Almost a quarter of the meat and poultry sold in U.S. grocery stores may be contaminated with drug-resistant forms of Staphylococcus aureus bacteria, also known as Staph, according to a study released Friday.

Researchers at the Translational Genomics Research Institute (TGen) examined 136 samples of meat and poultry from grocery stores in five cities around the country and found that 47 percent of them contained Staph bacteria, which can cause a variety of illnesses in humans, from skin infections to pneumonia or meningitis.

Out of all the strains of Staph found on these fresh meats, nearly half were multi-drug resistant, meaning they were resistant to at least three different antibiotics.

“This is the first time we’ve even recognized that drug-resistant staph is in the food supply,” said Dr. Lance Price, senior author of the study and director of TGen’s Center for Food Microbiology and Environmental Health, during a news conference.

The government currently monitors food for antibiotic-resistant strains of four foodborne pathogens: E. coli, Enterococcus, Salmonella and Campylobacter, but not for Staphylococcus, according to Price.

“So I think Staph is something that, given what we’re finding here, definitely needs to be added to that list,” he said.

Now that Staph has been shown to be so prevalent in food, Price said more research is needed about the danger this might pose to humans. “What we don’t know from this current study yet is whether these Staph are moving to people,” he said. “Now we really need to do those studies. The American consumer deserves to know what their risk is.”

Source of the Antibiotic-Resistant Staph: Animal Antibiotics?

What the authors say they do know, however, is why these drug-resistant strains of Staph are cropping up in food. At least they say they have an educated guess. The suspect? Animal antibiotics.

Researchers determined that the Staph bacteria found on meat in the study came largely from the animals themselves, rather than from outside contamination, which leads them to believe that drug-resistant Staph develops within animals, arising from antibiotics administered to them.

 “One of the major conclusions of the paper was that animals were the predominant source of the Staph that we were finding. So when we see a multi-drug resistance like we did, that points directly to problems with antimicrobial use on the farm,” said Price.

The use of antibiotics on food animals has raised concerns among some scientists and health officials that this practice is contributing to the rising number of antibiotic-resistant strains of disease in humans.

Two recent studies from Denmark and Canada showed that when farmers there stopped administering antibiotics to animals, corresponding antibiotic-resistant diseases decreased in humans.

“There is unequivocal evidence that decreasing antibiotic use in food animal production decreases antibiotic-resistant bacteria in the animals, decreases antibiotic-resistant bacteria on foods and decreases antibiotic-resistant bacteria in people,” said Price, referring to those previous studies.

In a letter last year, Thomas Frieden, director of the Centers for Disease Control and Prevention (CDC), made it clear that CDC is concerned about the repercussions animal antibiotics could have for human health.

“The CDC feels that there is strong scientific evidence of a link between antibiotic use in food animals and antibiotic resistant bacteria in humans,” he wrote.

This most recent research from TGen adds to the pile of evidence building up against animal antibiotics, which currently account for 80 percent of antibiotics sold in the United States in 2009, according to data from the FDA. 

“These are staggering figures and even more evidence that we must end the rampant overuse of antibiotics in agriculture,” said Rep. Louise Slaughter (D-NY), who is also a microbiologist, in a statement about the TGen study. “What we are witnessing is a looming public health crisis that is moving from farms to grocery stores to dinner tables around the country. Unless we act now, we will unwittingly be permitting animals to serve as incubators for resistant bacteria,” she said.

Meat Industry Responds

Meat and poultry producers responded to the TGen study by saying its small sample size makes it an inaccurate picture of the risks of Staph in fresh meat.

“This small sample is insufficient to reach the sweeping conclusions conveyed in a press release about the study,” the American Meat Institute (AMI) said

“Calling into question the safety of U.S. beef without conclusive evidence is careless and misleads consumers,” said Bill Donald, President of the Naitonal Cattlemen’s Beef Assiciation, in a statement.

AMI also cited data from CDC showing that Staphylococcus aureus is responsible for less than 1 percent of foodborne illnesses in the United States each year. 

And in response to the claim that animal antibiotic use leads to drug-resistant Staphylococcus, AMI points to studies that show that MRSA (antibiotic-resistant Staph) is rare, and is usually caused by human contamination.

AMI also notes that, while Staph found on meat in the study was antibiotic-resistant, it was not heat-resistant, and could be killed by cooking to an appropriate temperature.

However, Price doesn’t think the burden of preventing drug-proof Staph infections should fall to the consumer.

“Putting it on the consumer is like polluting the air and then telling everybody to wear gas masks,” he says. “We know where the drug-resistant bacteria are coming from and we know how to decrease them, and the single most effective way to reduce antibiotic-resistant bacteria in food is to stop feeding millions of animals antibiotics.”

The study was published in Clinical Infectious Diseases, and was sponsored by Pew Charitable Trusts