As Maryn McKenna and others have reported, a paper was released on Friday showing a high percentage of drug-resistant Staphylococcus aureus contaminating raw, retail-available meat products. There has been a lot of media coverage of this finding--so what does the study say, and what are its implications? More after the jump.
First, a bit about S. aureus itself, and why this study was carried out. Historically, Staph has been a relatively common cause of food poisoning. The bacterium produces toxins that can collect to a high level when prepared foods are left at room temperature, such as potato salad at picnics (a common vehicle for staph food poisoning). For much staph food poisoning, the ultimate source is generally thought to be a human--Aunt Bea who perhaps had S. aureus on her hands while cutting the potatoes, and then the bacterium grew and generated bacterial toxins while it was sitting out at the family gathering.
However, a number of recent studies have shown that S. aureus doesn't always have to come from people. Studies in the Netherlands and here in the US have shown the presence of S. aureus (including methicillin-resistant strains, or MRSA) on raw meat samples. That makes the epidemiology of S. aureus food poisoning much more difficult. Did Aunt Bea really contaminate the potato salad via her own S. aureus (about a third of us carry the organism, usually in our noses), or did she use the same cutting board for the potatoes that she'd just used to shape the turkey burgers, thereby cross-contaminating the potatoes that way? To try and figure out which had occurred, you could take a swab from Aunt Bea's nose and see if her staph matched what you isolated from the potato salad--but how can we be sure that Aunt Bea didn't also rub her nose while she was working on the turkey burgers, and spread that staph to herself as well? As you can see, it can be potentially messy.
That also introduces a second issue with staph in meat--it's not only the "classic" problem of toxin-mediated staph food poisoning, but we also have to consider the potential for food-borne S. aureus to be transferred from the meat to the food handler. If they're not careful with their procedures, they could end up introducing staph into any wounds on their hands (thereby possibly causing an infection), or rubbing their nose/eye/skin and introducing the bacterium that way, potentially leading to long-term carriage. Really, in that manner it's no different from picking up S. aureus at your local gym, or school, or hospital--you touch something that's contaminated with the bacterium, and then unknowingly colonize yourself with the germ.
So--for those reasons, there are two main concerns with the finding of staph in meat: classic food poisoning, and self-inoculation with subsequent potential spread to family and community. How often does either really occur? We have no idea at this time, and that's what makes studies like the current Price paper in Clinical Infectious Diseases critical.
For this study, they collected 136 meat samples (chicken, turkey, beef, and pork) from 5 large US cities: Flagstaff, Washington DC, Los Angeles, Chicago, and Fort Lauderdale. They tested all of the meat samples for S. aureus, and then did molecular typing on the isolates, and also determined what antibiotic resistances they demonstrated.
What did they find? A lot of staph, and a lot of drug resistance. From the paper, S. aureus contamination was most common among turkey samples (77%; 20/26) followed by pork (42%; 11/26), chicken (41%; 19/46), and beef (37%; 14/38). Almost all of the isolates were resistant to at least one antibiotic, and multidrug resistance (being resistant to at least 3 different classes of antibiotics) occurred among more than half of the isolates.
What about MRSA? Most of the stories don't specifically mention it, but Figure 1 in the paper shows that 3 of the tested isolates were MRSA (oxacillin resistant), for a prevalence of 2.2%. One was from a turkey sample originating in Washington, DC; another from a pork sample from Fort Lauderdale; and a third from a beef sample in LA. All three were also multi-drug resistant, meaning that the options would be limited if someone ended up infected with those and needed antibiotic treatment.
The other important finding was that most of the strains were not common "human" types of staph, and that each type of meat had its own unique "fingerprint" of staph types. Oddly enough, what we usually think of as "pig staph" (ST398, see also here) was identified more commonly in turkey samples than in pork, where the most common type was ST1. In chicken meat, ST5 was most commonly identified (to be expected; see this post by Ed Yong covering a study about how people gave staph to chickens decades ago). In beef, a number of different types were found.
Not surprisingly, the meat industry has criticized the study and the press accounts covering it. They note--rightly--that it's a small study, and there are a lot of unanswered questions as I describe above. A quote from the article (registration required):
"Authors of the new study, which involved a small number of samples from retail stores, claim that their findings suggest that a significant public health risk exists," AMI [American Meat Institute] said in a statement.
"However, federal data from the Centers for Disease Control and Prevention (CDC) show steady declines in foodborne illnesses linked to consumption of meat and poultry overall and indicate that human infections with Staphylococcus aureus comprise less than one percent of total foodborne illnesses, " AMI said.
When the U.S. Department of Agriculture studies the prevalence of bacteria, their work involves thousands of samples collected over long periods of time to ensure accuracy, AMI said.
The bacteria are destroyed through normal cooking procedures, AMI noted.
"While the study claims that the many of the bacteria found were antibiotic resistant, it does note that they are not heat resistant," AMI said.
