MRSA in meat: How much? Which? And more bad news.

(Sorry for the radio silence, constant listeners. It’s been a challenging few weeks at Casa Superbug, with a death in the family and the chaos afterward of catching up to the rest of life. But back now, with some interesting stuff planned for later this week.)

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, NARMS (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 ST 398, 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.

So, there’s that. Also at ASM, a team from Nicholls State University in Thibodaux, La. reported they found S. aureus in 50 percent of pork, beef and chicken from local grocery stores. In the abstract, they give the amount of MRSA they found as “many.”And in a much more complete abstract, a multi-national team led by North Dakota State University analyzed 136 nasal swabs from live animals at the university’s veterinary hospital and 150 supermarket meat samples (pork, beef, chicken) from Fargo, ND. They found S. aureus in 30 percent of the animals and 52.7 percent of the meat. There was MRSA in 8 percent of the pork samples, and it was all PVL-negative, a suggestion that it may have been ST 398, the livestock strain.

Meanwhile, a team from Korea reported plenty of S. aureus and MRSA in food from that country:  staph in 112 out of 376 samples of pork, beef, chicken, and sashimi, with 89 of them resistant to at least one antibiotic family and eight resistant to at least three antibiotic classes. There were four strains of MRSA present in the meat samples, again by analysis of mec type. (It doesn’t say how many of the isolates were MRSA, though.)

Finally, one little-noticed abstract at the meeting sounded an ominous note for future drug resistance in animals. A team from the University of Bern in Switzerland analyzed isolates from the routine monitoring for ST 398, pig MRSA, that is carried out in slaughterhouses in Switzerland. During 2009 and 2010, they report, 90 percent of the MRSA isolates showed a new resistance factor, to tiamulin and virginiamycin. This is unnerving.

Tiamulin is a veterinary drug that has no direct human analog, so no great distress there. Virginiamycin, though, has an analog in human medicine, a combination called  quinupristin/dalfopristin that is better known by the trade name Synercid. Synercid was approved by the FDA for human use in 1999, and is a drug of last resort for very serious infections including vancomycin-resistant Enterococcus or VRE. Scientists have been agitating for the FDA to withdraw its approval for veterinary virginiamycin almost since that human approval, but in 2004, the FDA declined to take action.

Virginiamycin use has been banned in the European Union since 1998. It is still used in animals in the US, and has been since 1974.

In the decade since Synercid’s human introduction, virginiamycin-resistant gut bacteria were periodically reported in animals, but the contention has always been that the percentage of resistance was low and the organisms’ movement into humans was rare. This new finding, though is of a new gene that appears to have migrated from enteric bacteria into staph (as vancomycin resistance did from VRE), and that occurs on a mobile genetic element that is capable of moving between bacteria. It is a preliminary finding  and requires more study, but it is unlikely to be good news.

See Also:



Leave a Reply

Your email address will not be published. Required fields are marked *