Seafood importation a potential pathway for antimicrobial resistance genes, US researchers warn

Researchers in the US have discovered colistin resistance genes in bacteria from imported seafood in Atlanta markets, particularly shrimp and scallops — highlighting a potential pathway for spreading antimicrobial resistance (AMR) genes through the global seafood trade.

The findings raise consumer health risk concerns and call for enhanced food safety monitoring to safeguard public health.

Colistin was introduced in the 1950s to treat Gram-negative bacterial infections caused by bacteria having an additional outer membrane outside their cell wall, making them more resistant to certain antibiotics. It was later discontinued in the US in the 1980s due to its increased risk of nerve and kidney damage in patients.

However, colistin was reintroduced due to its critical role in treating severe infections, with the WHO classifying it as a high-priority antibiotic.

Dr. Issmat Kassem from the University of Georgia, which conducted the study, tells Food Ingredients First that he has been working on colistin resistance since 2017, shortly after the discovery of the “mobile” colistin resistance genes (MCR) in 2016 — before which researchers believed colistin resistance was inherited, and could not “jump between different bacteria.”

“During our investigations in countries outside the US, we encountered MCR in aquaculture fish. Additionally, we knew from the literature that MCR genes occur in the exporting countries where the seafood is produced.”

“We also knew that MCR was isolated from seafood in these exporting countries. It became imperative to check MCR in bacteria in the seafood imported into the US. It is important to note that antibiotics (and associated ramifications such as pollution) are not tightly regulated in some countries.”

The findings, to be published soon in the American Society for Microbiology’s (ASM) journal mSphere, were presented this week in Los Angeles at ASM Microbe 2025, the ASM’s annual meeting.

AMR spread can affect humans, animals, and environments, which are all linked, says Kassem.

Growing public health risk

Antimicrobial resistance (AMR) is a rising public health threat, with estimates stating over 39 million people could lose their lives from antibiotic-resistant infections between 2025 and 2050. In 2019, AMR was directly responsible for approximately 1.27 million deaths and contributed to nearly five million deaths worldwide, according to the WHO.

Kassem says he suspected AMR might spread through the import and exportation of food. The food we eat can contain ingredients from various countries, some of which might not have strict regulations for using antibiotics in food animal production. This increases the likelihood of imported food being a “vehicle for transmission of resistance.”

“The science is clear. Learning from numerous examples in recent history, mobile antibiotic resistance genes can become hard to contain, spreading across the globe. The spread of AMR can affect humans, animals, and environments, which are all linked (One Health).”

The team screened seafood purchased from Georgia markets and found the same bacterial host, plasmids, and genes that they had previously identified in wastewater. 

Plasmids are small, circular DNA pieces that exist separately from a cell’s main DNA, and can move between cells, spreading traits like AMR.

“AMR does not recognize national borders, and if the plasmids favor the survival and fitness of the bacteria, then they will surely spread with and across different species. Foodborne bacterial infections can be very dangerous and problematic as is. Imagine if these infections become unresponsive to antibiotics.”

“Additionally, we often forget that many foodborne bacterial pathogens are also waterborne and/ or zoonotic. Therefore, we must be very concerned about foodborne pathogens acquiring AMR genes, regardless of the route of acquisition,” he warns.

The team found colistin-resistance genes in bacteria from imported shrimp and scallops in Atlanta markets.

Monitoring limitations

The US imports nearly 90% of its shrimp, and even though imported seafood is screened for contaminants, the scientists flag it may not detect AMR genes.

During the study, they found plasmid-borne and transmissible MCR genes in shrimp samples imported from Indonesia and Thailand, while a chromosomal MCR was found in a scallop imported from China, says Kassem.

“The US has robust monitoring systems for food safety and antibiotic resistance. However, it is difficult to monitor resistance in imported seafood, especially in unconventionally targeted bacteria such as Serratia, due to several factors, including the amount of imported seafood, expenses associated with testing, and manpower.”

“In 2022, the National Antimicrobial Resistance Monitoring System started to routinely monitor AMR in salmon, shrimp, and tilapia from retail grocery outlets in eight states. However, this monitoring remains limited in scope to specific bacterial species.”

Meanwhile, countries, including China, Thailand, and Indonesia, have banned colistin as a feed additive in food animal production. However, the status of colistin use and its extent in treating infections in farmed seafood is “unclear.”

The team only sampled imported seafood available in stores at the time, without focusing on the country of origin. “This means we do not have evidence to conclude that MCR does not occur on seafood sourced from other countries.”

Colistin use in seafood production persists in some countries despite bans.

Calls for global action

Kassem says current regulations and policies must include more robust monitoring systems and investments in alternative practices to contain AMR through the food supply.

“Also, they must target and prioritize the preservation of certain antibiotics that are considered critical to human and animal health.”

He expresses concerns over some countries still selling and marketing critical antibiotics as “vitamins for animal health” and their easy availability for animal farming in the black market.

“In addition to strict regulations and incentivization, we need everyone to be on the same page. We need a global collaborative effort that targets the misuse of antibiotics in food production.”

“Chiefly, intensive education programs are needed so that stakeholders recognize the dangers associated with AMR. Lastly, consumers and other stakeholders must engage with their official representatives to demand real action to combat AMR.”