The study, published in Environmental Monitoring and Assessment, arrives at a moment when the legacy of the 2019 spill remains uncertain. Tar balls chemically linked to that disaster continued to reach Brazilian beaches years afterward, echoing the pattern seen after the Prestige spill off Spain, where polycyclic aromatic hydrocarbons persisted in coastal ecosystems long after the acute emergency ended. PAHs are among the most worrying compounds released in oil spills because they are persistent, they bioaccumulate in fatty tissues, and several members of the family carry mutagenic and carcinogenic potential. Humans encounter them continuously through airborne particles, polluted water, and contaminated food, which makes reliable monitoring of seafood a genuine public health concern rather than an abstract environmental exercise.

Guanabara Bay is arguably the most challenging possible proving ground for such a method. The estuary, the second-largest bay on the Brazilian coast at roughly 384 square kilometers, is ringed by a metropolitan region of more than 16 million people and hosts a dense concentration of petrochemical infrastructure, including refineries and oil-storage terminals in its northwestern sector. Its sediments carry PAH burdens reflecting both chronic petrogenic inputs from oil handling and pyrogenic inputs from urban and vehicular combustion. Yet the bay also sustains artisanal mussel fisheries that provide an important protein source for local communities, creating an urgent need for affordable, routine contamination surveillance in exactly the kind of resource-limited setting where sophisticated analytical infrastructure is often unavailable.

Mussels are ideal sentinels for this task. As filter feeders, they ingest contaminants suspended in the water column and accumulate lipophilic pollutants in their tissues, and they tolerate a broad range of environmental conditions. Their track record in Guanabara Bay is well documented: after a fuel oil spill in 2000, PAH burdens in mussels collected from the pillars of the Rio-Niterói Bridge rose two to fivefold, tracking changes in the surrounding water closely. But extracting PAHs from lipid- and protein-rich mussel tissue has always been analytically awkward. Conventional approaches such as matrix solid-phase dispersion, accelerated solvent extraction with gel-permeation clean-up, or supercritical fluid extraction work, but they demand expensive equipment, large volumes of hazardous solvent, and considerable time per sample.

The new method borrows a trick from food safety chemistry. The QuEChERS approach, whose name stands for Quick, Easy, Cheap, Effective, Rugged and Safe, was originally devised in 2003 for extracting pesticide residues from produce. It relies on simple salting-out chemistry: homogenized tissue is vortexed with a mixture of acetonitrile and dichloromethane, then magnesium sulfate and sodium chloride are added to drive analytes into the organic layer. The researchers adapted this for mussel tissue and added a solid-phase extraction clean-up step using Florisil, silica gel, and alumina columns to strip out co-extracted lipids before instrumental analysis. The combination proved remarkably effective at minimizing the matrix interference that normally plagues PAH analysis in fatty biological samples.

The analytical team collected 210 mussels from four artisanal fishing communities around the bay, including Itaipu, Copacabana, Barra da Tijuca, and Recreio dos Bandeirantes, across four seasonal campaigns between September 2021 and June 2022, roughly two to three years after the spill. Each pooled sample of about twenty individuals was homogenized and analyzed in triplicate. The validated method achieved strong linearity, with coefficients of determination exceeding 0.995 for most compounds, and acceptable recoveries between 51.1 and 109.0 percent for 15 of the 18 target PAHs. Detection limits ranged from 0.009 nanograms per gram for naphthalene to just over one nanogram per gram for the most problematic analytes, figures comparable to those reported for far more expensive tandem mass spectrometry setups.

Not every target compound cooperated. Benzo(j)fluoranthene and benzo(k)fluoranthene share an identical retention time of 40.944 minutes on the DB-5MS capillary column used, a documented limitation of five-percent phenyl methylpolysiloxane phases that even laboratories running triple quadrupole instruments encounter. Both isomers were excluded from quantitative interpretation, a decision the authors note does not disadvantage the dataset because regulatory indices such as the EFSA-endorsed PAH4 already omit benzo(j)fluoranthene. Indeno(1,2,3-cd)pyrene yielded no quantifiable signal at any fortification level and was likewise excluded. Benzo(a)pyrene, the single most notorious carcinogenic PAH, showed precision marginally above the 20 percent threshold but was retained for reporting given its environmental significance. This transparent accounting of limitations, the researchers argue, is what makes the method genuinely useful for future monitoring programs.

