Aquaculture is an essential food production sector worldwide and is currently experiencing a significant growth. However, the pollution of surface waters with fecal bacteria from humans and terrestrial animals may pose a food and environmental safety risk in aquaculture settings relying on surface waters . This study aims at identifying sources of antimicrobial resistance carried by bacteria classically considered fecal indicators (Escherichia coli, Enterococcus faecium , Enterococcus faecalis), and at estimating their persistence in freshwater fish farms from north-west France.
Four freshwater fish farms were recruited for this longitudinal study: two farms operating as flow-through systems and two farms operating as recirculating aquaculture systems (RAS). For each type of system , we selected a farm located downstream from a waste water treatment plant. On t wo sampling dates per farm (autumn and winter) we collected fish feed, w ater, sediment, and biofilm samples from several points of the fish farm, following the sampling scheme designed by Novais et al (2018) . Bacterial isolates were identified with either MALDI-TOF or PCR. A ntimicrobial susceptibility testing was performed on a selection of 43 confirmed Escherichia strains with the broth microdilution method (determination of Minimum Inhibitory Concentrations ) against a set of 15 antibiotics.
Out of 234 samples, 64 were positive for Escherichia coli (25.6%) and 20 for Escherichia marmotae (8.5 %), inc luding water, sediment and biofilm . Concerning water, 26 out of 81 samples were positive for E. coli , which suggests that water is an important vehicle of this bacterium into fish farms. The fact that 31 out 85 biofilm samples carried E. coli suggests that it may persist in this ecological compartment. Furthermore, with the use of epidemiological cut-off values we found resistance to sulphonamides (34/43), ampicillin (4/43), tetracycline (4/43) , gentamicin (3/43) ciprofloxacin (2/43) and colistin (1/43). Nine Escherichia isolates showed an antimicrobial multiresistant profile. Enterococcus results are pending.
These preliminary results underscore the importance of monitorin g and addressing fecal pollution in surface waters used by aquaculture and aquatic leisure activities. Further testing is needed on the strains showing resistance to antibiotics commonly used in aquaculture such as tetracycline and sulphonamides : if these resistance phenotypes were conferred by mobile genetic elements transferable to bacterial fish pathogens, the effectiveness of these drugs in controlling bacterial infections in fish populations would be compromised. Understanding the broader implications of fecal pollution in surface waters is highly important for ensuring aquaculture sustainability and safeguarding public health.
Reference: Novais et al (2018) Water supply and feed as sources of antimicrobial-resistant Enterococcus spp. in aquacultures of rainbow trout (Oncorhyncus mykiss), Portugal. Science of the Total Environment 625: 1102–1112