October 31, 2014

MRSA Probably Developed in Food Animals

According to a new study by the Translational Genomics Research Institute (TGen), a strain of the antibiotic-resistant bacteria MRSA (methicillin-resistant Staphylococcus aureus) most likely developed that resistance in animals raised for food.

The study, published in the online journal mBio, focused on MRSA CC398. This particular strain is also known as “pig-MRSA” because it infects people who have close contact to pigs and other food animals. MRSA CC398 is found in almost half of the meat products in the United States.

The bacteria takes a convoluted evolutionary path to becoming antibiotic-resistant.

  • First, the bacteria starts in humans as methicillin-susceptible-S. aureus.
  • It makes the jump to animals.
  • In animals, it becomes resistant to methicillin and tetracycline, most likely because of the low level antibiotic dosing commonly used on farm animals.
  • Then the bacteria is passed back to humans.
  • The next step is for the bacteria to be passed between people, causing an outbreak.

One of the study’s authors, Paul Keim, said, “We can’t blame nature or the germs. It is our inappropriate use of antibiotics that is now coming back to haunt us.”

The Center for Disease Control (CDC) has admitted that the widespread practice of using low doses of antibiotics in farm animals may be a problem and a source of antibiotic-resistant bacteria. This study confirms that theory. In fact, antibiotic resistant pathogens were the source of some major foodborne illness outbreaks in 2011.

Representative Louise Slaughter (D-NY) (who is a microbiologist) has introduced legislation regulating antibiotic use in farm animals. Last week, she asked fast-food corporations to tell consumers if they use antibiotics in their farm-raised meat products.

The FDA has been slow to rule on the extralabel use of antibiotics in cattle, swine, chickens, and turkeys. They first decided not to withdraw the antibiotics penicillin and tetracycline from animal feed, then announced that ranchers and farms must restrict extralabel use of some antibiotics.

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