The environmental findings were as instructive as the analytical ones. Only five PAHs were quantified above the limit of quantification across all 210 samples: naphthalene, phenanthrene, 5-methylchrysene, benzo(a)anthracene, and benzo(a)pyrene. Concentrations were low and statistically homogeneous across sites and seasons, with benzo(a)pyrene reaching the highest mean at just 0.308 nanograms per gram, far below the European Union maximum of 5 nanograms per gram for bivalve molluscs. Crucially, the compositional pattern told a story: the total PAH burden of 4.27 nanograms per gram was dominated almost entirely by high-molecular-weight, combustion-derived compounds rather than the light petrogenic compounds, such as naphthalene and phenanthrene, that would signal lingering petroleum from the spill. The contamination signature points instead to diffuse, continuous pyrogenic inputs from vehicle emissions, industrial combustion, and atmospheric deposition.

That counterintuitive result carries a sobering implication. Benzo(a)pyrene is classified by the International Agency for Research on Cancer as a Group 1 confirmed human carcinogen, acting through reactive diol-epoxide metabolites that form covalent DNA adducts, while benzo(a)anthracene and 5-methylchrysene are Group 2B possible carcinogens. None of these genotoxic compounds has a recognized safe exposure threshold, and mussels remain a staple protein for the fishing communities surrounding the bay. The authors caution that without pre-spill baseline data or an uncontaminated reference site, low burdens cannot be read as recovery from the 2019 disaster, nor can the spill be disentangled from chronic urban-industrial pollution. What the study does establish is that the threat to these communities comes less from dramatic spill events than from the quiet, ceaseless combustion chemistry of a megacity.

The broader significance of the work lies in accessibility. Compared with pressurized solvent extraction, QuEChERS-based workflows have been shown to cut per-sample costs roughly fivefold while increasing throughput tenfold, and other studies report cost reductions approaching 97 percent relative to Soxhlet extraction. Because the new method uses matrix-matched calibration rather than costly isotope-dilution standards and relies on a single quadrupole mass spectrometer, it can be adopted by laboratories that could never afford tandem instrumentation. For oil-impacted tropical estuaries worldwide, from Jakarta Bay to the Gulf of Guinea, the message is that meaningful PAH biomonitoring no longer requires a flagship budget, only mussels, modest solvents, and a well-validated protocol.

Subject of Research: Development and validation of a QuEChERS-SPE-GC-MS method for PAH biomonitoring in Perna perna mussels from an oil-impacted tropical estuary

Article Title: Development of a short and sensitive QuEChERS-SPE-GC-MS method for polycyclic aromatic hydrocarbon (PAH) biomonitoring in Perna perna (Linnaeus, 1758) mussels from an oil-impacted tropical estuary

Article References: de Pinho, J. V., Tessaro, L., Silva Melo, P. T., de Andrade, J. C., Celano, M. R., de Almeida Rodrigues, P., Castro Cardoso de Almeida, A. E., Hauser-Davis, R. A., & Conte-Junior, C. A. (2026). Development of a short and sensitive QuEChERS-SPE-GC-MS method for polycyclic aromatic hydrocarbon (PAH) biomonitoring in Perna perna (Linnaeus, 1758) mussels from an oil-impacted tropical estuary. Environmental Monitoring and Assessment, 198(11), Article 1184. https://doi.org/10.1007/s10661-026-15980-0

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15980-0

Keywords: polycyclic aromatic hydrocarbons, QuEChERS, GC-MS, Perna perna, Guanabara Bay, biomonitoring, oil spill, mussels, benzo(a)pyrene, pyrogenic contamination, solid-phase extraction, environmental monitoring

Colistin, also called polymyxin E, occupies a special and precarious position in modern medicine. It is a cationic polypeptide that attacks the outer membrane of Gram-negative bacteria, and after decades of limited use because of kidney toxicity, it was reintroduced in recent years as a final defense against multidrug-resistant infections. With few new antibiotics in the development pipeline, clinicians increasingly depend on colistin when carbapenem-resistant pathogens strike hospital patients. The emergence of bacteria that resist this drug is therefore not a routine scientific concern but a genuine emergency in global public health, and the new study demonstrates that the ocean may be serving as an unexpected reservoir and highway for the genes responsible.

