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