Nasty Strain of MRSA

It's long been suspected that administering large amounts of antibiotics to livestock promotes antibiotic resistance.

Now a new paper in mBio describes how a particularly nasty strain of MRSA—the CC398 strain  found primarily in pigs but also in cattle and poultry—likely did that. 
Sequencing the genomes of 88 closely-related strains of S. aureus, the researchers found the CC398 strain likely originated as a harmless bacterium living in humans, which acquired antibiotic resistance only after it migrated into livestock. From there it migrated back to humans, where it now causes skin infections and sepsis, mostly in farm workers.
So far the strain has not evolved the ability to transmit between humans.