Antibiotics: they’re not just for curing the sick. Livestock farms in the U.S. regularly use antibiotic drugs as feed additives to boost animal growth, but a study in mBio this week reveals new evidence that adding antibiotics to pig feed stimulates gene exchange in the guts of these animals, a development that could move antibiotic resistance genes where they’re not wanted.
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 study by Allen et al. adds to what we know about what happens to the microorganisms that populate animal digestive tracts when they are exposed to these low, persistent levels of anitbiotics. Researchers at USDA’s National Animal Disease Center (NADC) in Ames, Iowa 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 impact bacterial fitness. Prophages can cut themselves out of the larger chromosome of 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.
Lead author Heather K. Allen says when pigs were fed antibiotics, the actual numbers of antibiotic resistance genes carried by the phages remained steady, but the microorganisms still reacted to the presence of antibiotics. Prophages underwent a significant increase in induction when exposed to antibiotics, indicating that medicating the animals led to increased movement of prophage genes among gut bacteria.
“Induction of the prophages is showing us that antibiotics are stimulating gene transfer,” says Allen. “This is significant because phages have previously been shown to carry bacterial fitness genes such as antibiotic resistance genes.”
Studies that explore the impacts of in-feed antibiotics most often focus on the bacterial residents of the gut, according to Allen, but phages and other viruses move a significant amount of genetic information around the community. This makes changes in prophage induction an important collateral effect of antibiotic treatment, she says. Resistance genes are the unit of currency among microbes experiencing the duress of an antibiotic, so following the movement of genes is arguably more important than following certain changes in bacterial communities. And if bacteria in humans acquire resistance genes from animals, there can be serious health consequences.
“What’s important is the transfer of a gene that could get into the wrong place at the wrong time,” says Allen. “Increased gene transfer is a critical event in the evolution of gut bacteria.”