More than 70 percent of the antibiotics consumed in the United States go to livestock farms, one of the main triggers driving a rising crisis of antibiotic resistance in human medicine.

On Tuesday, researchers from Ohio State University published an alarming finding in a peer-reviewed journal: On a US hog farm, they found bacteria that can withstand a crucial family of antibiotics. Carbapenems, as they are known, are a "last line of defense" against bacterial pathogens that can resist other antibiotics, the paper notes. Worse still, the gene that allowed the bacteria to resist carbapenems turned up in a plasmid—small chunks of DNA found in bacterial cells. Plasmid-carried genes bounce easily from one bacterial strain to another, meaning that carbapenem resistance is highly mobile—making it more likely to find its way into bacterial pathogens that infect people.

If this news sounds depressingly familiar, it's because something very similar happened with another last-ditch antibiotic, colostin. About a year ago, Chinese researchers alarmed global public health authorities when they found a "plasmid-mediated" strain of colistin-resistant E. coli on a Chinese hog farm. As predicted, it quickly went global, and it turned up in the United States in a patient in May, as well as in a pig intestine identified by US Department of Agriculture researchers. In September, Rutgers and Columbia University researchers found a strain of E. coli with plasmid-carried resistance to colostin and carbapenems. The new Ohio State study marks the first time plasmid-borne carbapenem resistance has been found on a US farm, though it has turned up in livestock operations in Asia and Europe, the researchers write.

To see whether carbapenem resistance is taking hold on US hog farms, the researchers settled on a 1,500-sow confined operation that follows "typical US production practices," which include giving newborn pigs a dose of an antibiotic called ceftiofur at birth, with the males getting a second dose when they're castrated at six days. Interestingly, carbapenems are banned from use in US farms. But ceftiofur is a member of the cephalosporin family of antibiotics, which kills bacteria in a similar way to carbapenems, and the authors speculate that those ceftiofur doses "may provide significant selection pressure" for the emergence of carbapenem resistance. They found it in swabs taken from the the surfaces of the farrowing and nursery pens.