They are correct that foodborne illness overall has decreased, and that cooking likely kills the organisms, but they completely gloss over the fact that 1) not all foods that will be in contact with Staph contamination from the meat products will be cooked (e.g., vegetables or anything that may be cross-contaminated during prep procedures), and 2) that there is the potential colonization aspect that I raised above. So some media outlets have oversold the risk, acting as if we have good evidence that Staph from meat is killing people (right now, we don't, though there are many cases of human infections with "animal" staph in people who lack contact with livestock). Others, such as AMI, are minimizing the risk by only focusing on one line of exposure (ingestion of cooked food that was contaminated with staph).
The bottom line is that we simply don't have the studies yet to tell us if Staph on food is riskier than Staph at your gym, or Staph at your school, or Staph directly from a pig, etc. However, there's nothing good or beneficial about having yet another source of S. aureus that's multi-drug resistant, and as such, it's yet another reminder to do what you can regarding safe food handling practices.
Waters, A., Contente-Cuomo, T., Buchhagen, J., Liu, C., Watson, L., Pearce, K., Foster, J., Bowers, J., Driebe, E., Engelthaler, D., Keim, P., & Price, L. (2011). Multidrug-Resistant Staphylococcus aureus in US Meat and Poultry Clinical Infectious Diseases DOI: 10.1093/cid/cir181
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Good synopsis ;-)
Thanks Tara for adding your invaluable perspective to this ongoing discussion!
Every year, millions of Americans get sick from food tainted with micro-organisms from salmonella to Campylobacter. While most episodes are short, I'll attest it can be misery that almost makes you wish you were dead. And about 1,800 die annually:
My two cents regarding food poisoning: I worry less about the particular micro-organism, or even their drug resistance, than teaching Aunt Bea to wash her hands between cutting chicken and dicing potatoes, to store foods in a sanitary fashion, to make sure the meats are cooked to the recommended temperatures, and to not leave food out for hours at room temperature. There are dozens of other nasties besides staphylococcus that justify the need for safe cooking.
To the issue of drug resistance in livestock, it seems to me the greater worry isn't food poisoning, but the possibility that those genes may find their way back into human vectors.
@Russell: I agree, washing seems like the big barrier to cross here. Anecdotaly, I don't think surface decontamination is a huge issue anymore (given the bewildering array of surface decontamination wipes on the market), but washing while preparing food. Often, its hard to wash between food preparation steps...
I would wonder, on that line, if Alaska has a higher incidence of food-born illness. The reason I wonder is because outside of Anchorage, most of the state views running water and indoor plumbing as a luxury. The number, last I saw, was around 70%, with population centres such as Fairbanks lagging behind truly rural villages (there's more aid available to develop village water supply than semi-urban). People have access to water, but it comes in jugs. Washing your hands becomes a lesser priority, because water becomes a difficult resource to acquire, and one that should be meted conservatively.
More after the jump. Unsubbing!
One thing I've always been worried about is how thoroughly you need to wash (your hands, tools or surfaces) to actually get them clean. I grew up with my mother immunocompromised, so I automatically do a fairly thorough scrub when cooking, don't wear rings or bracelets, keep my nails short, and so on. But how many people actually clean their nails properly in the kitchen, or take their rings off? I know I don't always these days, especially when I'm in a hurry. I think everyone is going, sooner or later, to give something a lick-and-a-promise clean, or forget and scratch their nose; the greater the proportion of contaminated food, the greater the risk that infected material and sloppy procedure will coincide. To throw all the responsibility onto the users (either domestic or commercial food preparers) is typical of business interests, but bad for public health.
Thanks, Tara! This sort of article is exactly why I'm so glad to see you blogging again.
Did the researchers note any correlation between a specific meat packing company and the drug resistance?
They don't describe anything like that. For researchers,I can attest to the fact that it's really difficult to get that kind of information--you can get the lot numbers etc. of the meat, but to trace that back to a packing plant takes the assistance of inspectors or someone similar to navigate. I know in our research we've not been able to obtain that information.
This blog was very interesting. I've always wondered how you get food poisining. I dont think washing your hands between seperate cooked foods will not realy help with sanitation, and not transfer staph between the seperate foods. I think that if you watch how you mix your foods, cook, and store them you will be fine.
Just wondering if you have done any research on how long staph can live: for example if you were to contaminate a dry dairy powder with staph then seal the bag, would the staph number's decrease and die off? if there was very little moisture?
Just wondering if you were to contaminate a dairy powder with staph ( say from your hands) and then seal the bag, how long could the staph live, would the numbers decrease?
I can attest to the fact that it's really difficult to get that kind of information--you can get the lot numbers etc. of the meat, but to trace that back to a packing plant takes the assistance of inspectors or someone similar to navigate.
Teresa, not sure. Staph generally live longer with a bit of moisture/humidity, so a dairy powder may be too dry for them to survive long. Just a guess though. A quick PubMed search didn't turn up anything close to that I could find.
I am doing some work on risk analysis on food intoxication by staph aurues. What probably the researcher needed also to know was Staph being present on meat doesnt automatically translate into illness. And so for them to assert that there is a high risk based on purely hazard identification is misleading.
Have you come accross any paper that talks of how many staph aureus colonies are needed in say meat to cause intoxication? If you please do me a favour and send it to me.