The mechanics of colistin resistance matter for understanding why the new results are so significant. Resistance can arise in two ways. Intrinsic resistance develops through chromosomal mutations that alter lipid A, the lipopolysaccharide component of the bacterial outer membrane that colistin targets, reducing the drug’s ability to bind. Acquired resistance, by contrast, depends on the horizontal transfer of mcr genes, usually carried on plasmids, which are mobile DNA elements that can shuttle between bacterial cells. Since the discovery of the mcr-1 gene in Escherichia coli plasmids a decade ago, researchers have identified ten mcr variants, from mcr-1 through mcr-10, and watched them disperse across continents, animal populations, and clinical settings. What remained poorly understood was how extensively these genes had colonized marine ecosystems.

To answer that question, the Brazilian team designed a natural experiment spanning roughly 330 kilometers of the Rio de Janeiro coastline plus the Fernando de Noronha Archipelago, a volcanic island group about 360 kilometers offshore in the Western Atlantic. Their sampling sites deliberately covered a gradient of human influence. At one extreme sat Bom Jesus Cove in Guanabara Bay, a tropical urban estuary receiving raw sewage, oils, and industrial contaminants from Rio de Janeiro city. At the other extreme lay the submarine caves of Fernando de Noronha, protected within a national marine park and accessible only by SCUBA diving. Between those poles, the researchers sampled the Cagarras Archipelago, a no-take marine protected area unfortunately positioned near the Ipanema submarine sewage outfall; the biodiverse waters of Arraial do Cabo, including the Gruta Azul submarine cave; and Ilha Grande Bay, one of the world’s largest tropical bays, which faces growing pressure from coastal settlements and maritime traffic.

The laboratory work was methodical and technically demanding. From 1,550 total bacterial isolates, the team focused on 308 Gram-negative bacilli identified as potential hosts of acquired colistin resistance determinants. Bacteria were cultured on Luria Bertani and MacConkey agar from water samples, washed and swabbed from pieces of floating plastic, and extracted from sponge tissue through serial dilution across four growth media. Identification proceeded by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry on a Bruker Microflex LT platform, and DNA was extracted using a thermal lysis protocol with Chelex 100 resin. Polymerase chain reaction screening then probed each strain for eight mcr variants, and any positive strains were tested further for beta-lactamase genes, quinolone resistance genes, aminoglycoside resistance genes, sulfonamide resistance genes, and class 1, 2, and 3 integron-integrase genes.

The results were striking in both breadth and detail. Of the 308 potential host strains, 101 carried at least one mcr variant, generating 128 total gene occurrences. Water samples contributed 59.4 percent of the positive strains, plastic litter 35.6 percent, and marine sponges 5.0 percent. The mcr-9 variant dominated with 33 occurrences, followed by mcr-3 with 23, mcr-2 with 18, and mcr-1 with 17. That mcr-9 topped the list is noteworthy because global metagenomic surveys have often ranked mcr-1 as the most dispersed variant. The mcr-9 gene is known to circulate on IncHI2 plasmids that interconnect human, animal, and environmental isolates, suggesting an efficient dissemination network that the ocean may now be extending.

The bacterial hosts carrying these genes were equally concerning. Among ten identified genera, Enterobacter, Acinetobacter, Vibrio, and Klebsiella predominated. Enterobacter species alone accounted for 31 strains and harbored the greatest mcr diversity, with every variant except mcr-5 and mcr-7 detected in the genus. More alarming still, mcr-positive Enterobacter was not confined to polluted Bom Jesus Cove but turned up in marine sponges and cave waters in Fernando de Noronha, an area with restricted human access. One Enterobacter hormaechei strain, isolated from a sponge in Sapata Cave, simultaneously carried mcr-2, mcr-9, and the sulfonamide resistance gene sul1. Four strains carried three mcr variants at once, including Klebsiella pneumoniae isolates from pristine sponges and from floating plastic, and an Enterobacter cloacae from Noronha water. Both K. pneumoniae and E. cloacae belong to the notorious ESKAPE group of pathogens, and their carriage of multiple resistance genes in aquatic matrices highlights the risk of waterborne transmission to humans.

