These resistance genes might be linked to the use of antibiotics in industrial agriculture: The researchers found 42 antibiotic-resistance genes that were shared between livestock-associated and human-associated bacteria, demonstrating a crucial link connecting pools of drug resistance in human and agricultural populations.
"Somehow, even though a billion years of genome evolution separate a bacterium living on a cow and a bacterium living on a human, both are accessing the same gene library," Alm says. "It's powerful circumstantial evidence that genes are being transferred between food animals and humans."
Moreover, the team identified 43 independent cases of antibiotic-resistance genes crossing between nations. "This is a real international problem," says microbiology graduate student Mark Smith, another lead author of the study. "Once a trait enters the human-associated gene pool, it spreads quickly without regard for national borders."
The practice of adding prophylactic antibiotics to animal feed to promote growth and prevent the spread of disease in densely housed herds and flocks is widespread in the United States, but has been banned in many European countries. According to the Federal Drug Administration, more than 80 percent of the 33 million pounds of antibiotics sold in the United States in 2009 was for agricultural use, and 90 percent of that was administered subtherapeutically through food and water. This includes antibiotics such as penicillins and tetracyclines commonly used to treat human illness.
The MIT researchers found that HGT occurs more frequently among bacteria that occupy the same body site, share the same oxygen tolerance or have the same pathogenicity, leading them to conclude that ecology or environmental niche is more important than either lineage or geographical proximity in determining if a transferred gene will be incorporated into a bacterium's DNA and passed on to its descendants.
|Contact: Kimberly Allen|
Massachusetts Institute of Technology