Study finds Staphylococcus aureus bacteria originated from cattle

A recent study suggested that strains of Staphylococcus aureus, or CC97, which causes skin and soft tissue infections in humans, actually originated from the cow and developed a drug-resistance once in the human body.

The study was led by Ross Fitzgerald of the Roslin Institute and the University of Edinburgh, Scotland. The research he and his team conducted was recently published in the American Society for Microbiology's online journal mBio.

Fitzgerald and his team studied the genomes of 43 different CC97 bacteria from humans, cattle, pigs and other animals. They then plotted the genomes' genetic relationship using a phylogenetic tree. The sequence pointed to cows being the ancestors of the CC97 today seen in humans.

"Bovine strains seemed to occupy deeper parts of the phylogenetic tree - they were closer to the root than the human strains," Fitzgerald said. "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 the research suggested that methicillin-resistant CC97 began after the bacteria made the jump to the human body. Fitzgerald hypothesized that mobile genetic elements encoded on the bacteria's genes allowed for the strain to evolve.

"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."

The study found that pigs also express methicillin-resistant CC97. Fitzgerald plans to conduct further research to study the relationships between CC97 in humans and pigs.

"We have a relatively small sample size, and the findings are robust, but we want to extend the study now to include a greater number of clones to get a bigger picture of what's going on across the S. aureus species," Fitzgerald said.