Last week was the General Meeting of the American Society for Microbiology (ASM). This is the conference at which, several years ago, Tara Smith’s team at the University of Iowa first announced they had found MRSA ST398 in pigs in the United States, so it always bears watching for new MRSA news, and this year it didn’t disappoint.
First: I’ve complained persistently because the federal system that monitors antibiotic-resistant bacteria in animals and food, NARSM (National Antimicrobial Resistance Monitoring System) doesn’t include MRSA among the pathogens that it tracks. It is possible that might be changing — because at ASM, a team from the Food and Drug Administration reported the results of a pilot study that looked for MRSA in retail meat in the US and found it.
According to the abstract, the team asked public health laboratories in nine states (Connecticut, Colorado, Maryland, New Mexico, Oregon, new York, Pennsylvania and Tennessee) to collect samples of ground beef, ground turkey, chicken breasts and pork chops, test them for the presence of S. aureus, and send the isolates to FDA for further testing. They received 311 staph samples, and 32 (10 percent) were MRSA.
After that, the analysis gets a little tricky, in the sense that it doesn’t match up to other studies. The team classified the isolates as MRSA based on the presence of the mecA gene (which confers resistance to beta-lactam antibiotics, of which methicillin, the M in MRSA, was the first). They then subdivided the isolates based on the short string of genetic material in which the mec gene resides, known as SCCmec (SCC stands for “staphylococcal chromosomal cassette”). There are seven SCCmec types so far, and they roughly correlate to what we think of as “hospital-acquired” and “community-associated” human MRSA infections — but they do not match up well to the results of multi-locus sequence typing, the method that’s been used over the past 7 years to identify ST398, or “livestock-associated” MRSA in animals and humans. (Used, for instance, in this important study that found drug-resistant staph in one in four meat samples.)
The FDA study found that 14 of the 32 samples (44 percent) were SCCmec type VI, which correlates to community-acquired staph, and that 12 (38 percent) were positive for the production of Panton-Valentine leukocidin or PVL, a cellular toxin generally produced by community-associated MRSA strains. That suggests, though there is no way to prove it, that the MRSA in those meat samples may be due to human contamination at slaughter or during packaging.
Regarding the other half of the isolates, the abstract doesn’t say anything. It’s worth noting that ST398 and the other livestock-associated MRSA strains tend to be PVL-negative. The abstract also doesn’t say whether susceptibility testing was performed, to see whether the isolates responded to drugs other than beta-lactams. A hallmark of livestock-associated MRSA has been its resistance to tetracycline, a drug that is little-used for human staph but commonly given to farm animals.