I’m flattered to have as a regular reader Dr. Peter Davies, a professor of swine health and production in the University of Minnesota’s Department of Veterinary Population Medicine. (Disclosure: I worked part-time at U Minn from mid-2006 to mid-2010, but in a different school.) In a comment on my previous post, he points out — perils of reading on a smartphone — an important point where I erred: The staph strain involved in the death of the French 14-year-old was not MRSA, but MSSA, drug-sensitive staph, that had picked up a resistance factor.
Unpacking that a bit: At a minimum, MRSA is resistant to all beta-lactam antibiotics — penicillin, the semi-synthetic penicillins (including methicillin, what the M in MRSA stands for), several generations of cephalosporins, monobactams, and carbapenems. It is also separately, but variably, resistant to macrolides (such as erythromycin), lincosamides (clindamycin), aminoglycosides (gentamicin), fluoroquinolones (ciprofloxacin) and tetracycline.
Livestock-associated MRSA, known as ST398 for its performance on a particular test (multi-locus sequence typing) was first identified as having a tie to pig-farming because it was also resistant to tetracycline, which was being given to the pigs on the farms where the first human carriers worked. (Hence its jocular name, “pig MRSA,” though it’s since been found in other animals.)
The ST398 strain involved in the French girl’s death does not have that broad array of resistance. Chiefly, it was not resistant to beta-lactams, and so can’t be considered MRSA. On analysis, it was resistant to the macrolides, of which the best-known are erythromycin and azithromycin (Zithromax or Z-Pak). Here’s something else intriguing: On another test (spa typing), the ST398 strain in the French girl was one known as t571; the ST398 that has spread from pigs to humans in the European Union, and subsequently to Canada and the United States, is usually t034.
Here’s why this is all so interesting: MSSA ST398 t571 was reported just a few years ago in New York City, in a Bronx community that has close ties to the Dominican Republic, and also in the towns in the Dominican Republic where those Bronx residents come from and visit. (Here’s my initial post on that finding from a medical meeting, and subsequent post when the paper was published.) In that case, the ST398 was fully drug-sensitive — and there was no visible link to pigs, though the authors speculated that livestock, perhaps poultry, might be playing a role on either side of the “air bridge” connecting the two communities.
In the paper (Bhat, Dumortier, Taylor et al., EID 2009, DOI: 10.3201/eid1502.080609), the authors expressed concern that, given staph’s promiscuous ability to acquire resistance — and the fact that ST398 is not regularly surveilled for — the ST398 in New York could become an undetected resistant strain:
Given ST398’s history of rapid dissemination in the Netherlands, its potential for the acquisition of methicillin resistance, and its ability to cause infections in both community and hospital settings, monitoring the prevalence of this strain in northern Manhattan and the Dominican Republic will be important to understand more about its virulence and its ability to spread in these communities.
And now it appears it has become resistant — but in France, not New York City or the Dominican Republic, and to macrolides, not beta-lactams. It’s one more reminder of staph’s genius at acquiring genetic defenses, and of how our lack of attention to its mutability and spread continues to allow it to take us by surprise.