"Their results are not unexpected," said Jennifer Koeman, director of public health for the National Pork Board. "The prevalence is comparable or less than seen in other countries."

But while nearly a quarter of the MRSA strains identified in the study were of the livestock variety, 50 percent were human strains, said Tara Smith, lead author of the study and an epidemiological researcher at the University of Iowa.

"These are common human strains," Smith said. "These strains do cause infections in people."

Staphylococcus aureus bacteria is one of the leading agents of food poisoning in the U.S., causing an estimated 185,000 cases every year. The bacteria has been associated with serious, sometimes deadly blood, skin and other organ infections in people. Research indicates that a growing number of these bacterial infections are resistant to standard antibiotic treatment.

MRSA, the antibiotic-resistant form of Staphylococcus aureus, was once associated only with hospital infections. But a growing number of cases are being found outside of hospitals, and community-associated MRSA is now considered the leading cause of skin and soft-tissue infections treated in emergency rooms.

Smith, who is also the interim director of the University of Iowa's Center for Emerging Infectious Diseases, said she was surprised at the prevalence of staph bacteria found in their samples.

Nearly two-thirds of the samples contained some form of the staph bacteria.

However, the researchers found no difference between the incidence of MRSA – the antibiotic-resistant form of staph – in conventionally produced pork products and those produced without the use of antibiotics.

“We were surprised to see no significant difference in antibiotic-free and conventionally produced pork,” Smith said. “Though it’s possible that this finding had more to do with the handling of the raw meat at the plant than the way the animals were raised, it’s certainly worth exploring further.”

According to Smith, unlike products that are stamped with the organic label, products with the label "raised without antibiotics" are not routinely verified by an independent third party. In addition, pork produced conventionally and alternatively can be processed at the same plants and, therefore, contamination can occur.

"It's hard to say, just taking the end product, where the contamination came from," she said.

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She said she worries that consumers may become too complacent with food handling when they see a product stamped with organic or antibiotic-free, because it doesn't mean it is free of bacterial contamination.

Liz Wagstrom, a scientist with the National Pork Producers Council, said that regardless of the health risk posed by some of these strains, it is "always a good idea to wash your hands."

This post also appears at California Watch, a content partner of KQED News.

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Resistant Bacteria Found in US Pork Products