Antimicrobial Use In Aquaculture and Microbial Diversity

             Antimicrobials are widely used in salmon aquaculture. This use in the aquatic environment can potentially decrease bacterial diversity by eliminating susceptible organisms and simultaneously selecting for resistant ones. These effects and the emergence of antimicrobial resistant bacteria are directly linked and proportional to the amounts of antibiotic used in a particular geographical location.

            Studies of salmon aquaculture in Chile strongly indicate that the amounts of some antimicrobials, including tetracyclines, quinolones and florfenicol, used in this industry are larger than those used in human medicine and other veterinary activities. This use in salmon aquaculture makes it the most important current and future selective pressure on the development of antimicrobial resistance in this country. Studies of sediments from salmon aquaculture-impacted and non-impacted sites indicate that these sediments appear to contain sufficient amounts of antimicrobials to exert selective pressure upon the bacteria contained in them.

            Molecular analysis of bacteria isolated from these sediments has revealed that their genomes contain a variety of antimicrobial resistance genes coding for resistance to tetracycline, quinolones and florfenicol. These resistant bacteria can be selected in vitro, and probably in situ, by the presence of residues of antimicrobials in the sediments. The occurrence of some of these genes in genetic elements such as integrons, coupled to the presence of residual antibiotics in the sediment, also indicate that the potential exists for dissemination of these resistance determinants among bacterial populations by horizontal gene tranfer. This potential ability is consistent with information indicating that bacteria from aquatic environments and terrestrial environments including human pathogens share antibiotic resistance determinants and the mobile genetic elements harboring them.

             In summary, injudicious use of antimicrobials in aquaculture decreases bacterial diversity, selects for bacteria resistant to these antimicrobials and is associated with potentially negative impacts on piscine and human health.

              Funded by the Lentfest Ocean Program/Pew Charitable Trusts