The two bacteria were present in the wound of a patient in a Michigan hospital in June 2002.

The confirmation came when researchers sequenced DNA from S. aureus and concluded that the DNA, which carried genes for vancomycin resistance, had almost certainly come from the bacterium Enterococcus faecalis. Apparently the resistance genes had “jumped” from one microbe to another.

Scientists have known that in the laboratory the two species can exchange genes for resistance to vancomycin. But the new study marks the first time scientists have documented how the exchange occurred in real life.

Scientists at the Centers for Disease Control and Prevention in Atlanta, Georgia, led the research. The findings appear in Science.

“The really important observation is that this [exchange] happened in a clinical environment,” says Steven R. Gill of The Institute for Genomic Research (TIGR) in Rockville, Maryland, where the DNA was sequenced.

S. aureus is already resistant to most antibiotics, and scientists had long-anticipated that the bacterium would become resistant to vancomycin, leaving physicians without a last-resort treatment.

More than a year ago, doctors’ fears came true when they discovered S. aureus resistant to vancomycin in a Michigan dialysis patient. Staph bacteria can still be treated with some antibiotics.

Bacterial genomes have large amounts of DNA that can move around on chromosomes, between organisms, and even jump between species. This enables them to pass traits such as drug resistance from one to another.

Antibiotic resistance is a growing health concern, which led the U.S. Department of Health and Human Services to launch a campaign in September called “Get Smart: Know When Antibiotics Work” to alert doctors and patients to be cautious in using antibiotics.