The review, led by Alisha Sinha, Jogesh Murmu and colleagues at the ICMR-National Institute of Health Research in Bhubaneswar, India, set out to map where colistin resistance occurs in contaminated aquatic systems and how it travels. The team followed PRISMA guidelines and searched five major databases up to December 2024, retrieving a staggering 26,740 records. After removing duplicates, 20,691 unique citations were screened, 497 full-text reports were assessed, and just eight observational studies ultimately met the eligibility criteria. Three additional studies surfaced through manual searching of Google Scholar and reference lists, completing the set of eight taken forward for synthesis. The review was prospectively registered with PROSPERO under identifier CRD42024613277.

The eight studies spanned Germany, Brazil, Japan, the United States, China and Vietnam, and together they examined an unusually diverse set of aquatic matrices: slaughterhouse wastewater, municipal wastewater treatment plant influents and effluents, urban streams and rivers, coastal aquaculture zones stretching more than 5,000 kilometers of Chinese coastline, and wastewater-derived bioaerosols and sludge. What emerged is a picture of resistance genes threading through the water systems that humans, livestock and food chains all depend on.

One of the most striking findings came from Germany, where researchers recovered 185 Klebsiella isolates from slaughterhouse wastewater and municipal treatment plants and found that 60 of them, or 32.4 percent, were phenotypically resistant to colistin. Many belonged to high-risk multidrug-resistant clones. In Brazil, a single Escherichia coli isolate from an urban stream used for recreation carried mcr-1.1 alongside beta-lactamase genes blaCTX-M-2 and blaCMY-2, all hitching rides on plasmids and a Tn21-like transposon. In China, scientists screening 1,500 bacterial isolates from the Funan River in Chengdu found 24 mcr-positive isolates, most carrying mcr-1 and the rest mcr-3, with resistance gene abundance highest near sewage outfalls.

The molecular biology behind these findings is what makes them genuinely alarming. Colistin resistance was long assumed to be chromosomal, arising from mutations in regulatory systems such as pmrAB and phoPQ or inactivation of the mgrB gene, which alter the charge of the bacterial outer membrane and prevent the drug from binding. That changed in 2015 with the discovery of mcr-1, the first plasmid-mediated colistin resistance gene, found in E. coli from animals in China. Since then, ten mcr variants have been identified worldwide, and because they sit on mobile genetic elements such as plasmids, transposons and integrons, they can jump between bacterial species through horizontal gene transfer. The review found mcr-1, mcr-3, mcr-5, mcr-8 and mcr-9 among the included studies, often on epidemiologically important plasmid types such as IncX4 and IncI1, and frequently co-located with other resistance genes, including extended-spectrum beta-lactamases.

That co-localization matters because of a phenomenon called co-selection. Aquatic environments contaminated with antibiotic residues, heavy metals and disinfectants can maintain and even enrich mcr-carrying bacteria without any direct colistin exposure, since a single selective pressure targeting a different resistance gene on the same mobile element will keep the whole package alive. The review’s authors point out that wastewater systems, with their high microbial density, abundant nutrients and constant influx of contaminants, provide near-ideal conditions for genetic exchange. Coastal aquaculture told a particularly sobering story: Chinese researchers detected mcr-1 by qPCR in all 27 water samples taken along a vast stretch of coastline, although the single high-level resistant isolate they characterized, Ralstonia pickettii, carried chromosomal rather than plasmid-mediated resistance, a distinction the review takes care to preserve.

Chromosomal resistance was not absent, however. In Japan, ten colistin-resistant Klebsiella pneumoniae and K. quasipneumoniae isolates recovered from municipal wastewater influents carried disruptions in mgrB and ramR rather than mcr genes, and one strain belonged to the epidemic ST29 clone, implicating human gut bacteria as the source. Unlike plasmid-mediated resistance, chromosomal resistance spreads through vertical inheritance and clonal expansion, meaning resistant lineages can persist and multiply in environments under continuous selective pressure. The review also flagged an underappreciated pathway: wastewater treatment processes such as aeration and sludge handling generate bioaerosols, and a US study found mcr sequences, particularly mcr-5 and mcr-8, in bioaerosols and liquid sludge from three treatment plants, at 0 to 0.5 percent of annotated antimicrobial resistance gene reads. Airborne dissemination of resistance genes, the authors note, is a dimension of environmental surveillance that is almost never measured.

Perhaps the most honest and important conclusion of the review is what it cannot say. The eight studies measured resistance in fundamentally non-comparable ways: some counted resistant isolates, others reported sample positivity by qPCR, and still others reported metagenomic relative gene abundance, which detects genetic material but cannot confirm viable resistant bacteria or actual phenotypic resistance. A single-isolate report from Brazil, rated high risk of bias, cannot yield a prevalence at all. Because denominators differ and several are tiny, the authors deliberately report raw numerators and denominators rather than forcing a common prevalence estimate. Quality appraisal using JBI checklists rated four studies as low risk of bias, three as moderate and one as high. The result is a narrative synthesis rather than a meta-analysis, and the authors are explicit that no quantitative environmental prevalence can currently be calculated.

The public health implications, where they can be cautiously drawn, are considerable. Urban rivers harboring mcr-positive bacteria are used for recreation in Brazil and Vietnam, where mcr-1 abundance correlated strongly with fecal pollution indicators such as E. coli and crAssphage, pointing to human waste as a plausible source. Aquaculture systems raise the specter of food-chain transmission to consumers and workers, and in regions with inadequate water treatment, resistance genes can migrate into drinking water supplies. The review also highlights a glaring surveillance gap: no eligible study from India was identified, despite the country’s intense antibiotic use and wastewater challenges, and the authors stress that this absence reflects missing data rather than absent resistance.

The way forward, the authors argue, lies in standardized, integrated surveillance that combines culture-based methods, molecular detection and genomic or metagenomic characterization within a One Health framework connecting human, animal and environmental health. Harmonized sampling frames, uniform molecular methods, and reporting of outcomes with explicit numerators and denominators would finally make results comparable across sites and continents. As colistin resistance genes continue to ride plasmids through rivers, fish ponds and treatment plants worldwide, the review makes clear that the environment is not a passive victim of antimicrobial resistance but an active reservoir and highway for it, and that protecting the last-resort status of this drug will require looking well beyond the hospital ward.

Subject of Research: Environmental occurrence and dissemination of colistin resistance genes in contaminated aquatic systems

Article Title: Environmental occurrence and dissemination of colistin resistance in contaminated aquatic systems: a systematic review of eight observational studies

Article References: Sinha, A., Murmu, J., Singh, S., Pal, A., Mallick, U., Mishra, P., Sahoo, K. C., Sahu, M. C., & Pati, S. (2026). Environmental occurrence and dissemination of colistin resistance in contaminated aquatic systems: a systematic review of eight observational studies. Frontiers in Antibiotics, 5, Article 1970443. https://doi.org/10.3389/frabi.2026.1970443

Image Credits: AI Generated

DOI: 10.3389/frabi.2026.1970443

Keywords: colistin resistance, mcr genes, antimicrobial resistance, wastewater, aquatic environments, horizontal gene transfer, plasmids, aquaculture, bioaerosols, One Health, environmental surveillance, systematic review