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Drug-Resistant Staph Linked to Animal Antibiotics

A study published today in MiBio 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, a drug in the beta-lactam class of antibiotics, which are used to treat Staph infections in humans.  

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 bacteria 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 bacteria, 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 Staph infections in patients allergic to the penicillin class of antibiotics, which includes methicillin.   

Because both tetracycline and penicillins are commonly administered to food animals, the study finds that it is likely that the use of these drugs in livestock gave this Staph bacteria 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.  

Both of these drugs are crucial for treating human Staph infections, says Dr. Lance Price, lead author of the study and Director of TGen’s Center for Food Microbiology and Environmental Health.

“Methacyline’s a really good antibiotic for treating these kinds of infections,” he told Food Safety News in an interview. “But a substantial portion of the population is allergic to penicillin and they need alternative drugs like tetracycline. But 69% of the staph we see is resistant to tetracycline.”

Price says that animal feeding operation provide the perfect setting for the growth of antibiotic-resistant bacteria. According to the Centers for Disease Control and Prevention, MRSA is easily transmitted in settings where factors it calls the “5 Cs” are present: crowding, frequent skin-to-skin contact, compromised skin (cuts or abrasions), contamination or lack of cleanliness. 

“A CAFO (Concentrated Animal Feeding Operation) is the place where these 5 Cs are most prevalent,” says Price. “And the single worst thing you could do is add antibiotics to that environment. It’s the cocktail for creating superbugs.”

Once these superbugs are created, there’s no telling where they can go, Price says. While most cases of CC398 come from direct contact with livestock, some human cases cannot be traced to live animals, raising the question of where they came from. Other humans? Contaminated meat?

Several studies have shown MRSA to be prevalent on our meat and poultry. A study published in the Journal of Food Protection in October of last year found Staph bacteria in 29 percent of grocery store ground meats. And a study led by Price released earlier that year discovered Staph on almost one half of grocery store meats and poultry after putting samples in a bacteria enrichment broth to make Staph more detectable. Half of these contaminated meats were carrying a multi-drug resistant strain of Staph.

Price says the transmission of MRSA from meat to humans is an area that needs more research.

“There is definitely drug-resistant Staph in our food supply and probably in high concentrations. We still don’t know whether it’s a good route of exposure for people,” he explains. “I think that’s something we’ve really got to dedicate some research to.” 

For now though, Price says the upshot of his research is that antibiotics should be saved for treatment, not growth or prevention.  

“We should be using antibiotics prudently, both in human antibiotics and animal production,” he notes. “We should only be using them to treat infections – and it should be a last resort – because we’re heading towards a time when our antibiotics won’t work anymore, and we have to do everything we can to preserve the ones we have.”

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