A mysterious journey of antibiotic resistance, spanning from farmyards to distant water bodies, unfolds on the wings of wild birds.
In the endless battle between humans and infectious bacteria, colistin has long been our last line of defense—a crucial antibiotic reserved for treating infections when all others fail. Yet this final fortress is under siege, and the messengers of its downfall are surprisingly overhead: wild birds.
Recent discoveries in Egypt have revealed that migratory and resident wild birds are silently carrying and spreading genes that confer resistance to colistin, presenting a complex public health challenge that transcends human and animal boundaries. This silent transmission underscores the interconnectedness of ecosystem health and human medicine in what scientists call the One Health framework.
Colistin belongs to the polymyxin class of antibiotics and remains one of the few effective treatments against multi-drug resistant pathogens, particularly those that have developed resistance to carbapenems and other advanced antibiotics. For patients fighting severe infections caused by Gram-negative bacteria like Klebsiella pneumoniae and Pseudomonas aeruginosa, colistin is often the final treatment option 1 3 .
The effectiveness of this last-resort antibiotic is now threatened by the emergence of mobile colistin resistance genes, known as mcr genes. These genes are particularly alarming because they're located on plasmids—small, mobile pieces of DNA that can easily transfer between different bacterial species, rapidly spreading resistance capabilities 1 .
To date, ten mcr genes (mcr-1 to mcr-10) have been identified worldwide. The mcr-1 gene, discovered in China in 2015, was the first known plasmid-mediated colistin resistance mechanism. Shortly after, mcr-2 was identified in Belgium. What makes these genes so formidable is their mobility—they can jump between bacteria found in humans, animals, and the environment, creating an interconnected web of resistance 2 .
Wild birds, particularly those that frequent agricultural and urban areas, can acquire resistant bacteria through contaminated water, soil, or food sources. Their high mobility, especially among migratory species, enables them to disperse these resistant bacteria across vast distances, making them ideal vectors for antibiotic resistance transmission 4 .
A 2024 systematic review examining 51 studies confirmed that wild birds worldwide carry resistant E. coli, with higher prevalence in low and middle-income countries. The review highlighted that wild birds can acquire resistant strains when foraging in polluted environments or contaminated surface water 4 .
The situation in Egypt presents a particularly interesting case study. Situated along major bird migration routes, Egypt hosts numerous resident and migratory species that interact with both agricultural and natural environments, creating ideal conditions for the acquisition and spread of antibiotic resistance 4 .
In a comprehensive study conducted from 2017-2018, Egyptian researchers designed an investigation to determine whether wild birds were carrying colistin-resistant bacteria and to explore potential transmission pathways 1 3 .
fecal samples from wild birds (including 80 resident and 60 migratory birds)
surface water samples from areas where bird trapping occurred
human stool samples from farmers living near the sampling sites 3
The researchers processed these samples through conventional culture techniques and biochemical identification to isolate various bacteria. They then used multiplex polymerase chain reaction (PCR) to detect the presence of mcr-1 and mcr-2 genes in all positive isolates 3 8 .
To confirm their findings, the team sequenced the mcr-1 genes from three randomly selected E. coli isolates—one from a migratory bird, one from water, and one from a human 3 .
The bacteriological examination revealed isolates of Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, and P. aeruginosa. The PCR results demonstrated that E. coli was the most prevalent bacterium harboring the mcr genes 1 .
The data revealed two particularly important patterns: migratory birds showed higher prevalence of mcr-1 compared to resident birds, and water sources showed the highest prevalence of mcr-2 across all samples 1 3 .
When researchers sequenced the mcr-1 gene from E. coli isolates from migratory birds, water, and humans in the same locality, they found a 100% genetic similarity between the sequence from a migratory bird and surface water, indicating a direct transmission link 4 . This genetic evidence strongly suggests that wild birds are acquiring resistant bacteria from contaminated water sources and potentially spreading them through their fecal deposits.
Understanding how scientists detect and track these resistance genes reveals the sophistication of modern microbiology techniques.
| Research Tool | Function | Application in mcr Studies |
|---|---|---|
| Multiplex PCR | Simultaneous detection of multiple genes | Identifying mcr-1 and mcr-2 in isolates |
| API 20E/NE Kits | Biochemical profiling | Confirming bacterial species identity |
| Broth Microdilution | Phenotypic resistance testing | Determining colistin MIC values |
| DNA Sequencing | Genetic characterization | Confirming gene identity and relationships |
| Plasmid Analysis | Studying mobile genetic elements | Understanding resistance transmission |
These tools enabled researchers to not only detect the presence of resistance genes but also to trace their movement between different reservoirs. The broth microdilution method is particularly important as it remains the gold standard for determining colistin resistance, since the antibiotic's poor diffusion makes agar-based methods unreliable 7 .
The detection of mcr genes in wild birds represents more than just a scientific curiosity—it signals a serious public health threat with implications that extend far beyond ornithology.
The One Health approach recognizes that the health of humans, animals, and ecosystems are interconnected. The transmission of colistin resistance exemplifies this connection: antibiotics used in human medicine and livestock production select for resistant bacteria in the environment, which wild birds then acquire and spread through their migratory patterns, potentially introducing resistance genes into new geographic regions 4 .
This silent spread is particularly concerning because mcr genes can exist alongside other resistance mechanisms. Researchers have observed mcr genes on plasmids containing other resistance genes, including those encoding carbapenemases and extended-spectrum β-lactamases, potentially creating pan-resistant bacteria that could defy all available antibiotics 3 .
The Egyptian study found that despite having no direct exposure to antibiotics, wild birds carried resistant bacteria at rates comparable to humans living in the same areas, highlighting how environmental contamination—possibly from agricultural runoff or inadequate waste management—can drive resistance in wildlife populations 1 3 .
The discovery of colistin resistance genes in wild birds serves as both a warning and an opportunity. It warns of the far-reaching consequences of antibiotic misuse, demonstrating how resistance can escape clinical settings and agricultural operations to circulate freely in wildlife populations. Simultaneously, it offers the opportunity to adopt more comprehensive surveillance strategies that monitor wildlife and environmental reservoirs as early warning systems for emerging resistance threats.
Responsible use in human medicine and agriculture
Preventing environmental contamination
Integrating human, animal, and environmental monitoring
Addressing this challenge requires coordinated action across multiple sectors. As the Egyptian study demonstrates, the health of wild birds is inextricably linked to our own. Protecting the efficacy of our last-resort antibiotics will require looking beyond hospital walls and farm fences to consider the entire ecosystem—including the feathered couriers overhead.