A study recently published in MBio lends further weight to the growing theory that using animal antibiotics in livestock contributes to drug resistance among human bacteria.
Methicillin-resistant Staphylococcus aureus, or MRSA, is a strain of Staph that's resistant to methicillin - the drug most commonly used to treat Staphylococcal infections.
Using a detailed DNA mapping technique, researchers at the Translational Genomics Research Institute (TGen) in Arizona were able to trace one of these superbugs - MRSA CC398 - to its origins, discovering that the human strain of this pathogen developed its drug resistance in animals rather than in people.
Often referred to as "pig-MRSA" or "livestock-associated MRSA," the strain is known to affect humans who have been exposed to live animals, such as farmers or veterinarians. But this study found that CC398 was originally a human bacterium, susceptible to antibiotics, before it spread to animals and then back to people. By the time it returned to humans it had picked up two souvenirs: resistance to methicillin and resistance to tetracycline - a drug often used to treat Staphylococcus aureus infections in patients allergic to the penicillin class of antibiotics, which includes methicillin.
Because both tetracycline and penicillin are commonly administered to food animals, the study finds that it is likely that the use of these drugs in livestock gave Staphylococcus aureus the exposure it needed to develop resistance to these drugs.
In 2010, Tetracycline - used to promote growth and prevent the spread of disease - comprised over 42 percent of all antibiotics administered to food-producing animals in the United States. That year 12,328,520 pounds of the drug were given to animals, while just over 100,000 pounds of the drug are sold for human use. And while over 1.9 million pounds of penicillin were sold for animal use in 2010, approximately 1.5 million pounds are distributed for human use.