Drug-resistant staph bacteria strains have been found in U.S. grocery store beef, pork and poultry at “unexpectedly high” rates, according to a recent study, but meat industry groups said the research is misleading and overstates risks to human health.

According to the Translational Genomics Research Institute, 47 percent of meat samples in the study were contaminated with Staphylococcus aureus, bacteria linked to a wide range of human diseases. Fifty-two percent of those bacteria were resistant to at least three classes of antibiotics.

But staph aureus is a common bacterium found on the skin of people and animals, so it would have been unusual to have found not found it on meat, University of Minnesota researchers Peter Davies and Liz Wagstrom wrote in responses to the study.

Staph bacteria usually does not cause illnesses in humans, most often affecting people with compromised immune systems, open wounds or invasive devices, which is why the vast majority of staph infections happen in health care facilities, the Minnesota researchers said.

Additionally, the Translational Genomics study didn’t measure how much staph bacteria was found on samples, the Minnesota researchers said in an e-mail forwarded by the National Pork Producers Council.

“Simply finding low levels of staph is no indication of the meat’s potential to cause illness,” they said.

Translational Genomics’ study was based on 136 samples covering 80 brands of beef, chicken, pork and turkey from 26 retail grocery stores in Chicago, Fort Lauderdale, Fla., Flagstaff, Az., Los Angeles and Washington, D.C.

The small sample size is “insufficient to reach the sweeping conclusions” made in the study, the American Meat Institute said in an April 15 statement. By contrast, U.S. Department of Agriculture bacteria surveys involve thousands of samples collected over long periods of time, the Meat Institute said.

While the study claims many of the bacteria were antibiotic resistant, it noted that they are not heat resistant, the Meat Institute said. “These bacteria are destroyed through normal cooking procedures, which may account for the small percentage of foodborne illnesses linked to these bacteria,” according to the Meat Institute, which represents most U.S. beef, pork and poultry processors.

Staph infections account for less than 1 percent of total foodborne illnesses, the Meat Institute said, citing government data.

Methicillin-resistant Staphylococcus aureus, or MRSA, can cause a range of illnesses from minor skin infections to life-threatening diseases, such as pneumonia, endocarditis and sepsis.

Staph bacteria have grown resistant to a widening range of the antibiotic drugs used to fight them. While U.S. food safety regulators routinely test retail meat for four types of drug-resistant bacteria, S. aureus is not among them, meaning a more comprehensive inspection system is needed, according to Translational Genomics.

According to the Translational Genomics study, DNA testing suggests that the food animals themselves were the major source of contamination. Densely-stocked industrial farms, where food animals are steadily fed low doses of antibiotics, are ideal breeding grounds for drug-resistant bacteria that move from animals to humans, according to the study.

“For the first time, we know how much of our meat and poultry is contaminated with antibiotic-resistant Staph, and it is substantial,” said Lance B. Price, senior author of the study and director of Translational Genomics’ Center for Food Microbiology and Environmental Health.

Although staph bacteria can be killed with proper cooking, they still may still pose a risk to consumers through improper food handling and cross-contamination in the kitchen.

“The fact that drug-resistant S. aureus was so prevalent, and likely came from the food animals themselves, is troubling, and demands attention to how antibiotics are used in food-animal production today,” Price said in a statement on the Phoenix-based institute’s Website.

The study, which was published April 15 in the Clinical Infectious Diseases journal, was supported through a grant from the Pew Charitable Trusts, a non-profit organization that has been critical of antibiotic use and other large-scale agriculture practices.

The U.S. pork industry has funded research to determine if MRSA is present in the domestic swine herd and supports additional epidemiology research and strong surveillance systems in hospitals and healthcare communities to monitor the disease, National Pork Producers Council spokesman Dave Warner said in an e-mail

“The U.S. pork industry supports the responsible and judicious use of antibiotics, which are essential to the health and well-being of animals and to producing safe pork products,” Warner said.