Ashley O'Brien in Tara Smith's UI lab has a new study out in PLoS ONE looking at MSSA and MRSA in fresh retail pork samples. Tara has a comprehensive blog post covering this study and their prior work leading up to this paper, so check it out if you want a lot of the details.
Briefly, they collected 395 samples from 36 stores in IA, MN and NJ. 300 samples were from "conventional" pork and 95 were from pork labeled "raised without antibiotics." S. aureus was found in 67% of conventional samples and 57% of antibiotic-free samples, while 6.3% of conventional pork and 7.4% of antibiotic-free pork were contaminated with MRSA.
This is a relatively small study, so it likely should be repeated. Also, as Tara mentioned in her post, in the states included in this study, very few USDA-certified organic products were available unfrozen, and they targeted sampling of fresh meats. It is hard to speculate why MSSA and MRSA didn't differ significantly between the types of pork, but contamination during processing or at retail is certainly possible. Could it be that conventional and antibiotic-free pigs are raised in close proximity?
O'Brien AM et al. PLoS ONE January 2012