Imported Seafood and the Spread of Colistin Resistance

Colistin is a last-resort antibiotic used to treat people with severe bacterial infections that no longer respond to other drugs. However, it is not foolproof. Resistance to colistin is spreading globally. This reduces treatment options and raises the risk of infection.

Researchers at the University of Georgia recently identified a new way for colistin-resistance genes to spread: imported seafood. In a new study, microbiologist Issmat Kassem, Ph.D., and his team reported the first detection of colistin-resistance genes in bacteria found in imported shrimp and scallops. These samples were purchased from eight food stores in Atlanta, Ga.

Kassem will present the findings this week in Los Angeles at ASM Microbe 2025, the annual meeting of the American Society for Microbiology. A related study will be published in the ASM journal mSphere.

We love our seafood.

Issmat Kassem, Microbiologist, University of Georgia

Many people do not realize that most seafood eaten in the United States is imported. That includes about 90 % of shrimp, he added. Imported seafood is screened for pollutants, but current methods do not detect everything. This includes antimicrobial resistance genes.

“The bacteria that were carrying colistin resistance genes are not normally screened,” Kassem noted.

Kassem and his team also found that some resistance genes are carried on plasmids. These are circular pieces of genetic material that can move from one bacterium to another.

Antimicrobial resistance is an ongoing public health concern. Antibiotic-resistant infections cause hundreds of thousands of deaths worldwide each year. Colistin was first developed in the 1950s to treat infections caused by Gram-negative bacteria. However, it can seriously affect patients, increasing the risk of kidney and nerve damage. In the 1980s, it was phased out in the United States.

Kassem noted that other countries still use colistin in agriculture. It is used to treat animal diseases and promote growth. Because it was one of the few effective treatments for some bacterial infections, colistin was later reintroduced for human use. The World Health Organization classifies it as a high-priority, critically important antibiotic for severe infections.

In 2016, researchers identified a mobile colistin resistance gene, known as mcr. It was called “mobile” because it could move between bacterial plasmids through lateral transmission. According to Kassem, before that discovery, scientists believed colistin resistance was inherited, not shared. “Which means it could not jump between different bacteria.”

Researchers have now found at least ten mcr genes and several variants, known as alleles. Kassem, who has studied antimicrobial resistance for two decades, suspected that the genes could spread through food imports and exports.

Kassem added, “Our food is sourced from different places. If you go out to lunch today, your plate might have ingredients from 6, 7, 8 countries. 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.”

In an earlier study, Kassem’s lab found mcr genes in wastewater samples from Georgia. They also identified the bacterial host carrying the plasmid with those genes. This type of testing is not routinely performed on imported food, he explained. Later studies found mcr genes in plasmids in other locations.

When the team tested seafood bought from markets in Georgia, they identified the same bacterial host, plasmids, and genes they had previously seen in wastewater.

“The good news is that we did not find it in locally produced seafood,” Kassem noted.

He noted that the scientists identified one source of colistin resistance. However, there may be others, and these are likely to spread.

Kassem concluded, “We live in a very connected world. We move a lot, we travel a lot, our food travels, and we are going to spread whatever emerges, even across national borders. So, it is important to invest in monitoring systems and expand them and collaborate, especially on the global level, on the issue of antimicrobial resistance.”