The co-occurrence of resistance genes painted an even darker picture. Nearly half of the mcr-positive strains, 46.5 percent, also carried other antimicrobial resistance genes or integron-integrases, with sulfonamide resistance genes, particularly sul1, the most frequent companions, followed by the beta-lactamase genes blaTEM and blaSHV. In Bom Jesus Cove, the team detected the carbapenemase gene blaKPC alongside mcr-3 in Enterobacter bugandensis and Raoultella ornithinolytica strains, a combination that effectively eliminates two of the last therapeutic options for infections caused by these organisms. Integron-integrases, genetic platforms that capture and shuffle gene cassettes, co-occurred with mcr exclusively in the polluted cove, suggesting bacterial adaptation to intense anthropogenic selective pressure. Because changes in membrane permeability in mcr-positive bacteria can reduce sensitivity to multiple drugs, these combinations can compound into true multidrug resistance, complicating treatment of hospital-associated infections.

The gradient of pollution left a measurable fingerprint on the data. In Bom Jesus Cove, 27 of 104 water-isolated strains tested positive for mcr, and 36 of 82 strains from floating plastic litter carried the genes, a prevalence of roughly 44 percent that represents the highest in the study. The finding positions plastic debris as both a reservoir and a vector for antimicrobial resistance, offering microbes a stable, drifting substrate on which biofilms can form, exchange genes, and travel with currents. At the Cagarras Archipelago, 40 percent of water-isolated strains were positive despite formal protection, a legacy of the nearby sewage outfall. Arraial do Cabo showed 36.5 percent prevalence among water isolates and Ilha Grande Bay 36.3 percent. In Fernando de Noronha, prevalence dropped to about 5 percent of candidate host strains, with two of 17 water isolates and five of 15 sponge isolates positive, proving that resistance persists even where human pressure is minimal.

What emerges from the study is a warning that antimicrobial surveillance cannot remain confined to clinics and farms. Marine sponges, submarine caves, and drifting plastic are now documented hotspots of colistin resistance, and the genes involved sit on mobile elements capable of jumping into human pathogens. The Brazilian coastline, with its juxtaposition of dense urban pollution and remote protected archipelagos, offered an ideal natural laboratory, but the pattern it revealed is almost certainly global. The researchers argue for integrated monitoring programs that treat the ocean as a critical node in the antimicrobial resistance network, alongside conservation strategies that reduce sewage discharge and plastic pollution. As colistin remains a last resort for patients out of options, every mcr gene circulating in seawater, attached to plastic, or sheltering inside a sponge represents a card stacked against future medicine, and the ocean, it turns out, is dealing them freely.

Subject of Research: Distribution of mobile colistin resistance (mcr) genes in bacteria from Brazilian marine environments

Article Title: Widespread occurrence of mobile colistin resistance genes in Brazilian marine environments

Article References: Brunelli, R. C., de Jesus Carvalho Baptista, T. V., de Oliveira Nithack Marques, M., da Silva Oliveira Alves, G., Abdon, B. B., Mello, M. P., Paranhos, R., Gallo, M. N., Vinzon, S. B., Lage, A., Sandes, J., Muricy, G., Klautau, M., Lopes, M. V., Dias, G. R., Canellas, A. L. B., & Laport, M. S. (2025). Widespread occurrence of mobile colistin resistance genes in Brazilian marine environments. Ocean Microbiology, 1(1), Article 3. https://doi.org/10.1186/s44375-025-00003-z

Image Credits: AI Generated

DOI: 10.1186/s44375-025-00003-z

Keywords: antimicrobial resistance, colistin, mcr genes, marine microbiology, plastic pollution, Guanabara Bay, Fernando de Noronha, Enterobacter, Klebsiella pneumoniae, integrons, marine sponges, One Health