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

The marine plastisphere, a term that denotes the ecosystem formed around plastic debris in marine environments, has emerged as a new niche for microbial communities. This habitat presents unique challenges and opportunities for microbial evolution, including the development of antibiotic resistance. In a seminal study, scientists successfully extracted a strain of A. beijerinkii, referred to as MPE71, from this intriguing environment. Genetic and phylogenetic analyses highlighted the unsettling similarity between MPE71 and several known human pathogenic strains of Acinetobacter, raising significant alarm bells regarding the potential transmission of resistance features from marine environments to human populations.

One of the most concerning findings related to the MPE71 strain was its resistance profile, which was evaluated using Minimum Inhibitory Concentration (MIC) assays for ten different antibiotics. The results painted a grim picture; MPE71 demonstrated unwavering drug resistance across the board. Particularly noteworthy was its high MIC against polymyxin B, a last-resort antibiotic. With a resistance threshold set at a staggering 200 µg/mL, the study marks this as the first documented instance of high-level polymyxin B resistance in an Acinetobacter strain linked to the marine plastisphere.

To further investigate the genomic underpinnings of this multidrug resistance, the research team engaged in an extensive genomic resistance gene analysis. This scrutiny uncovered a plethora of multidrug efflux pump genes embedded within the genetic framework of MPE71. Efflux pumps serve as critical players in the antibiotic resistance landscape, actively expelling toxin compounds from bacterial cells and thus rendering antibiotics less effective. The study effectively shines a spotlight on these mechanisms, revealing that Acinetobacter strains from marine environments can and do possess sophisticated resistance features akin to their terrestrial counterparts.

To take the investigation a step further, the researchers adopted a transcriptomics approach to delve deeper into the resistance mechanisms at play. Transcriptomic analysis allowed for an exploration of gene expression levels, uncovering a significant up-regulation of genes associated with membrane biosynthesis, multidrug efflux systems, and associated periplasmic proteins. The response was notably dose-dependent, indicating a finely-tuned biological mechanism that allows MPE71 to thrive in the presence of polymyxin B and other antimicrobial agents.

An unexpected but crucial breakthrough was made when the researchers explored the role of the proton motive force (PMF) in the resistance mechanism of MPE71. They employed a compound known as carbonyl cyanide 3-chlorophenylhydrazone (CCCP), a known inhibitor of PMF. The inhibition of PMF resulted in a marked degradation of MPE71’s resistance to polymyxin B. This finding substantiates the hypothesis that PMF-dependent efflux pumps are indispensable in the survival of resistant strains in antibiotic-contaminated environments.

The results of this study not only elucidate the resistance mechanisms employed by marine Acinetobacter but also raise significant ecological concerns. The presence and persistence of multidrug-resistant bacteria in marine environments, particularly around anthropogenically derived plastics, pose a risk to both marine life and human health. As these bacteria can potentially transfer their genetic material to more virulent strains, the implications for public health are daunting.

Researchers emphasize the necessity for increased surveillance in marine ecosystems to monitor the emergence and spread of antibiotic-resistant strains. This means employing innovative techniques, such as metagenomic studies and continuous ecological monitoring, to better understand how human activity is influencing microbial resistance patterns in oceanic environments. The rapid evolution of resistance genes within marine microflora necessitates a proactive approach to prevent a potential public health crisis stemming from the seas.

As scientists unravel these multidimensional layers of resistance, policymakers must consider regulatory steps to mitigate the impact of plastic pollution in marine environments. Such initiatives may include better waste management practices and stricter regulations on antibiotic usage in agriculture and aquaculture. Ultimately, addressing these systemic issues requires collaboration across scientific disciplines, environmental organizations, and public health authorities to protect both ecosystems and human health.

The discovery of MPE71 emphasizes a critical gap in our understanding of antibiotic resistance within natural ecosystems. It serves as a clarion call to researchers, policymakers, and the general public about the interconnectedness of our actions and the global health ramifications they may incur. As the ocean continues to absorb the consequences of human activity, it is evident that our responsibility extends beyond land; it reaches deep into the waters that sustain life on Earth.

In summary, this groundbreaking research opens the door to a new understanding of how marine environments can serve as reservoirs for multidrug-resistant pathogens. As we grapple with the complexities of antibiotic resistance, investigations like these offer essential insights into the adaptive strategies of bacteria. The implications are profound, underscoring the urgent need for a coordinated global response to the threat of antibiotic-resistant organisms emerging from our oceans.

In conclusion, the story of Acinetobacter beijerinkii strain MPE71 serves as a powerful reminder of the hidden dangers lurking in our oceans and the critical importance of understanding the full scope of antibiotic resistance in all its forms. With continued research and vigilance, we may yet stem the tide of antibiotic resistance and protect both human health and marine ecosystems for generations to come.

Subject of Research: Marine-derived antibiotic resistance mechanisms in Acinetobacter species.

Article Title: High-level polymyxin B resistance and underlying mechanism in a multidrug-resistant Acinetobacter strain isolated from the marine plastisphere.

Article References:

Qin, P., Ding, W. & Zhang, W. High-level polymyxin B resistance and underlying mechanism in a multidrug-resistant Acinetobacter strain isolated from the marine plastisphere.
J Antibiot (2025). https://doi.org/10.1038/s41429-025-00888-7

Image Credits: AI Generated

DOI: 19 December 2025

Keywords: Antibiotic resistance, Acinetobacter, marine plastisphere, multidrug resistance, polymyxin B, efflux pumps, public health.