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Microscopic image of Staphylococcus Aureus

An international research team has charted the recent emergence of a new superbug that has resulted from the routine use of antibiotics to boost livestock production.

For many years, scientists have been warning that agricultural practices are fuelling the growth of antibiotic-resistant strains of bacteria. But this latest research is especially worrisome because the new superbug can do serious harm to humans, not just animals.

It is a strain of methicillin-resistant Staphylococcus aureus, or MRSA, which can produce potentially fatal infections that don't respond to standard antibiotic treatment. Its official name is ST398. When the strain was first discovered in livestock in 2004, many experts assumed it would have a hard time colonizing humans. But that's not been the case – and it has spread from animals to farm workers, veterinarians and their families.

Scientists at the Translational Genomics Research Institute (TGen) in Arizona, the Statens Serum Institut in Denmark and several other institutions decoded the genome of ST398 and related strains in order to recreate the evolutionary history of the superbug.

The analysis revealed that ST398 probably originated in people as a strain of S. aureus that was still susceptible to antibiotics. Once it crossed into livestock, it picked up some new genes and became resistant to tetracycline and methicillin – two important antibiotics for quelling Staph infections, according to the findings published in the online journal mBio.

"What this study shows is that the boundaries between humans and animals are not as strict as we had once believed," said the lead researcher Lance Price of TGen, who added: "When we overuse antibiotics in animals, we are threatening our own health."