A feared strain of 'superbug' that causes skin and soft tissue infections in humans originally came from cattle, according to a study by researchers who published their work through American Society for Microbiology. However, the researchers discovered the antibiotic-resistant bacteria only acquired their drug-resistant properties after crossing over into humans. 

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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 humans, which is believed to have happened around forty years ago. Today, methicillin-resistant S. aureus (MRSA) strain CC97 is an emerging human pathogen in Europe, North and South America, Africa, and Asia.

The team's findings highlight the potential for cows to serve as a reservoir for bacteria with the capacity for pandemic spread in humans. There have been a series of warnings from food policy groups over the increase in intensive livestock production around the world, especially in areas where biosecurity precautions are not strictly enforced, and the potential public health problems this poses, as well as the environmental impacts.  

The researchers made their discovery by sequencing and then studying the genomes of the bacteria, from humans, cattle and other animals.

Ross Fitzgerald of the Roslin Institute and the University of Edinburgh in Scotland, and who worked on the study, explained, "Strains of CC97 found in cows appear to be the ancestors of CC97 strains from humans." He elaborated, "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."

However, the estimates of Fitzgerald's team as to when the jump by two strains of bacteria between cattle and humans occurred are less precise. The team estimate the jump of the first 'clade' occurred somewhere between 1894 and 1977 and the second between 1938 and 1966.

After they made the jump, the human CC97 strains acquired some new capabilities, according to Professor Fitzgerald. One of these new capabilities, the most problematic for humans, is the ability to resist methicillin, an important antibiotic for fighting staphylococcal infections. Only human strains of CC97 are 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.

However, the Professor pointed to the case of a different S. aureus strain from pigs, which scientists established in 2012 had evolved the ability to resist methicillin before the bacteria crossed over into humans.

The cattle study's authors concluded, however, that "Any number of factors could create these differences, making pigs - but not cattle - a source of a drug-resistant bacterium. At this point… 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."

Professor Fitzerald said he hopes the findings will one day contribute to stopping crossovers, preventing the rise of diseases which could threaten humanity's reliance on antibiotics.