By Jill U. Adams, ScienceNOW

If you're paying premium prices for pesticide- and antibiotic-free meat, you might expect that it's also free of antibiotic-resistant bacteria. Not so, according to a new study. The prevalence of one of the world's most dangerous drug-resistant microbe strains is similar in retail pork products labeled "raised without antibiotics" and in meat from conventionally raised pigs, researchers have found.

Methicillin-resistant Staphylococcus aureus (MRSA), a drug-resistant form of the normally harmless S. aureus bacterium, kills 18,000 people in the United States every year and sickens 76,000 more. The majority of cases are linked to a hospital stay, where the combination of other sick people and surgical procedures puts patients at risk. But transmission also can happen in schools, jails, and locker rooms (and an estimated 1.5% of Americans carry MRSA in their noses). All of this has led to a growing concern about antibiotic use in agriculture, which may be creating a reservoir of drug-resistant organisms in billions of food animals around the world.

Tara Smith, an epidemiologist at the University of Iowa College of Public Health in Iowa City who studies the movement of staph bacteria between animals and people, wondered whether meat products might be another mode of transmission. For the new study, published this month in PLoS ONE, she and colleagues bought a variety of pork products—395 packages in all—from 36 different stores in two big pig farming states, Iowa and Minnesota, and one of the most densely populated, New Jersey.

In the laboratory, the team mixed meat samples "vigorously" with a bacterial growth medium and allowed any microbes present to grow. MRSA, which appears as mauve-colored colonies on agar plates, was genetically typed and tested for antibiotic susceptibility.

The researchers found that 64.8% of the samples were positive for staph bacteria and 6.6% were positive for MRSA. Rates of contamination were similar for conventionally raised pigs (19 of 300 samples) and those labeled antibiotic-free (seven of 95 samples). Results of genetic typing identified several well-known strains, including the so-called livestock-associated MRSA (ST398) as well as common human strains; all were found in conventional and antibiotic-free meat. (The label "antibiotic-free" is not regulated, and the products were not "certified organic.")

Smith says she was surprised by the results. In a related investigation, which has not been published, her group tested pigs living on farms and found that antibiotic-free pigs were free from MRSA, whereas the resistant bug is often found on conventional pig farms.

The study reveals an important data point on the path from farm to fork, yet the source of the MRSA on meat products is unknown, Smith says. "It's difficult to figure out." Transmission of resistant bugs might occur between antibiotic-using and antibiotic-free operations, especially if they're near each other, or it could come from farm workers themselves. Another possibility is that contamination occurs at processing plants. "Processing plants are supposed to be cleaned between conventional and organic animals," she says. "But how well does that actually happen?"

In another recent study, researchers from Purdue University in West Lafayette, Indiana, found that beef products from conventionally raised and grass-fed animals were equally likely to be contaminated by antibiotic-resistant Escherichia* coli*. In a second study by the same group, poultry products labeled "no antibiotics added" carried antibiotic-resistant E. coli and Enterococcus (another bacteria that causes invasive disease in humans), although the microbes were less prevalent than on conventionally raised birds.

"The real question is, where is it coming from, on the farm or post-farm?" says Paul Ebner, a food safety expert who led the Purdue studies. And the biggest question of all, he says, "Is it impacting human health?"

"There's a tremendous amount of interest in this issue—feeding antibiotics to food animals," says Ellen Silbergeld, an expert on health and environmental impacts of industrial food animal production at the Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland. "Thus, determining when amending that practice makes a difference is important."

"The definitive study would take every bacterium and follow that along until it gets in humans—from food supply to causing a certain disease," Smith says. "It would be a huge and costly study that no one's going to do, but that's what the meat producers" say is missing." Meanwhile, Smith says she will continue her investigations of MRSA, one potential transmission point at a time.

This story provided by ScienceNOW, the daily online news service of the journal Science.

Image: Even pigs raised antibiotic-free may harbor MRSA bacteria. (Janice Haney Carr/CDC)

Updated: Feb. 1, 2012 at 10:20 a.m. EST. All original references to "organic" have been replaced by "antibiotic-free" because the meat used in this study was not certified organic.