Imported seafood is spreading antibiotic resistance genes

Some imported seafoods on American dinner plates are carrying an invisible threat: bacteria equipped with genes that can disarm a high-priority, critically important antibiotic called colistin.

Colistin is reserved for life-threatening infections that have outwitted every other drug. When it fails, clinicians have nowhere left to turn.

In research presented at ASM Microbe 2025, scientists at the University of Georgia found colistin-resistance genes – known as mcr – in shrimp and scallops purchased from eight Atlanta-area grocery stores.

It is the first evidence that U.S. seafood shoppers are unknowingly buying products harboring bacteria capable of shrugging off one of medicine’s last lines of defense.

Seafood’s global supply chain

Lead investigator Issmat Kassem has spent two decades tracing the global spread of antimicrobial resistance.

Sampling from local supermarkets, his team ran genetic screens on bacteria living in the imported seafood including shellfish and uncovered multiple plasmids – circular snippets of DNA – that carry mcr.

These plasmids make resistance “mobile,” because the genes can hop from one bacterium to another.

When Kassem talks about the popular appeal of shellfish, he puts it simply: “We love our seafood.”

And most Americans do: about 90 percent of the shrimp consumed in the United States are raised overseas. Many exporting nations still rely on antibiotics, including colistin, to treat or fatten farmed animals.

That practice creates selective pressure for resistant bacteria, which can end up in packing plants, shipping containers, and ultimately U.S. kitchens.

The last-resort antibiotic

Colistin debuted in the 1950s, then largely disappeared from U.S. hospitals because it can scar kidneys and nerves. Its toxic profile forced doctors to shelve it in favor of safer drugs.

That calculus reversed when so-called “superbugs” evolved resistance to nearly every other antibiotic. Colistin returned as a last resort, but its revival was overshadowed by news in 2016 of the first plasmid-borne mcr gene.

Until then, many scientists assumed colistin resistance evolved only through slow, vertical inheritance. The plasmid discovery rewrote that script, proving resistance could not jump between different bacteria – an assumption Kassem now calls dangerously outdated.

Today, at least ten mcr variants have been catalogued worldwide, and Kassem’s new findings reveal a pathway for them to enter U.S. ecosystems.

In earlier work, his lab found the same plasmids and bacterial hosts in Georgia wastewater. The match confirms that agricultural practices abroad can shape microbial landscapes at home.

A supply chain problem

Although federal inspectors test imported seafood for chemical residues and visible pathogens, they do not routinely test for resistance genes.

“The bacteria that were carrying colistin resistance genes are not normally screened,” Kassem warned. Standard checks, designed decades ago, fail to catch this genomic stowaway.

“Our food is sourced from different places,” he said. “If you go out to lunch today, your plate might have ingredients from six, seven, eight countries.” These complex supply chains enable a hidden circulation of microbes.

“Some countries do not have strict regulations for using antibiotics in food animal production, so imported food can be a vehicle for transmission of resistance.”

Domestic aquaculture operations appear free of mcr for now, suggesting U.S. oversight can be effective when consistently applied.

No borders for seafood “superbugs”

Resistance that travels by plasmid does not respect borders. “We live in a very connected world,” Kassem emphasized.

“We move a lot, we travel a lot, our food travels, and we are going to spread whatever emerges, even across national borders.” In other words, the United States cannot isolate itself from a problem embedded in international trade.

That reality underscores the need for better surveillance. “It’s important to invest in monitoring systems and expand them and collaborate, especially on the global level, on the issue of antimicrobial resistance,” Kassem said.

Enhanced screening at ports of entry could catch resistant bacteria before they enter the consumer market. Equally critical is global cooperation to reduce unnecessary antibiotic use in aquaculture and livestock production.

Hidden risks in the kitchen

Cooking shellfish thoroughly destroys live bacteria, but plasmid DNA may survive and flow into wastewater systems.

Consumers should handle raw imported seafood with the same caution they would raw poultry. This includes washing hands, sanitizing cutting boards, and avoiding cross-contamination with ready-to-eat foods. Yet individual diligence cannot solve a systemic problem.

The study shows that antimicrobial resistance is no longer confined to hospitals or far-off farms – it can ride quietly into suburban kitchens on ice.

A global health warning

If policymakers act swiftly, the United States can reinforce its defenses by updating import regulations and requiring genetic testing for high-risk resistance genes. It can also partner with exporting nations to curb antibiotic misuse.

For now, Kassem’s message is a wake-up call. Colistin remains a critical, if imperfect, guardian against multidrug-resistant infections.

Allowing its defenses to erode – even incrementally – brings the world closer to a post-antibiotic era where routine surgeries and minor wounds become life-threatening.

The microbes in our seafood are sending a warning. Whether we heed it will shape the future of global health.

The study is published in the journal mSphere.

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