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A recent study by a research group in Arizona found, in a wide sampling of meat from grocery stores, that almost a quarter of them contained strains of staph bacteria resistant to multiple forms of antibiotics. The report raises the specter of MRSA – the increasingly common and difficult to treat “superbug” form of staph – infecting meat consumers, but another recent report by the Wisconsin Food Research Institute found that such cases are exceedingly rare.

The Arizona study, conducted by the nonprofit Translational Genomics Research Institute and commissioned by the Pew Charitable Trusts, collected 136 meat samples from 26 grocery stores in five U.S. cities (Chicago, Washington D.C., Fort Lauderdale, Fla., Los Angeles and Flagstaff, Ariz.). Tests found that 47 percent of the samples were contaminated with staph bacteria – known for causing skin and internal infections in hospitals – and out of those, 52 percent were resistant to three or more forms of antibiotics.

Turkey samples were the most prone to antibiotic-resistant contamination, followed by pork, beef and chicken.

But only three of the samples (2 percent of those collected) contained staph bacteria resistant to methicillin, the antibiotic most commonly used to treat staph infections. Methicillin resistance is what makes a staph strain a MRSA “superbug.” MRSA infections, which can be deadly, have become more common in hospitals in recent years.

The MRSA meat samples came from one beef product, one turkey product and one pork product. The study notes, “Our sample size was insufficient to accurately estimate prevalence rates.” It adds that higher concentrations have been detected in meat samples in the Netherlands.

“The public health relevance,” it says, of widespread antibiotic-resistant staph contamination “is unclear.”

A report released earlier this year by three UW-Madison researchers (including two from the university’s Food Research Institute) and funded “in part by the American Meat Institute Foundation,” a group funded by meat producers, suggested the risk of contracting a MRSA infection from eating meat or some other food product is extremely low.

According to the report, only two foodborne outbreaks of MRSA have been recorded. In the first, which occurred in Tennessee in 2000, three people were sickened after eating coleslaw bought at a convenience store. In the second case, 41 people were sickened between 1992 and 1993 (and five died) during an outbreak at a Dutch hospital. In both cases, the source of the bacteria was believed to be the food handlers, not the food itself.

MRSA is found in livestock, particularly in pigs, in both Europe and the United States, although testing in this country has been limited. In recent European tests, about 27 percent of pigs tested positive for MRSA. Infection is believed to be less common in the United States, but testing at American pig farms has found concentrations ranging from zero to 33 percent, according to the UW report.

It says meat can become contaminated by both slaughtering infected animals and from the meat processors themselves. The CDC estimates that about half of Americans are staph carriers, and about 1.5 percent are MRSA carriers. (Staph often colonizes inside the human nose.) Other staph carriers include dogs (10 percent), zoo animals and even pet parrots.

The Arizona study suggests that the antibiotics given to farm animals may foster antibiotic resistant strains, a conclusion echoed by the UW researchers, who say, “Some suspect that the use of antibiotics in pig farming may have played a role in the evolution of (livestock-associated MRSA).”

They say that most cases of food poisoning caused by staph are caused by toxins produced by the bacteria that remain in food after the bacteria themselves are killed during cooking. But such cases of food poisoning are short-lived and relatively mild.

More serious intestinal infections are typically limited to infants, adults with compromised immune system or people who have depleted healthy, competing bacteria in their intestines by taking long courses of antibiotics. These infections are relatively rare, they say, and those caused by MRSA are even rarer.

Still, the Food Research Institute notes that the presence of MRSA in “retail meat samples raises concerns” and ongoing monitoring “would be prudent.”

Translational Genomics Research Institute didn’t return a call seeking comment.

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