Dosing livestock with antibiotics can be bad for farmers' health. A strain of MRSA that causes skin infections and sepsis in farm workers evolved its resistance to antibiotics inside farm animals.
The ST398 strain of MRSA first appeared in 2003 and is prevalent in US livestock. Humans who pick it up from animals can become dangerously ill, but it cannot yet spread from human to human.
A team led by Paul Keim of the Translational Genomics Research Institute in Phoenix, Arizona, sequenced the genomes of 88 closely related strains of Staphylococcus aureus, the bug that can become MRSA. Their findings suggest that ST398 was originally a harmless strain living in humans, which migrated into livestock where it acquired antibiotic resistance.
For some time microbiologists have been concerned that giving large amounts of antibiotics to livestock can promote antibiotic resistance. Keim's study provides a powerful example, says Ross Fitzgerald of the University of Edinburgh, UK.
Earlier this year the US Food and Drug Administration announced new restrictions on using preventive antibiotics in livestock, but the rules cover a small subset of drugs constituting just 0.2 per cent of antibiotics used on farms. As a result they are not expected to do much good.
Journal reference: mBio, in press