April 15, 2011 -- There’s a new reason to be careful when handling raw meat at mealtimes.
Researchers testing raw turkey, pork, beef, and chicken purchased at grocery stores in five different cities across the U.S. say that roughly one in four of those samples tested positive for a multidrug antibiotic-resistant “superbug” bacterium.
“The findings were pretty shocking,” says study researcher Lance B. Price, PhD, director of the Center of Food Microbiology and Environmental Health at the Translational Genomics Research Institute in Flagstaff, Ariz. “We found that 47% of the samples were contaminated with Staph aureus, and more than half of those strains were multidrug resistant, or resistant to three or more antibiotics.”
The presence of drug-resistant staph bacteria, a category that includes methicillin-resistant Staphylococccus aureus (MRSA), in farm animals and food has been a closely watched problem in Europe, where it has been traced to outbreaks of human disease.
But less has been known about its prevalence in the U.S. food supply.
“We haven’t looked at this before in the United States,” says Price. “What we don’t know is whether people can pick it up through meat. This is the first time that we’ve even recognized that it’s there.”
“We don’t know where these are coming from, and it’s really something that we have to understand,” he says.
Meat Producers Respond
Food producers say their products are safe.
“Staphylococcus aureus is a very common bacteria found in the environment, and is one of the most common found on human hands. It rarely causes any health problems,” says Hilary Thesmar, PhD, RD, director of scientific and regulatory affairs for the National Turkey Federation in Washington, D.C., in a statement.
“Contamination by human hands is a likely source of contamination of the products in this study,” Thesmar says. “The most important message for consumers is to follow proper food safety methods, such as washing your hands and cooking meat and poultry thoroughly. Following good food safety practices will ensure that consumers continue to enjoy safe, high-quality, and nutritious turkey products.”
Others agree with that assessment.
“Staph aureus is common in everything. It’s common in people. Something like 30% of people carry it in their nasal passages, and it’s on your skin. Finding that in food products wouldn’t have been anything out of the usual,” says Dave Warner, director of communications for the Pork Producers Council in Washington, D.C.
What the Study Found
For the study, researchers collected 136 meat and poultry samples from 26 grocery stores in five cities: Chicago; Washington, D.C.; Fort Lauderdale, Fla.; Los Angeles; and Flagstaff, Ariz.
DNA testing confirmed the presence and specific types of S. aureus bacteria. The bacteria were exposed to antibiotics from different classes to determine which drugs could kill the germs and which could not.
Of all the meat types tested, turkey samples were the most frequently contaminated; 20 out of 26 samples (77%) tested positive for S. aureus. Turkey was also the most likely to have bacteria that couldn’t be killed by at least three classes of antibiotics; 79% of the turkey samples that tested positive for staph were multidrug resistant.
That compares to roughly 40% of pork, chicken, and beef cuts that tested positive for the bacteria. Of the positive samples of those meats, 64% of pork samples, 35% of beef, and 26% of chicken were multidrug resistant.
Three samples, or slightly more than 2%, tested positive for MRSA. The strains of MRSA identified in the study were resistant to antibiotics that have never been approved for food production, suggesting that the MRSA strains may have come from people who were handling the meat.
The study is published in Clinical Infectious Diseases.
Learning From Europe
In 2006, scientists in the Netherlands were the first to trace infections in a farming family caused by MRSA from people back to their pigs.
By 2007, the same group of Dutch scientists reported that the strain they found, called ST398, was responsible for more 20% of all the MRSA found in people in that country.
In 2009, a study that tested more than 2,217 samples of raw meat in the Netherlands found nearly 12% were contaminated with MRSA, with 85% of the MRSA bacteria belonging to the ST398 strain.
“What we’re learning from Europe is that there are these strains of multidrug-resistant and methicillin-resistant Staph aureus that can move from food animals to farm workers and then to their families and get established in the community that way,” Price says.
Consumer advocates say this new study may help to explain the growing number of drug-resistant staph infections that arise from the community, rather than a health care setting.
“Normally we think of MRSA exposure in a hospital setting, but clearly, it’s coming into your home on raw meat. You could be getting it through handling raw meat,” says Caroline Smith DeWaal, JD, director of food safety for the nonprofit Center for Science in the Public Interest (CSPI) in Washington, D.C.
DeWaal recently co-authored a white paper for CSPI on the problem of antibiotic resistance in food-borne pathogens, but she was not involved in the current research.
“The best precaution would be to handle meat and poultry with gloves, especially if they have any wounds on their hands,” DeWaal says.
The danger is that drug-resistant bacteria could enter the body, usually through a small cut in the skin, causing an infection that's tough for doctors to treat.
Infectious disease, experts, however are quick to point out that any tie between drug-resistant staph bacteria in meat and human infections, at least in the U.S., is still circumstantial.
“The study is fairly small,” says Pascal James Imperato, MD, dean of the School of Public Health at the SUNY Downstate Medical Center, in Brooklyn. “We don’t know, when you look at such a small sample, how valid the conclusions are. One really wants to see much larger numbers.”
“I don’t see the contaminated meat as having been proven, yet, to be a major public health problem. We don’t have any data on that, frankly,” Imperato says.