MRSA infections in humans may originate from cows

Friday 16 Aug 13


Robert Skov, MD
Areahead for Bacteriologic Surveillance and Infection Control
Statens Serum Institut

Tel. +45 32 68 83 48

A type of MRSA bacteria—found in humans all over the word—has been documented to originate from cows. By examining bacteria genes, scientists from University of Edinburgh, DTU Food and Statens Serum Institut, among others, have for the first time been able to identify the exact origin of the infections

MRSA is a type of staphylococci bacteria that have mutated to become resistant to penicillins and other similar antimicrobial agents. MRSA has been known for many years, but until about ten years ago, it was almost exclusively a hospital problem where the people affected were already sick.

But over the past ten years, the world has seen an increase in non-hospital-related cases. This type of so-called community-acquired MRSA cases has involved other MRSA strains that are even more virulent due to their ability to infect people not already weakened by other diseases.

By examining genes from staphylococci from cows and people from four continents, researchers from University of Edinburgh, DTU Food and Statens Serum Institut—among others—have been able to demonstrate that 40 years ago, one of the bacterial strains was transmitted from cows to humans in two separate cases.

Until now, it was not possible to document the origin of community-acquired MRSA types, but the study findings provide an explanation. It is a very important discovery which offers a biological understanding of how bacteria can spread and—in the long term and through further research—could provide an answer to how to prevent and combat the spread of MRSA.

The bacteria mutated to infect people

After transmitting to people, the staphylococci bacteria adapted to its new host by assimilating new genes and thus properties which improved its ability to spread among people. For example, the bacteria were sensitive to antimicrobial agents in animals while they developed resistance to a number of antimicrobial agents once they had spread to humans. In addition, the bacteria acquired genes which enable the staphylococci to suppress the human immune system.

The strain the researchers have tracked is called CC97—one of approx. ten major global strains of CA-MRSA, which infect otherwise healthy people without contact to a hospital environment. In Denmark, CC97 is not among the most frequent MRSA strains, but more and more people are becoming infected. It was first identified in Denmark in 2007 with two infected—in 2012, the number had increased to 30.

Unique Danish MRSA records

Headed by a research group in Edinburgh, the survey of staphylococci genes is a collaboration between universities in Edinburgh and Cambridge, DTU Food, Statens Serum Institut and its UK counterpart, Health Protection Agency. 

Since 2007, data have been recorded and bacteria stored from all Danes who have been infected with MRSA. Denmark is thus the only place in the world with a full overview of MRSA infections in humans, and the Danish contribution has therefore been instrumental in ensuring the ability to document that MRSA infections in humans originate from cows.

Read more

The study has just been published in the scientific journal mBio: Staphylococcus aureus CC398: Host Adaptation and Emergence of Methicillin Resistance in Livestock.