The research team, led by María Josefina Gonzalez of the Instituto Nacional de Limnología (INALI, CONICET-UNL) together with colleagues from the Universidad Nacional del Litoral, the Universidad Autónoma de Entre Ríos, the Universidad Nacional del Litoral’s engineering faculty, and the Universidad de Buenos Aires, isolated a total of 684 E. coli strains: 437 from water samples and 247 from sediments collected at sites surrounding Santa Fe City. When the researchers tested these strains against a panel of antibiotics using standards established by the Clinical and Laboratory Standards Institute, they found that 36.1 percent were resistant to at least one drug. The resistance profile was dominated by older, widely used antibiotics: ampicillin, tetracycline, and the combination trimethoprim–sulfamethoxazole headed the list, followed by ciprofloxacin, a fluoroquinolone that remains a mainstay of clinical practice.

What makes the study particularly consequential, however, is what the team found at the harder end of the therapeutic spectrum. Resistance to colistin, an antibiotic often described as a last-resort drug for infections caused by multidrug-resistant Gram-negative bacteria, was detected in both water and sediment isolates. Colistin resistance is a global concern because the mobilized colistin resistance (mcr) genes that often underpin it can transfer between bacterial species, effectively spreading the ability to shrug off one of medicine’s final options. The study also documented resistance to third-generation cephalosporins at 40 percent of the sampled sites, including locations where the researchers detected strains producing extended-spectrum β-lactamases, or ESBLs. These enzymes confer resistance to most beta-lactam antibiotics, including penicillins and many cephalosporins, and ESBL-producing E. coli are a leading cause of difficult-to-treat urinary tract and bloodstream infections worldwide.

The geography of resistance within the floodplain proved as revealing as its biology. Sites associated with waste elimination, particularly those located in socioeconomically vulnerable urban areas where informal settlements lack adequate sanitation infrastructure, stood out on two fronts simultaneously. First, these locations showed E. coli concentrations that exceeded recreational water quality standards set by the U.S. Environmental Protection Agency, the benchmark commonly used to judge whether water is safe for swimming and other contact activities. Second, the strains recovered from these hotspots displayed the highest proportions of multidrug resistance, meaning individual bacteria were simultaneously resistant to several antibiotic classes. The convergence of fecal contamination, high bacterial loads, and dense resistance profiles marks these environments as the most dangerous points in the landscape from the standpoint of potential human exposure.

To translate laboratory findings into public health terms, the researchers conducted a screening-level assessment of exposure during recreational water activities. The analysis, which the authors are careful to frame as an estimate of potential exposure rather than a quantitative infection risk assessment, produced a striking result: at the most contaminated sites, a person could potentially ingest E. coli resistant to up to five different antibiotic classes during a single swimming session. While such estimates depend on assumptions about water ingestion volumes and bacterial densities, the exercise underscores a point that environmental microbiologists have been making for years: recreational waters are not just a nuisance exposure but a plausible route by which resistance genes and resistant bacteria move from the environment into human populations.

The sediments of the floodplain add another dimension to the problem. Aquatic sediments are known to act as long-term repositories for fecal indicator bacteria, where microorganisms can persist far longer than in the water column above, protected from sunlight and grazing by the physical structure of the sediment matrix. Studies in urban estuaries elsewhere have shown that benthic-pelagic coupling, the exchange of organisms and nutrients between sediment and water, can repeatedly reseed overlying waters with fecal bacteria even after the original contamination source has been reduced. The detection of resistant strains in the Paraná floodplain sediments, including resistance to colistin and third-generation cephalosporins, suggests that these deposits could serve as a persistent reservoir that continues to supply resistant bacteria to the water long after pollution events occur, complicating any remediation effort.

The drivers behind this pattern are consistent with what global research on antimicrobial resistance in urban waters has identified. Antibiotic misuse, weak regulatory frameworks governing pharmaceutical discharge, and inadequate sanitation all contribute to the environmental dissemination of resistance. Argentina is far from alone: pharmaceutical pollution of the world’s rivers is now documented on every inhabited continent, and surveillance studies in Europe have used urban wastewater as a window onto resistance circulating in human populations. What the Santa Fe study adds is a detailed picture from a large South American river floodplain, a landscape type that dominates much of the Paraná basin and supports dense human settlement alongside productive fisheries and heavy recreational use. In such settings, the boundary between environmental and clinical resistance is unusually porous.

The equity dimension of the findings deserves particular attention. The hotspots of multidrug resistance were concentrated in socioeconomically vulnerable urban areas, places where residents often have no alternative water sources for washing, fishing, or recreation. This pattern echoes research from informal settlements on other continents, which have been described as hotspots of antimicrobial resistance where environmental transmission is difficult to curb. From an environmental justice perspective, the communities least equipped to protect themselves from exposure to resistant bacteria are those living closest to the contamination sources. The authors situate their work explicitly within the One Health framework, which recognizes that human health, animal health, and environmental health are interconnected and that managing resistance requires interventions across all three domains rather than in hospitals alone.

The practical implications of the study extend to water management policy. The researchers emphasize that recreational waters should be regarded as potential exposure pathways within environmental health management, not merely as amenities to be monitored for conventional fecal indicator counts. Their data suggest that routine surveillance focused solely on E. coli density may miss the qualitative dimension of the problem: two sites with similar bacterial counts can differ dramatically in the resistance profiles of the strains they carry. Incorporating resistance testing into recreational water quality programs, prioritizing sanitation infrastructure investment in the vulnerable neighborhoods identified as hotspots, and managing waste elimination practices around the floodplain emerge as concrete steps that could reduce both contamination and the selective pressures that maintain resistance in these ecosystems.

For a region where the Paraná floodplain shapes the daily life of hundreds of thousands of people, the study transforms a diffuse global concern into a local, measurable reality. The image it leaves behind is sobering but actionable: the same waters that cool swimmers in the Argentine summer and sustain riverside communities also harbor bacteria armed against ampicillin, tetracycline, ciprofloxacin, colistin, and the cephalosporins that hospitals rely on when first-line drugs fail. Screening-level exposure estimates, however preliminary, make clear that a single swim at the wrong site can deliver a cocktail of multidrug-resistant organisms. As antimicrobial resistance continues to climb the global public health agenda, studies like this one demonstrate that the front lines of the crisis run not only through intensive care units but through the muddy, everyday waters at the edges of cities.

Subject of Research: Antibiotic-resistant Escherichia coli in urban floodplain waters and sediments of the Paraná River near Santa Fe, Argentina

Article Title: Urban aquatic systems as reservoirs and exposure pathways for resistant Escherichia coli in a South American river floodplain (Argentina)

Article References: Gonzalez, M. J., Molina, F. M. R., Mayora, G., Gonzalez, S. M., López, M. S., Ghiglione, B., & Di Conza, J. (2026). Urban aquatic systems as reservoirs and exposure pathways for resistant Escherichia coli in a South American river floodplain (Argentina). Environmental Geochemistry and Health, 48(14), Article 558. https://doi.org/10.1007/s10653-026-03453-9

Image Credits: AI Generated

DOI: 10.1007/s10653-026-03453-9

Keywords: antimicrobial resistance, Escherichia coli, Paraná River, urban aquatic systems, ESBL, colistin resistance, recreational waters, water quality, One Health, sediments, Argentina, public health