According to Science Daily, American scientists from the University of Georgia, led by microbiologist Dr. Issmat Kassem, reported the first isolation of colistin-resistance genes in bacteria found in imported shrimp and scallops. An accompanying paper will be published in the ASM journal mSphere.
Colistin is a last-resort antibiotic used only to treat people with life-threatening bacterial infections that have developed resistance to other drugs, and it is gradually losing its effectiveness due to the emergence of mobile resistance genes known as MCR. The discovery of colistin resistance genes in imported seafood reveals a new pathway for the emergence of deadly and untreatable bacterial infections, promoting the spread of transmissible colistin resistance.
Kassem and his group analyzed seafood samples purchased from eight food markets around Atlanta, Georgia. They found, for the first time, bacteria with MCR genes in these samples. "The good news is that we didn't find it in locally produced seafood," Kassem said, according to Science Daily.
Most seafood consumed in the US is imported, including about 90% of shrimp. "Our food is sourced from different places. If you go out to lunch today, your plate might have ingredients from 6, 7, 8 countries," Kassem added, according to Science Daily.
Imported seafood is screened for contaminants, but the process doesn't catch everything, especially antimicrobial resistance genes. "The bacteria that were carrying colistin resistance genes are not normally screened," Kassem noted. This oversight could lead to dangerous pathogens slipping through import inspections undetected.
Antimicrobial-resistant infections kill hundreds of thousands of people globally every year. The World Health Organization categorizes colistin as a high priority critically important antibiotic, which means it is an essential option for treating serious human infections.
Colistin was first introduced in the 1950s to treat infections by pathogenic Gram-negative bacteria but was discontinued in the United States in the 1980s due to its potential side effects, including increased risk of damage to the nerves and kidneys. It was eventually reintroduced to human medicine because it was one of the few options available to treat certain bacterial infections. Other countries continued to use colistin in agricultural settings, both to treat infections and to promote animal growth.
In 2016, researchers discovered a mobile colistin-resistant gene, or mcr, that was "mobile" because it could be transferred via lateral transmission in plasmids passed among bacteria. Before then, researchers believed colistin resistance was inherited, not shared. "Which means it could not jump between different bacteria," Kassem explained, according to Science Daily. Since then, researchers identified at least 10 mcr genes and many alleles, or variations.
Kassem and his group found that some of the resistance genes are carried on plasmids—round bits of genetic material that can be transmitted from bacteria to bacteria. They also identified the bacterial host that was carrying the plasmid containing the genes. "The bacterial host wasn't normally screened in food coming into the US," Kassem said.
In previous work, Kassem's group found mcr genes in samples from wastewater in Georgia. When they screened seafood purchased from markets in Georgia, they found the same bacterial host, the same plasmids, and the same genes that they'd previously identified in wastewater. In studies published since then, researchers have found mcr genes in plasmids elsewhere, indicating the global spread of colistin resistance.
Kassem cautioned that while they identified one source of colistin resistance, there could be others, and they're likely spreading. He suspected that colistin resistance might spread through the importing and exporting of food. "We live in a very connected world," he said, according to Science Daily. "We move a lot, we travel a lot, our food travels, and we are going to spread whatever emerges, even across national borders."
"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," he explained, according to Science Daily. "So, it's important to invest in monitoring systems and expand them and collaborate, especially on the global level, on the issue of antimicrobial resistance."
Colistin is not foolproof; worldwide, resistance to colistin is spreading, further diminishing treatment options and putting infected people at higher risk. "Antimicrobial resistance is a rising public health menace," Kassem warned. "
Assisted by a news-analysis system.