MRSA, Meat, and Motown

It's been not even a month since the last paper looking at MRSA in meat, and up pops another one. So far here in the US, we've seen studies in Rhode Island (no MRSA found); Louisiana (MRSA found in beef and pork, but "human" types: USA100 and USA300); the recent Waters et al study sampling in California, Florida, Illinois, Washington DC, and Arizona, finding similar strains (ST8 and ST5, associated with USA300 and USA100, respectively). Now a new study has collected MRSA samples in Detroit, collecting 289 samples from 30 retail stores in the city.

For this study, they collected only beef, turkey, and chicken--a bit odd, since pork has been the meat product typically linked to MRSA to date. The paper is short on methods so it doesn't say how the sampling was done, which is a bit frustrating as they found levels of S. aureus that were quite a bit lower than those found in the Waters paper. Unlike the Pu and Waters papers, *all* of the Detroit samples were USA300. No typing data was given for the S. aureus that were susceptible to methicillin.

There's also something interesting about some of the USA300 isolates--they're resistant to tetracycline. Resistance to this antibiotic is relatively rare in human S. aureus isolates, but it was found in 3 chicken samples--all a molecular type called t2031. The other isolates were resistant to erythromycin, and one was additionally resistant to ciprofloxacin and levofloxacin, suggesting (like the Waters paper) that multi-resistant S. aureus are present in our meat supply. Unfortunately, there's no information letting us know whether these positive isolates--especially the unique t2031 strains--were from the same brands of meat product, same stores, etc.

So what's going on here? The authors suggest that human contamination is probably at play here, and that's quite possible. No ST398 ("livestock-associated") MRSA has been found yet in published papers examining U.S. meat, though Waters did find ST398 in their S. aureus which were methicillin-susceptible. That suggests that farm-origin Staph can make it through the processing chain, but is human contamination along the line a bigger issue in the U.S.? This is different than the situation in The Netherlands, where they found ST398 MRSA almost exclusively in the meat products they tested. But--the prevalence of humans carrying MRSA in that country is also much, much lower than it is in the U.S., so it may simply be an issue of relative colonization rates (more MRSA in Dutch animals versus their human population, while we may have more in American humans versus our animals--but additional surveillance would be needed to confirm that).

So what we're left with here is another piece of the puzzle, but one that unfortunately doesn't yet add a whole lot to the bigger picture.

Bhargava K, Wang X, Donabedian S, Zervos M, da Rocha L, Zhang Y. (2011). Methicillin-resistant Staphylococcus aureus in Retail Meat, Detroit, Michigan, USA Emerging Infectious Diseases : 10.3201/eid1706.101095

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For translational science...what does it mean for consumers?

cook the meat well; wash your hands with basic soap [not antibacterial ones] before and after handling

many people turn to organic, but with human contamination being perhaps the biggest factor, does it really matter so much? Are the strains different between organic and non-organic?

or is local better for minimizing strain spread in food chain--less variance in DNA for swapping

thanks for writing.

Obviously I'm no expert, but my hunch would be that going organic would be of no help against supply-chain contamination by unwitting humans. Organic livestock aren't supposed to get routine antibiotics, which should reduce the risk of breeding resistant bacteria, but there's no guarantee they won't still get colonized by the resistant bacteria anyway.

I think the most important thing is to practice good food safety in the kitchen. Handwash regularly with soap and water, wash all counter surfaces with soapy water before and after handling meat, do not mix raw meats on the counter, keep the meat cold until you are ready to cook it, cook it thoroughly, and religiously segregate cooked and raw meat (as well as the implements used to handle them) to avoid rendering the entire process pointless. Also, buy a few meat thermometers (remembering to keep them clean too and not let them become a vector for cross-contamination -- they're cheap) and check the internal temperature of your meat against tables for how hot it should be getting (remembering to always check temperature in the middle of the thickest part of the meat).

By Calli Arcale (not verified) on 17 May 2011 #permalink

There is MRSA and there is MRSA, they are not all the same. I am a pediatrician in rural NC. We are seeing a lot of MRSA abcesses. This community acquired MRSA is not associated with the hospital scenario of immunocompromised or elderly patients, people with multiple antibiotic use, etc. The reports I am getting is that 1/3 or more of children in the US are colonized with this type of MRSA. It is aggressive, and children who are susceptible and colonized develop infections after otherwise minor and unremarkable skin barrier transgressions (diaper rash, insect bite, playing outside). Spider bites are often implicated, but this is nonsensical, hysterical arachnophobia, just outright hatred of octapeds! If 1/3 or more of kids are colonized, perhaps bovids are colonized too? OR adults working in the meat trades? It may not mean much that the bacteria is on the meat, since it is so prevalent in the community. If they get a chance to grow, if the meat is poorly handled, the bacteria may grow, produce toxins and become a source of food poisoning. Are they a source of infection?

By Robert Solomon (not verified) on 17 May 2011 #permalink

Robert, I agree with much of what you say, but

The reports I am getting is that 1/3 or more of children in the US are colonized with this type of MRSA.

Where are you getting those numbers? Most studies show about a tenth of that.