'Pig MRSA' came from humans, evolved via farm drugs
This week, writing in mBio, the open-access journal of the American Society for Microbiology, a team from the Translational Genomics Research Institute in Flagstaff provide an answer. Using whole-genome sequencing of 89 MRSA and drug-sensitive (MSSA) isolates from around the globe, they establish that ST398 originated as a human MSSA strain which jumped to pigs — where it acquired both the hallmark methicillin resistance (actually resistance to several dozen drugs in the beta-lactam category) and also tetracycline resistance as a result of farm antibiotic exposure — and then jumped back to humans.Just to underline that key point: acquired resistance as a result of farm drug exposure. To be clear, MRSA ST398 represents what proponents of large-scale confinement agriculture contend does not exist: an indication that farm antibiotic use breeds resistance that moves off the farm and subsequently affects humans.The important development in the story of ST398 is its move back off the farm into humans, causing first asymptomatic carriage in that original family, and then illnesses in other Dutch residents, and then outbreaks in healthcare settings, and then movement across oceans, and then appearance in retail meat, and then infections in people who had no connection whatsoever to farming — all from an organism with a distinctive agricultural signature.Maryn McKenna, "‘Pig MRSA’ Came From Humans, Evolved Via Farm Drugs," WIRED, Feb. 23, 2012. http://www.wired.com/wiredscience/2012/02/pig-mrsa-origin/