How did MRSA, a persistent and deadly bug, become the drug-resistant bacteria vexing medical and public health experts?

The answer is in the genes. Researchers have pinpointed how a common strain found in livestock, called Staphylococcus aureus CC398 bounced from humans, when it was treatable, to animals where it became antibiotic resistant.

Drug-resistant staph infection has been linked to the over use of antibiotics in livestock. This is the first study to chart the genetic link between that and its consequences in humans.

MRSA, methicillin-resistant Staphylococcus aureus, is a type of staph bacteria that is resistant to common antibiotics. Staph infection can be especially deadly when it occurs in people who have weak immune systems. But it’s usually treatable through antibiotics. The problem with MRSA is that it's a superbug – unable to be treated or cured with the usual antibiotics.

MRSA has resulted in 278,000 hospitalizations and more than 18,000 deaths in 2005, according to one study.

In a study published Tuesday in online journal mBio, researchers from Translational Genomics Research Institute in Arizona, Statens Serum Institut in Denmark, and several other institutions, sequenced the genes of this particular bacteria.

The strain, Staphylococcus aureus CC398 started in humans and was still treatable with antibiotics, said Lance Price, director of the Center for Food Microbiology and Environmental Health Translational Genomics Research Institute in Arizona.

It spread to livestock, which are usually pumped with antibiotics, to keep them healthy. “The lineage appears to have undergone a rapid radiation in conjunction with the jump from humans to livestock, where it subsequently acquired tetracycline and methicillin resistance,” according to the study.

Methicillin and tetracycline are among the most common antibiotics used to treat staph infections.

The problem is in the way animals are raised and pumped with antibiotics, said Price, the lead author of the study. He said that farmers and ranchers give millions of pounds of antibiotics to farm animals to make them grow faster and to prevent - rather than treat - diseases.

It’s a controversial practice that has been banned in the European Union since 2006 because it contributes antibiotic resistance.

Farmers and workers surrounded by livestock are exposed to MRSA, which thrives in cluttered, congested and unclean feeding operations, Price said. They tend to be “overcrowded, have tons of contact, compromised skin, and they’re filthy. Now think about introducing an antibiotic in that setting,” he said.

“I couldn’t engineer a better system for creating drug-resistant bacteria or a superbug, than introducing antibiotics into a concentrated animal feeding operation,” Price said. “It’s begging for disaster.”

Earlier this year, the FDA prohibited one class of antibiotics in cattle, pigs, chickens and turkeys starting April 5, 2012, but some critics say this only restricts one class of drugs and that more regulation is needed to prevent more deadly strains.

“It’s underscoring that as we’re using millions of antibiotics, we’re selecting for drug resistant bacteria to come back and haunt us,” Price said.