A team of researchers from the Northern Arizona University have discovered that a strain of the potentially life-threatening antibiotic-resistant bacteria MRSA jumped from livestock animals to humans.

The researchers discovered that a strain of methicillin-resistant Staphylococcus aureus (MRSA), which humans contract from livestock was originally a human strain, but it developed resistance to antibiotics once it was picked up by farm animals and can now transfer back to humans. The Arizona team, who published their findings in the journal mBio this month, said the disturbing findings further strengthen the link between misuse of antibiotics and the rise of potentially lethal and difficult to cure human infections.

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Although in the EU efforts are underway to curb the irresponsible use of antibiotics in medicine and farming, with the adoption last year of a 12 point program aimed at tackling resistance to antibiotics, in the United States, the FDA is fighting big pharma over its decision to tighten regulations on cephalosporins (antibiotics), other antibiotics are still widely used as growth promoters in livestock.

A range of governments, including the UK, France and Germany’s authorities have announced their own measures to tackle resistance and overuse of antibiotics, however, progress appears slow. In 2010, after sustained lobbying from drug companies and industrial farming interests, Defra rejected a decision to prevent companies advertising their drugs direct at farmers, which would have brought the country’s regulations in line with the rest of Europe.

The research team, from the Translational Genomics Research Institute (TGen) in Arizona, said their study draws a line between the exposure of S. aureus to antibiotics on farms and the development of a resistant form of MRSA that can threaten human lives. They said that, although this correlation has long been suspected but has been difficult to study directly.

Through genome sequencing, the team found that the bacteria changed rapidly, switching genes once it had found a home in livestock, whereupon it differentiated into different types, including the ST398 studied, which is resistant to a number of antibiotics.

Ross Fitzgerald, of the University of Edinburgh, who reviewed the paper, said, "Most of the ancestral human strains were sensitive to antibiotics, whereas the livestock strains had acquired resistance on several independent occasions."  He said that the study highlighted the fact that humans and animals rely on a finite number of antibiotics and concluded, "Intensive farming practices could promote the transfer of bacteria between [the] different host species."

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