Contaminated Seawater Drives Resistant Bacteria Through the Oyster Supply Chain

Oysters are prized…

The findings from Florianópolis add an important dimension to ongoing debates about how antimicrobial resistance moves through food systems. While much of the global attention on resistant bacteria has focused on hospitals, livestock production, and terrestrial agriculture, coastal aquaculture occupies a distinctive position at the interface between human wastewater, environmental reservoirs, and direct food consumption. Oysters are particularly informative sentinels in this setting because their filter-feeding behavior concentrates particles and microorganisms from large volumes of seawater, effectively integrating the microbial signature of the surrounding environment over time. When that environment carries fecal contamination from urban runoff or inadequate sewage treatment, the bacteria it harbors, including strains carrying extended-spectrum β-lactamase genes and other resistance determinants, can be taken up and retained by the animals destined for human consumption.

The observation that identical sequence types and resistance profiles appeared at multiple points along the supply chain, from seawater through farmed oysters to commercially sold product, suggests that contamination is not randomly distributed but follows traceable routes of dissemination. This kind of genomic concordance is a powerful epidemiological signal. Whole genome sequencing allows researchers to distinguish between merely similar organisms and truly related clones, and when the same clone with the same resistance gene complement is recovered from water and from oysters offered to consumers, the most parsimonious explanation is that the water source seeded the product. It also implies that interventions targeting water quality at the farm level would have downstream benefits that extend all the way to the point of sale, whereas interventions applied only at later stages would leave the upstream reservoir intact.

The prominence of Escherichia coli and Klebsiella pneumoniae among the 167 resistant isolates deserves particular attention. Both species are members of the Enterobacterales order and are common inhabitants of the human and animal gut, which makes their presence in coastal waters a reliable indicator of fecal pollution. Klebsiella pneumoniae, in particular, is classified by the World Health Organization among the critical priority pathogens for which new treatments are urgently needed, largely because of its capacity to acquire and stack resistance genes, including carbapenemases in some lineages. Finding such organisms in an aquaculture environment intended for raw consumption underscores the stakes involved. The detection of extended-spectrum β-lactamase genes is especially concerning because these enzymes degrade penicillins and cephalosporins, the antibiotic classes most heavily used in both human medicine and, in some regions, animal husbandry. The additional identification of determinants conferring resistance to quinolones, aminoglycosides, tetracyclines, sulfonamides, and trimethoprim reflects the broader pattern seen in environmental Enterobacterales, where resistance rarely arrives as a single trait but instead arrives as a package of linked genes.

The plasmid association of many of these determinants is central to understanding why. Plasmids are mobile genetic elements that can replicate independently of the bacterial chromosome and transfer horizontally between strains and even between species. A resistance gene embedded on a broad-host-range plasmid can move from a relatively benign commensal organism into a more dangerous pathogen under the right selective conditions. Coastal waters receiving mixed inputs of human sewage, animal waste, and agricultural runoff provide exactly those conditions, bringing diverse bacterial populations into sustained contact while antimicrobial residues, even at subinhibitory concentrations, favor the maintenance of resistance plasmids. In this sense, the oyster supply chain functions not merely as a conduit for resistant bacteria but as a potential arena for genetic exchange, where resistance determinants circulating in the environment can recombine and redistribute among bacterial hosts before reaching consumers.

The regulatory context highlighted in the study reveals a structural gap that extends well beyond Brazil. The major shellfish-producing nations, including the United States through its National Shellfish Sanitation Program, the European Union through Regulation No 853/2004, China through its national food safety standards, and Australia through its Shellfish Quality Assurance Program, all anchor their monitoring frameworks on Escherichia coli or fecal coliform counts. These indicators are effective at signaling fecal contamination and have demonstrably reduced the burden of classic shellfish-associated illnesses. However, they were never designed to detect antimicrobial resistance. A water sample can meet the regulatory threshold for E. coli concentration while still harboring a subpopulation of resistant strains, and current classification schemes provide no mechanism to flag that risk. The Brazilian regulation, Portaria SDA/MAPA No. 884 of 2023, follows the same indicator-based logic. The consequence is that oysters harvested from waters deemed suitable for cultivation can nonetheless carry WHO critical-priority pathogens bearing clinically significant resistance genes, a scenario the Florianópolis data appear to confirm directly.

Against this backdrop, the finding that depuration reduced both resistant bacteria and the prevalence of resistance genes in oysters carries substantial practical weight. Depuration exploits the natural filtration and clearance physiology of bivalves by holding them in tanks of clean, treated, and monitored water, during which they gradually expel accumulated microorganisms. The process is already mandated or recommended in several regulatory systems for harvesting areas with moderate contamination, but its effectiveness against antimicrobial-resistant organisms specifically had been less well characterized. The study’s results suggest that depuration does not merely reduce total bacterial load but preferentially diminishes the resistant fraction and the resistance genes they carry, which is a more meaningful food safety outcome from an antimicrobial resistance perspective. This positions depuration as one of the few practical, commercially scalable interventions currently available that addresses the resistance dimension of shellfish safety rather than only the conventional microbiological indicators.

Several caveats and avenues for further work naturally follow from these findings. Depuration operates on a defined timescale, and the persistence of certain bacterial strains or genes beyond typical depuration periods remains an open question, as does the possibility that some organisms internalize in tissues in ways that resist clearance. The relationship between seawater contamination levels and resistance levels in marketed oysters, which the study observed at sites with higher environmental ARB burden, points toward water quality management as the upstream lever with the greatest potential impact. Improving sewage treatment in coastal watersheds, controlling agricultural and urban runoff, and monitoring resistance genes directly in aquaculture waters would complement depuration rather than replace it. From a One Health perspective, the study illustrates how human health outcomes at the seafood counter are connected to environmental conditions in the bay, to wastewater infrastructure on shore, and to the microbial ecology of coastal ecosystems, and it demonstrates that genomic surveillance along the supply chain can make those connections visible in actionable detail.

Subject of Research: Antimicrobial-resistant bacteria in the oyster supply chain and the effect of depuration

Article Title: Microbiological and genomic surveillance of WHO critical priority pathogens along the oyster supply chain: water quality as a driver of persistence and depuration as a mitigating strategy in a One Health context

Article References: Rocha, G., Ribas, M. R., de Assunção, I. B. F., Dal Pizzol, J. L., Wolleck, V. F., de Jesus, J. R., da Silva, G. S., Palmeiro, J. K., & Sincero, T. C. M. (2026). Microbiological and genomic surveillance of WHO critical priority pathogens along the oyster supply chain: water quality as a driver of persistence and depuration as a mitigating strategy in a One Health context. Frontiers in Bacteriology, 5, Article 1876999. https://doi.org/10.3389/fbrio.2026.1876999

Image Credits: AI Generated

DOI: 10.3389/fbrio.2026.1876999

Keywords: antimicrobial resistance, oysters, depuration, water quality, Escherichia coli, Klebsiella pneumoniae, ESBL, whole genome sequencing, One Health, food safety, plasmids, WHO priority pathogens