MRSA strain in humans originally came from cattle

Published on: Aug 13, 2013

A strain of bacteria that causes skin and soft tissue infections in people originally came from cattle, according to a study to be published in mBio, the online open-access journal of the American Society for Microbiology. However, the researchers who conducted the genetic analysis of strains of Staphylococcus aureus known as CC97 said these strains developed resistance to methicillin after they crossed over into people around 40 years ago.

The researchers sequenced the genomes of 43 different CC97 isolates from humans, cattle and other animals, and plotted their genetic relationships in a phylogenetic tree. Corresponding author Ross Fitzgerald of the Roslin Institute and the University of Edinburgh in Scotland said strains of CC97 found in cows appear to be the ancestors of CC97 strains from humans.

"Bovine strains seemed to occupy deeper parts of the phylogenetic tree — they were closer to the root than the human strains. This led us to conclude that the strains infecting humans originated in cows and that they had evolved from bovine to human host jumps," Fitzgerald said.

After they made the jump, the human CC97 strains acquired some new capabilities, Fitzgerald noted, thanks to genes encoded on portable pieces of DNA called mobile genetic elements.

"It seems like these elements, such as pathogenicity islands, phages and plasmids, are important in order for the bacterium to adapt to different host species," Fitzgerald said. "The reverse is true as well: the bovine strains have their own mobile genetic elements."

Perhaps the most problematic new capability the human strains acquired is the ability to resist methicillin, an important antibiotic for fighting staphylococcal infections. Only human strains of CC97 were able to resist the drug, which indicates that the bacteria acquired resistance after they crossed over into humans, presumably through exposure to antibiotics prescribed for treating human infections.

This sequence of events contrasts with the case of a S. aureus strain from pigs, Fitzgerald pointed out, since a study in 2012 revealed that MRSA ST398 strains evolved the ability to resist methicillin before they crossed over into humans (http://mbio.asm.org/content/3/1/e00305-11). Any number of factors could create these differences, making pigs - but not cattle - a source of a drug-resistant bacterium. At this point, though, there isn't enough information to say whether differences in the S. aureus strains, differences between pigs and cattle, or differences between swine and dairy farming practices might be responsible.