Doctors always suspected that our heavy reliance on antibiotics is what spurred the rise MRSA, Methicillin-resistant Staphylococcus aureus, the staphylococcus "super germ." Turns out it may not have been us, but rather our porcine population.
According to a new study by researchers Paul Keim and Lance Price, both from Northern Arizona University, and published in the journal mBio, MRSA actually began as a nonresistant strain that infected humans.
The duo employed sequenced the genomes of 89 types of animals — including turkeys, chickens, pigs, and humans-samples from four continents.
They discovered that MRSA only developed its resistance after jumping from humans to pig populations within our food production chain. While floating amongst the hogs, it became immune to two antibiotic medications — tetracycline and methicillin — that pigs are commonly administered. These medications are also commonly prescribed to fight human staph infections. From there, the strain appears to have then jumped back to humans, bringing it's new-found defenses with it.
As the study posits,
The human-associated isolates from the basal clades carried phages encoding human innate immune modulators that were largely missing among the livestock-associated isolates. Our results strongly suggest that livestock-associated MRSA CC398 originated in humans as MSSA.
"Our findings underscore the potential public health risks of widespread antibiotic use in food animal production," Price said in a statement. "Staph thrives in crowded and unsanitary conditions. Add antibiotics to that environment and you're going to create a public health problem."
The CC398 strain, as it's known first appeared in cattle, pig, and poultry populations around 2003. The study argues that the mixture of growth hormones, antibiotics, and other medications employed to increase production and make the animals more suitable for the crowded conditions industrial food production requires are to blame for creating an ideal setting for the bacteria to gain resistance. "The most powerful force in evolution is selection. And in this case, humans have supplied a strong force through the excessive use of antibiotic drugs in farm animal production," said Paul Keim, a co-author on the study. [PopScience - Inside NAU - mBio]
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