identified from all isolates, These species were Vibrio alginolyticus (24 strains), Vibrio vulnificus (12 strains), Vibrio parahaemolyticus (7 strai ns), Vibrio logei (4 strains), Vibrio pelagius II (2 strains), Vibrio fluvi alis (1 strain) and Vibrio meditterranei (1 strain). The three dominant spe cies (V. alginolyticus, V. vulnificus and V. parahaemolyticus) were confirm ed to be virulent to sea bream by experimental challenge. All isolates were screened for plasmid DNA by agarose gel electrophoresis a nd tested for susceptibility to 16 antibiotics by the agar dilution method. Of the 51 isolates examined, all strains were sensitive to ceftriaxone, st reptomycin, nalidixic acid and rifampicin, and almost all were sensitive to ceftazidime, netilimicin, chloramphenicol and sulfamethoxazole, Most isola tes were resistant to ampicillin (60.8%), cefuroxime (66.7%), amikacin (55% ), kanamycin (58.8%) and trimethoprim (76.5%), Fifteen of the 51 isolates h arboured 1-4 plasmids, with sizes ranging from 9 to 123 kb, Both the plasmi ds and the associated antibiotic resistance (ampicillin, cefuroxime and tri methoprim) of 9 isolates could be transferred to a recipient by single-step conjugation, However, the frequencies were very low, ranging from 10(-11) to 10(-9). The present results indicate that resistance to these antibiotic s is chromosomal, (C) 1999 Elsevier Science Ltd. All rights reserved.