Speculation has long abounded that overuse of antibiotics by factory farmers has been a major contributing factor in the development of so-called “superbugs” like MRSA or Methicillin-resistant Staphylococcus aureus. Now, according to a report from Mother Jones, there is scientific proof.

According to a paper in the American Society of Microbiology’s newsletter mBio, researchers have sequenced the genomes of 88 closely-related strains of Staphylococcus aureus. They have concluded that one “particularly nasty” strain, CC398, began as a fairly harmless human bacterium known as MSSA, but evolved after colonizing the systems of pigs, chickens and other livestock.

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Inside the animals, the bacterial strain was bombarded by an array of broad-spectrum antibiotics, drugs commonly used by factory farmers to reduce infections and disease in animals kept in close quarters. According to mBio, this allowed the germs to become resistant to antibiotics like tetracycline and methicillin, as well as allowing the microorganisms to become “bidirectional,” meaning that they can freely be transmitted between humans and livestock.

The resistant CC398 strain first appeared in livestock in 2003, but is now widespread among U.S. farm animals and has been causing sepsis and skin infections, mostly in farm workers. So far, the infection has not been able to transmit from human to human.

The Food and Drug Administration announced in January that it is placing new restrictions on the wholesale use of some antibiotics in farm animals. Mother Jones reports, however, that according to the journal New Science, these regulations cover a paltry .02 percent of the drugs commonly used on animals.

(image via WikiMedia Commons)