A new study led by the University of Georgia’s Center for Food Safety (UGA’s CFS) suggests that imported seafood—particularly shrimp and scallops—can introduce resistance genes for last-resort antibiotics into the U.S. food system.

The project was partially funded by grants from UGA’s CFS. The study was authored by researchers from the Food and Agriculture Organization of the United Nations (FAO) and the Yale University School of Medicine, in addition to those from UGA’s CFS. The full study, published in Applied and Industrial Microbiology, can be read here.

The Threat of Antimicrobial Resistance (AMR) and its Emergence via Drug Use in Food Animals

Colistin is a last-resort antibiotic used to treat infections caused by multidrug-resistant bacteria in humans. The emergence of mobile colistin resistance (mcr) genes—and the rise of antimicrobial resistance (AMR) as a whole—is a critical threat to global public health. For instance, the U.S. Centers for Disease Control and Prevention (CDC) estimate that more than 2.8 million AMR infections occur in the country yearly, resulting in the deaths of 35,000 people.

The use of antibiotics in food animals is common practice worldwide, but their imprudent use contributes to the issue of AMR. Although colistin is not approved for agricultural use in the U.S., recent sampling suggests that mcr genes may be entering the U.S. food supply through vehicles like imported food products.

The U.S. imports approximately 65–85 percent of its seafood, and surveillance programs like the National Antimicrobial Resistance Monitoring System (NARMS) have limited scope in seafood monitoring, per the researchers. Additionally, the prevalence of mcr genes in seafood-exporting countries has been noted.

In this context, the researchers aimed to investigate whether imported seafood serves as a vehicle for introducing transmissible mcr genes into the U.S. food supply.

Resistance to Last-Resort Drug With Transfer Capabilities Identified in U.S. Seafood Imports

For the study, a total of 45 imported and 18 domestic seafood (shrimp and scallop) samples were collected from eight retail stores in Georgia. Bacterial colonies were isolated from the samples and subjected to antimicrobial susceptibility testing. Isolates were analyzed via polymerase chain reaction (PCR) and whole genome sequencing (WGS) to provide genetic and phenotypic characterization.

Colistin-resistant colonies were found in 20 percent of imported samples and 16.6 percent of domestic samples. The researchers randomly selected 48 isolates to screen for mcr-1 to mcr-10 using gene-specific PCR. The analyses detected mcr in 12.5 percent (six) of the tested isolates, which were retrieved from 13.4 percent (six) of the imported samples. Specifically, one mcr-3-positive isolate was retrieved from scallops imported from China, while five mcr-9-positive isolates were found in shrimps imported from Indonesia and Thailand.

The mcr-positive isolates were further analyzed using WGS to identify bacterial species, antimicrobial resistance genes, virulence genes, and plasmid types. WGS analyses corroborated the PCR results and identified one mcr-3-positive Aeromonas salmonicida and five mcr-9-positive Serratia nevei. The isolates exhibited high resistance to colistin. Additionally, the mcr-9-positive S. nevei were multidrug-resistant (also exhibiting resistance to penicillin, ampicillin, amoxicillin, tetracycline, erythromycin, and streptomycin).

Single nucleotide polymorphism (SNP) analysis revealed that isolates from Indonesia and Thailand had SNP distances of 23–37, suggesting clonality.

Sequencing confirmed the plasmid localization of the mcr-9 genes, and heat shock assays demonstrated successful transfer of the mcr-9-carrying plasmids to Escherichia coli DH5-alpha.

The researchers also tested and confirmed the ability of the mcr-9-positive S. nevei to form biofilms. After 12 days, colonies were mcr-9-positive, indicating the persistence of the mcr-9-carrying plasmid in S. nevei biofilms.

Moreover, using PathogenFinder v.2.1 and VirulenceFinder v.2.0, all the mcr-9-positive S. nevei were predicted to be potential human pathogens and carried virulence genes. The researchers note that S. nevei is closely related to S. marcescens, an opportunistic enteric pathogen known to cause gastrointestinal infections.

Overall, the detection of plasmid-borne mcr-9 in imported seafood—capable of horizontal gene transfer, biofilm persistence, and functional resistance expression—underscores the public health risk posed by the rise of AMR facilitated by the global food trade. The researchers recommend that NARMS surveillance of seafood imports be expanded, and underline the need for policy development to address antimicrobial use in aquaculture, as well as international collaboration to mitigate the global spread of AMR through food systems.