A common strain of methicillin-resistant Staphylococcus aureus (MRSA) made the jump to humans from cattle, according to a study published in mBio. Researchers who conducted the genetic analysis of strains of S. aureus known as CC97 say these strains developed resistance to methicillin after they crossed over into humans around forty years ago. Today, methicillin-resistant S. aureus strain CC97 is an emerging human pathogen in Europe, North and South America, Africa, and Asia. The findings highlight the potential for cows to serve as a reservoir for bacteria with the capacity for pandemic spread in humans.
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, PhD of the Roslin Institute and the University of Edinburgh in Scotland says strains of CC97 found in cows appear to be the ancestors of CC97 strains from humans. Although the CC97 strains from animals were quite genetically diverse, the human isolates cluster together in two tight, distinct “clades,” or relatedness groups, indicating that S. aureus CC97 in cattle crossed over into humans on two separate occasions.
Using mutation rates as a molecular clock, the authors 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 the ability to resist methicillin, presumably through exposure to antibiotics. Read the article.
