A study to be published in the online journal mBio® on November 29 shows that adding antibiotics to swine feed causes microorganisms in the guts of these animals to start sharing genes that could spread antibiotic resistance.
Livestock farms use antibiotic drugs regularly, and not just for curing sick animals. Antimicrobial drugs are used as feed additives to boost animal growth, a profitable but controversial practice that is now banned in the European Union and under scrutiny here in the United States. Using antibiotics in animal feed saves farms money, but opponents argue the practice encourages antimicrobial resistance among bacteria that could well be consumed by humans. Today, livestock producers in the U.S. use an estimated 24.6 million pounds of antimicrobials for nontherapeutic purposes every year. The U.S. Government Accountability Office recently urged the federal government to follow up on plans to evaluate the impacts of the use of antibiotics as growth promoters.
The study by Heather Allen and her colleagues at the USDA National Animal Disease Center (NaDC) in Ames, Iowa, adds to the sum of knowledge about what happens to the microorganisms that populate animal digestive tracts when they are exposed to low, persistent levels of antibiotics. The researchers studied how two in-feed antibiotic formulations affect prophages, segments of DNA found in bacteria that can encode antibiotic resistance genes and other genes that bacteria may use.
Prophages can cut themselves out of the larger chromosome of bacterial DNA in a process called induction, then replicate and package themselves as viruses. These viruses explode the cell from the inside then move on to infect other organisms and deliver their genes.
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