Extended antibiotic treatment in salmon farms select multiresistant gut bacteria with a high prevalence of antibiotic resistance genes

Abstract

The high use of antibiotics for the treatment of bacterial diseases is one of the main problems in the mass production of animal protein. Salmon farming in Chile is a clear example of the above statement, where more than 5,500 tonnes of antibiotics have been used over the last 10 years. This has caused a great impact both at the production level and on the environment; however, there are still few works in relation to it. In order to demonstrate the impact of the high use of antibiotics on fish gut microbiota, we have selected four salmon farms presenting a similar amount of fish of the Atlantic salmon species (Salmo salar), ranging from 4,500 to 6,000 tonnes. All of these farms used treatments with high doses of antibiotics. Thus, 15 healthy fish were selected and euthanised in order to isolate the bacteria resistant to the antibiotics oxytetracycline and florfenicol from the gut microbiota. In total, 47 bacterial isolates resistant to florfenicol and 44 resistant to oxytetracycline were isolated, among which isolates with Minimum Inhibitory Concentrations (MIC) exceeding 2048 μg/mL for florfenicol and 1024 μg/mL for oxytetracycline were found. In addition, another six different antibiotics were tested in order to demonstrate the multiresistance phenomenon. In this regard, six isolates of 91 showed elevated resistance values for the eight tested antibiotics, including florfenicol and oxytetracycline, were found. These bacteria were called “super-resistant” bacteria. This phenotypic resistance was verified at a genotypic level since most isolates showed antibiotic resistance genes (ARGs) to florfenicol and oxytetracycline. Specifically, 77% of antibiotic resistant bacteria showed at least one gene resistant to florfenicol and 89% showed at least one gene resistant to oxytetracycline. In the present study, it was demonstrated that the high use of the antibiotics florfenicol and oxytetracycline has, as a consequence, the selection of multiresistant bacteria in the gut microbiota of farmed fish of the Salmo salar species at the seawater stage. Also, the phenotypic resistance of these bacteria can be correlated with the presence of antibiotic resistance genes.

Occurrence of multiple antibiotic resistance and genotypic characterization inEdwardsiella tardaisolated from cage‐cultured hybrid red tilapia (Oreochromissp.) in the Ping River, Northern Thailand

“…These observations suggest that there is a general trend of increasing resistance to oxytetracycline and sulpha‐trimethoprim of E. tarda isolates in aquaculture. It has been well documented that consistent long‐term exposure to antibiotics or overuse of antibiotics in aquaculture may result in the development of bacterial resistance (Higuera‐Llanten et al, ; Santos & Ramos, ; Usui et al, ). A study by Rico et al () reported that the commonly used antibiotics for the prevention and treatment of bacterial infection in Thailand were oxytetracycline, amoxicillin and sulpha‐trimethoprim.…”

High prevalence of class 1 integrase and characterization of class 1 integron gene cassettes in multiresistant bacteria isolated from the gut microbiota of extended antibiotic treated Salmo salar fish farms

“…The other strains containing the genes sulI and qacΔE , in the typical structure of a class 1 integron, showed no gene cassettes in the variable region, which implies that the system has a great potential to incorporate and express new antibiotic resistance genes. Nonetheless, it is important to note that none of the antibiotic resistance genes to florfenicol and oxytetracycline described in both banks by Higuera-Llanten et al 2018 [14] are found in the integrons of the isolated bacteria. Currently, a complete metagenomic analysis is being carried out to find the relationship between these resistance determinants and these genetic elements that could probably be established in non-culturable bacteria.…”

“…For this study, characterized banks of resistant bacteria to florfenicol and oxytetracycline isolated in our lab were used [14]. Shortly, four Atlantic salmon ( Salmo salar ) fish farms were chosen, located in the Aysén Region, North Patagonia, Chile.…”

“…Both banks were taxonomically characterized by amplification and sequencing of the 16S gene. The bank of bacteria resistant to florfenicol (FB) consists of 47 isolates, while the bank of bacteria resistant to oxytetracycline (OB) consists of 44 isolates [14]. This study was carried out in accordance with law 20,380 regarding animal welfare, as set out by the Chilean Health Ministry in the use of wild or protected animal species in biomedical research and approved by the National Fisheries Service (SERNAPESCA) and the Pontificia Universidad Católica de Valparaíso Bioethical Committee.…”

“…The administration of medicated food fundamentally affects the gut microbiota of the fish [12], since the constant exposure to antimicrobials leads to the selection of resistant bacteria and the increase of horizontal gene transfer (HGT) of those elements containing Antibiotic Resistance Genes (ARGs) [13]. In a recent article published by our laboratory, we have described that subsequent treatments with antibiotics select multiresistant bacteria with a high prevalence in ARGs both to florfenicol and oxytetracycline in the gut microbiota of fish [14].…”