TGen-led study suggests origins of MRSA strain in food animals

Antibiotic-resistant strain of Staph may have evolved after jumping from humans to animals and back to humans

Editorial Note: Due to questions surrounding the broad statements made in the original press release, we have modified several quotes to more accurately reflect the data in the published study. The updated release is below.

FLAGSTAFF, Ariz. - Feb. 21, 2012 - A strain of the potentially deadly antibiotic-resistant bacterium known as MRSA has jumped from food animals to humans, according to a new study led by the Translational Genomics Research Institute (TGen).

The study published today in the online journal mBio focuses on MRSA CC398, a strain of methicillin-resistant Staphylococcus aureus. The study suggests that MRSA CC398 probably started as a non-resistant (antibiotic-susceptible) strain in humans before it spread to food animals where it subsequently became resistant to several antibiotics.

Whole genome sequencing, a cutting-edge testing method in which billions of molecules of DNA are spelled out in exquisite detail, enabled the scientists to trace the likely history of MRSA CC398.

Researchers at 20 institutes joined forces to study 89 genomes from humans and animals spanning 19 countries and four continents.

The study's international team of scientists found that the jump of Staph from humans to food animals was followed by the bacterium becoming resistant; first to tetracycline, and then later to methicillin - two important antibiotics for treating Staph infections.

MRSA CC398 is often referred to as "pig-MRSA" or "livestock-associated MRSA" because it most often infects people with direct exposure to swine and other food animals.

"Our results strongly suggest that food animals-associated MRSA CC398 originated in humans as MSSA (methicillin-susceptible S. aureus)," the report concludes. Once in animals, the microbe became resistant to tetracycline and methicillin, likely as a result of the routine antibiotic use that characterizes modern food-animal production, the study said.

Dr. Lance Price, the study's lead author and Director of the TGen's Center for Food Microbiology and Environmental Health, said, "Our findings underscore the potential public health risks of widespread antibiotic use in swine production. Staph thrives in crowded and unsanitary conditions. Add antibiotics to that environment and you're going to create a public health problem.

"Retracing the evolutionary history of MRSA CC398 is like watching the birth of a superbug - it's simultaneously fascinating and disconcerting," added Dr. Price. "MRSA CC398 was discovered less than a decade ago and it appears to be spreading very quickly."

Dr. Paul Keim, one of the study's authors and Director of TGen's Pathogen Genomics Division, said the study describes evolution in action: "The most powerful force in evolution is 'selection.' And, in this case, humans have supplied a strong force through the use of antibiotic drugs in European swine production.

"We can't blame nature or the germs. The good news is that this study clearly shows the way to change evolution and remove this and other similar threats to our health," said Dr. Keim, who also is a Regents Professor of Biology at Northern Arizona University, and Director of NAU's Center for Microbial Genetics and Genomics.

Dr. Frank M. Aarestrup, Head of the Microbial Genomics and Antimicrobial Resistance unit at the Technical University of Denmark, said the study provided valuable information about the origins and future risks of this MRSA strain.

"Given its rapid emergence and trajectory of increasing importance in humans, the evolutionary history of MRSA CC398 has relevance for the epidemiology of MRSA and global health," said Dr. Aarestrup, who also is head of both the World Health Organization's Collaborating Centre for Antimicrobial Resistance among Foodborne Pathogens and of the European Union (EU) Reference Laboratory for Antimicrobial Resistance.

Dr. Robert Skov, M.D., the study's Senior Microbiologist and a member of the Department of Microbiological Surveillance and Research at the Statens Serum Institut in Copenhagen, Denmark, said more studies are needed to further assess the risks.

"Further analyses are required to estimate the number of independent genetic events leading to the methicillin-resistant sub lineages, but the diversity of the (MRSA) subtypes is suggestive of strong and diverse antimicrobial selection associated with food-animal production," Dr. Skov said.

TGen collaborators on this project included:

  • Center for Microbial Genetics and Genomics, Northern Arizona University, Flagstaff.
  • Department of Microbiological Surveillance and Research, Statens Serum Institut, Copenhagen, Denmark.
  • National Food Institute, Technical University of Denmark, Lyngby, Denmark.
  • Österreichische Agentur für Gesundheit und Ernährungssicherheit GmbH, Bereich Humanmedizin, Institut für Medizinische Mikrobiologie und Hygiene, Graz, Austria.
  • Veterinary Faculty, Institute of Microbiology and Parasitology, University of Ljubljana, Slovenia.
  • Istituto Zooprofilattico Sperimentale delle Regioni Lazio e Toscana, Rome, Italy.
  • Department of Swine Diseases, National Veterinary Research Institute, Pulawy, Poland.
  • Institute of Farm Animal Genetics, Friedrich-Loeffler-Institut (FLI), Neustadt-Mariensee, Germany.
  • Department of Pathology, Bacteriology and Poultry Diseases, Faculty of Veterinary Medicine, University of Ghent, Merelbeke, Belgium and Unit of General Bacteriology, Veterinary and Agrochemical Research Centre, Brussels, Belgium.
  • Anses, Ploufragan Plouzané laboratory, Ploufragan, France.
  • Interdisciplinary Centre of Research in Animal Health, Faculty of Veterinary Medicine, Lisboa, Portugal.
  • Centro de Vigilancia Sanitaria Veterinaria (VISAVET), Universidad Complutense de Madrid, Madrid, Spain.
  • University Paris-Diderot and National Reference Center, Bacterial resistance in the commensal flora, Hôpital Bichat-Claude Bernard, Paris, France.
  • Center for Emerging Infectious Diseases, University of Iowa, Coralville, IA, USA
  • Department of Microbiology, University of Mississippi Medical Center, Jackson, Miss.
  • Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada.
  • Department of Preventive Veterinary Medicine, San Marcos Major National University, Lima, Peru.
  • Department of Laboratory Medicine, the First Affiliated Hospital of Wenzhou Medical College, Wenzhou, China
  • Laboratory of Bacteriology - Hopital de la Croix Rousse, National Reference Center for Staphylococci, Lyon, France.

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