The characterization of antibiotic-resistant vibrios isolated from shellfish aquaculture is necessary to elucidate the potential transfer of resistance and to establish effective strategies against vibriosis. With this aim, we analyzed a collection of bacterial isolates obtained from 15 failed hatchery larval cultures that, for the most part, had been treated experimentally with chloramphenicol to prevent vibriosis. Isolates were obtained during a 2-year study from experimental cultures of five different clam species. Among a total of 121 Vibrio isolates studied, 28 were found to be chloramphenicol resistant, suggesting that the shellfish hatchery had been using a sublethal concentration of the antibiotic. Interestingly, chloramphenicol-resistant vibrios showed also resistance to tetracycline and amoxicillin (group A; n = 19) or to streptomycin (group B; n = 9). Chloramphenicol-resistant vibrios were subjected to a PCR amplification and DNA sequencing of the chloramphenicol acetyltransferase genes (cat), and the same approach was followed to study the tetracycline resistance markers (tet). 16S ribosomal RNA (rRNA) gene sequencing revealed that chloramphenicol-resistant vibrios pertained mostly to the Splendidus clade. Conjugation assays demonstrated that various R-plasmids which harbored the cat II/tet(D) genes and cat III gene in groups A and B respectively, were transferred to E. coli and bivalve pathogenic vibrios. Most interestingly, transconjugants exhibited the antibiotic resistance patterns of the donors, despite having been selected only on the basis of chloramphenicol resistance. This is the first report carried out in a bivalve hatchery elucidating the persistence of resistant vibrios, the mechanisms of antibiotic resistance, and the transfer of different R-plasmids.
Keywords: Antibiotic resistance genes; Bivalve cultures; R-plasmids; Shellfish hatchery; Vibrio spp..