Apr 15, 2011 (CIDRAP News) – Scientists report that they found the common pathogen Staphylococus aureus on close to half of 136 raw meat and poultry samples from five US cities and that more than half of the isolates were resistant to several antibiotics, but they say the public health significance of the findings is not clear.
"Our findings indicate that multidrug-resistant S aureus should be added to the list of antimicrobial-resistant pathogens that routinely contamination our food supply," says the report, published online today by Clinical Infectious Diseases.
But the researchers, from the Translational Genomic Research Institute (TGen) in Flagstaff, Ariz., say the public health relevance of their results remains to be sorted out, because little is known about the risk of human S aureus infections associated with meat and poultry products.
Up to 25% of healthy people carry S aureus on their skin or in their nostrils, according to the Centers for Disease Control and Prevention (CDC). The pathogen causes a wide range of skin infections as well as serious illnesses such as pneumonia, meningitis, endocarditis, and sepsis.
Foodborne S aureus outbreaks are not uncommon. Last December about 100 S aureus illnesses in Illinois and Wisconsin were tied to contaminated pastry. In an August article in Morbidity and Mortality Weekly Report, the CDC said 21 outbreaks of illness from foodborne staphylococcal toxins were reported in 2007, with 286 cases.
In a press release, the TGen study authors noted that proper cooking kills the pathogen, but it can still pose a risk to consumers through improper food handling and cross-contamination.
First national assessment
In what they call the first national assessment of antibiotic-resistant S aureus in the US food supply, the investigators collected and tested a total of 136 samples from 26 grocery stores in five cities: Chicago, Washington, Fort Lauderdale, Los Angeles, and Flagstaff. The products included ground beef, chicken, pork, and turkey, representing 80 different brands.
The pathogen was found in 47% of the samples overall, the report says. Contamination was most common in turkey, 77%; followed by pork, 42%; chicken, 41%; and beef, 37%.
Tests for antimicrobial susceptibility showed that 52% of the isolates had intermediate or complete resistance to three or more classes of antibiotics. Such multidrug resistance was most common in isolates from turkey (22 of 28, 79%), with lower rates in pork (7 of 11, 64%), beef (6 of 17, 35%), and chicken (6 of 23, 26%).
Two strains predominant
In examining genetic sequences, the researchers found 15 sequence types among the isolates, but two types, ST5 and ST398, were dominant because they were very prevalent in chicken and turkey samples. ST398, discovered in 2003, is a multidrug-resistant strain that mainly colonizes workers in food animal production and now accounts for many cases of community-acquired methicillin-resistant S aureus (MRSA) in the Netherlands, the report notes.
Two other sequence types, ST1159 and ST1, accounted for 29% of the beef isolates and 55% of the pork isolates, respectively.
"The distinct S aureus populatons on each product type suggest that food animals are the predominant source of contamination," the article says. "While a portion of the S aureus isolates may have been the result of human contamination, a uniform pattern of human-associated strains was not observed."
The authors also report that they found MRSA in one sample each of beef, turkey, and pork. In addition, fluoroquinolone-resistant isolates were "uniquely prevalent" among chicken samples, perhaps because fluoroquinolones were used in US broiler production from 1995 to 2005.
As for what the findings mean for public health, the authors say it is known that ST398 can colonize and infect humans, "but few studies have investigated the risk of human colonization and infection with S aureus from meat and poultry products."
The European Food Safety Authority judged that the risk for MRSA infection from food handling and consumption was low, but that was based on a small number of studies, the report says. Also, the agency didn't look at the risk of methicillin-resistant but multidrug-resistant S aureus, which is more common than MRSA in food, it adds.
Industrial agriculture to blame?
The authors go on to note that billions of animals are raised in concentrated animal feeding operations (CAFOs), where they are routinely fed antibiotics. In the press release, they comment that CAFOs are "ideal breeding grounds for drug-resistant bacteria that move from animals to humans."
"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," senior author Lance B. Price, PhD, commented in the press release. The study was funded by the Pew Charitable Trusts as part of the Pew Campaign on Human Health and Industrial Farming.
A growing literature
Craig Hedberg, PhD, a foodborne disease expert at the University of Minnesota, said the study is not the first to find S aureus in raw meat. There is a growing body of literature on the pathogen, and MRSA in particular, in food animal populations and raw meat and poultry products, he said.
Hedberg, an associate professor of occupational and environmental health, said the study suggests that food animals may be an important reservoir for S aurueus, including MRSA, and that use of antibiotics in industrial agriculture may be contributing to the developoment of multidrug resistant S aureus.
"Both of these may be true, but the implications are not clear," he said.
"Staph aureus are very common organisms that spread from person to person. In terms of foodborne transmission, food-handler contamination of ready-to-eat foods such as salads seems a greater concern to me than does cross-contamination from raw meat," Hedberg said. He added that in his view the concern is not so much the risk of foodborne illnesses as the threat of other kinds of Staph infections that are potentially more dangerous.
Hedberg said the possible role of food animals in the development of multidrug-resistant S aureus merits more research and magnifies the importance of prudent-use guidelines for antibiotics in humans and animals. "However, these preliminary findings do not allow us to conclude what role food animals may play in the epidemiology of [S aureus] infection in the population at large."
He also noted that S aureus produces toxins that are not destroyed by cooking, "but toxin production requires considerable growth of the organisms under conditions that would not be typical for handling of raw meat or poultry."
Waters AE, Contente-Cuomo T, Buchhagen J, et al. Multidrug-resistant Staphylococcus aureus in US meat and poultry. Clin Infect Dis 2011 May 15;52 (early online publication) [Full text]
Apr 15 TGen press release
Dec 28, 2010, CIDRAP News story about pastry-linked S aureus outbreak