Abstract

The most important problem of Chilean scallop culture is the occurrence of high larval mortalities caused by the specific development of pathogenic bacteria mainly belonging to Vibrio, Halomonas and Aeromonas species.^4-6 The use of florfenicol is frequent in Chilean scallop hatcheries, but no studies have been developed to evaluate the presence of resistance determinants in the resistant microbiota isolated from reared larvae under farming conditions. The present study was undertaken to gain information of the occurrence of floR and tet genes encoding for florfenicol and tetracycline resistance, respectively among florfenicol resistant strains. A number of 52 representative florfenicol-resistant isolates were recovered from larval cultures, identified by using the 16S rRNA gene sequence analysis,⁷ tested for susceptibility to 12 antimicrobials² and screened for the presence of the floR¹ and tet³ (tetA, tetC, tetD and tetE ) genes by PCR amplification. Most of the studied strains mainly belonged to the Pseudomonas, Pseudoalteromonas and Halomonas genus and were also resistant to streptomycin, chloramphenicol, oxytetracycline and co-trimexazole. Fifty-two percent of the isolates carried 1 or more of the tet genes examined. The distribution was as follows. The tetCgene was found in 7 isolates, the tetE gene was found in 14 isolates, whereas 6 strains carried both the tetC and tetEgenes. From these, 15 strains also harbored the floR gene, whereas 12 strains resulted negative for this gene. Otherwise, 14 strains carried only the floR gene, whereas in 11 strains none of the studied genes were detected. It is well stated that a main factor leading to the spread of antimicrobial resistance in aquaculture is the horizontal transfer of resistance genes between bacteria in animals or their environment, so the high prevalence of tet genes detected in florfenicol-resistant bacteria from reared scallop larvae ensures co-selection and persistence of the tetracycline resistance genes, contributing to rapid spread and increase in bacterial resistance to tetracyclines in scallop hatchery environments in Chile. This study was supported by FONDECYT (grant 1090793).

Acknowledgements

The authors thank the National Fund for Development of Science and Technology (FONDECYT) of Chile for financial support of this study (grant 1090793).

References

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