Researchers have nailed down something scientists, government officials and agribusiness proponents have argued about for years: whether antibiotics in livestock feed give rise to antibiotic-resistant germs that can threaten humans.

A study in the journal mBio, published by the American Society for Microbiology, shows how an antibiotic-susceptible Staph germ passed from humans into pigs, where it became resistant to the antibiotics tetracycline and methicillin. And then the antibiotic-resistant Staph learned to jump back into humans.

"It's like watching the birth of a superbug," says Lance Price of the Translational Genomics Research Institute, or TGen, in Flagstaff, Arizona.

Price and colleagues in 19 countries did whole-genome analysis on a Staph strain called CC398 and 88 closely related variations. CC398 is a so-called MRSA, or methicillin-resistant Staphylococcus aureus, that emerged within the past decade in pigs and has since spread widely in cattle and poultry as well as pigs.

The genetic analysis allowed the study authors to trace the lineage of the livestock bug back to its antibiotic-susceptible human ancestors. Price says it shows beyond doubt that the animal bacterium jumped back into humans with close exposure to livestock.

Many livestock groups say there's no evidence that using antibiotics in livestock feed creates a human health problem.