The Use of Antimicrobials in Salmon Aquaculture in Chile

In salmon aquaculture, infections caused by bacteria, viruses and other marine microbes are a frequent cause for huge mortality in salmon and ultimately revenue to fish farmers. Chile is the second largest producer of farmed salmon and trout in the world and their main strategy to prevent infection is the increased use of large quantities of antimicrobials. One problem that can arise from such frequent use of antimicrobials is the fact that not all the feed is ingested or digested and the active antimicrobials can end up in the surrounding environment. Once in the environment antimicrobials can select for resistant bacteria by increasing horizontal gene transfer (HGT) and through the recombination of antimicrobial resistance genes (ARG), leading to antimicrobial resistance (AR) in surrounding bacterial populations. Past studies have shown significant increases in bacteria resistant to certain antimicrobials in sediments from aquaculture sites. They have also found some evidence of similar ARGs in unselected bacteria from aquaculture and surrounding non-aquaculture control sites. This study by Shah et al. (2014) wanted to extend this study by looking at phenotypic resistance of a much larger range of antimicrobials in 200 unselected bacteria from sediment in aquaculture areas and surrounding non-aquaculture control areas as well as genotyping ARGs in these bacterial isolates.

Bacterial isolates were collected from aquaculture and non-aquaculture control sites in the Southern Chile (2008/09). Bacterial isolates were tested for antibiotic resistance to tetracycline, trimethoprim, oxolinic acid, amoxicillin, streptomycin, chloramphenicol, florfenicol, and sulfamethizole by disc diffusion. Identification of ARG was carried out using Southern hybridization as well as PCR and species of several unselected bacteria with AR were identified.  Finally the conjugative transfer of AR genes was carried out using E. coli as the recipient.

The results showed resistance to one or more antimicrobials was present in 81% of the isolates, irrelevant of what site they were from. They showed that AR in the isolates taken from sediments in Chilean aquaculture sites was much higher than other farming sites around the world. Take just tetracycline for instance, in the Chilean site 32% of the bacteria showed AR resistance to this antimicrobial whereas in a similar study in Washington (where far less antimicrobials are used annually) only 3-9% of the isolates showed tetracycline resistance. Unlike previous studies this study showed no significant difference in the proportion of AR bacteria between aquaculture and non-aquaculture sites. 9 isolates were tested for conjugation and 2 from the aquaculture site transferred AR to E. coli.

Chile’s reliance on antimicrobials, along with the lack of the use of vaccines and more long term methods to improve infection rates in aquaculture, mean that the results of this study are perhaps unsurprising with our current understanding of antibacterial resistance. This study reaffirms our understanding of this topic and underlines the importance of more long term methods of seafood safety. One limitation of this study maybe that only a few genes were looked at when comparing ARG inside and outside of aquaculture and so going forwards it could be beneficial to look at a wider range of genes.

Reviewed Paper  

Shah, S.Q., Cabello, F.C., L'Abée‐Lund, T.M., Tomova, A., Godfrey, H.P., Buschmann, A.H. and Sørum, H., 2014. Antimicrobial resistance and antimicrobial resistance genes in marine bacteria from salmon aquaculture and non‐aquaculture sites. Environmental microbiology, 16(5), pp.1310-1320.