According to a team of scientists led by Dr Ross Fitzgerald from the Roslin Institute and the University of Edinburgh in Scotland, cows may be a source of MRSA CC97 – an epidemic methicillin-resistant strain of Staphylococcus aureus that causes skin and soft tissue infections in humans.
Scanning electron micrograph of methicillin-resistant Staphylococcus aureus, yellow. Credit: NIAID.
The methicillin-resistant Staphylococcus aureus (MRSA) strain CC97 is an emerging human pathogen in Europe and the United States.
Dr Fitzgerald with colleagues sequenced the genomes of 43 different CC97 isolates from humans, cattle and other animals, and plotted their genetic relationships in a phylogenetic tree.
Their findings, reported in the open-access journal mBio, show 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,” Dr Fitzgerald explained.
Although the CC97 strains from animals were quite genetically diverse, the human isolates cluster together in two tight, distinct clades, indicating that MRSA CC97 in cattle crossed over into humans on two separate occasions.
Using mutation rates as a molecular clock, the scientists determined that the ancestor of clade A jumped from a bovine host to humans between 1894 and 1977 and clade B made the jump between 1938 and 1966.
“After they made the jump, the human CC97 strains acquired some new capabilities 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. The reverse is true as well: the bovine strains have their own mobile genetic elements,” Dr Fitzgerald said.
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 MRSA CC97 strains 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.
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Bibliographic information: Laura E. Spoor et al. 2013. Livestock Origin for a Human Pandemic Clone of Community-Associated Methicillin-Resistant Staphylococcus aureus. mBio vol. 4, no. 4; doi: 10.1128/mBio.00356-13
