It is one of the first uses of genome sequencing to study the dynamics of a food-borne outbreak and provides further evidence that genomic tools can be used to investigate future outbreaks and provide greater insight into the emergence and spread of infectious diseases.
The study, conducted in collaboration with groups at the Pasteur Institute in France, Assistance Publique-Hôpitaux de Paris, and Statens Serum Institut in Denmark, appears on February 6, 2012 in an advance online edition of Proceedings of the National Academy of Sciences.
“A genome contains the record of a strain’s evolutionary history, so by looking at the differences between the genomes of multiple bacteria from an outbreak we can get really useful clues about what happened in the outbreak. In this way, tracking outbreaks is like detective work, and this approach will be a powerful tool in trying to understand future outbreaks,” said lead author Yonatan Grad, a research fellow in the Center for Communicable Disease Dynamics, Department of Epidemiology at HSPH and infectious disease physician at Brigham and Women’s Hospital in Boston.
“This work is a testament to the power of genome sequencing and analysis to shed light on the mechanisms that drive disease outbreaks,” said co-senior author Deborah Hung, a core faculty member at the Broad Institute, an assistant professor at Massachusetts General Hospital and Harvard Medical School, and an infectious disease physician at Brigham and Women’s Hospital. “We can see things that we simply couldn’t see before, and that holds promise for improving public health.”
The outbreak in Germany, which was caused by the strain E. coli O104:H4, led to around 4,000 cases of bloody diarrhea, 850 cases of hemolytic uremic syndrome (HUS), which can lead to kidney failure, and over 50 deaths. The source of the outbreak was traced to sprouts from an organic farm in Germany. In France, where 15 people were sickened with bloody diarrhea that progressed to HUS in nine people, the source of the outbreak was sprouts, germinated from seeds purchased at a garden retailer, that were served at a children’s community center buffet. European investigators, using traditional epidemiological methods, traced the outbreaks to a shipment of seeds from Egypt that arrived in Germany in December 2009.
The researchers, led by Grad and senior authors Hung and William Hanage, associate professor of epidemiology at HSPH, analyzed isolates of E. coli bacteria from both the German and French outbreaks. Based on conventional molecular epidemiological analysis, the E. coli strains from the outbreaks in Germany and France appear identical.
However, by harnessing the Broad’s expertise in whole-genome sequencing and analysis, the researchers were able to determine that there were small, but measurable, differences among the isolates. They made two surprising findings: All the strains connected to the larger German outbreak were found to be nearly identical, while the strains in France showed greater diversity; and the German isolates appeared to be a subset of the diversity seen in the French isolates.
“If genomes have fewer differences than we expect, like the German outbreak, it suggests that the outbreak might have passed through a bottleneck. A bottleneck might be something like disinfection procedures that killed most but not all of the bugs, or maybe passage through a single infected individual,” said Hanage.
Another hypothesis offered by the researchers is that there was uneven distribution of diversity in the original shipment of contaminated seeds.
As costs for genomic sequencing decline, these tools, combined with traditional epidemiological techniques, can provide greater insight into the emergence and spread of infectious diseases and will help guide preventive public health measures in the future.
Support for this study was provided by the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health (NIH); Infectious Disease Program of the Broad Institute; NIH’s National Institute of General Medical Sciences Models of Infectious Disease Agent Study (MIDAS) Award; Danish Council for Strategic Research; Institut de Veille Sanitaire.
About Harvard School of Public Health
Harvard School of Public Health is dedicated to advancing the public’s health through learning, discovery and communication. More than 400 faculty members are engaged in teaching and training the 1,000-plus student body in a broad spectrum of disciplines crucial to the health and well being of individuals and populations around the world. Programs and projects range from the molecular biology of AIDS vaccines to the epidemiology of cancer; from risk analysis to violence prevention; from maternal and children’s health to quality of care measurement; from health care management to international health and human rights. For more information on the school visit www.hsph.harvard.edu.
HSPH on Twitter: http://twitter.com/HarvardHSPH
HSPH on Facebook: http://www.facebook.com/harvardpublichealth
HSPH on YouTube: http://www.youtube.com/user/HarvardPublicHealth
HSPH home page: http://www.hsph.harvard.edu
About the Broad Institute of MIT and Harvard
The Eli and Edythe L. Broad Institute of MIT and Harvard was founded in 2003 to empower this generation of creative scientists to transform medicine with new genome-based knowledge. The Broad Institute seeks to describe all the molecular components of life and their connections; discover the molecular basis of major human diseases; develop effective new approaches to diagnostics and therapeutics; and disseminate discoveries, tools, methods and data openly to the entire scientific community.
Founded by MIT, Harvard and its affiliated hospitals, and the visionary Los Angeles philanthropists Eli and Edythe L. Broad, the Broad Institute includes faculty, professional staff and students from throughout the MIT and Harvard biomedical research communities and beyond, with collaborations spanning over a hundred private and public institutions in more than 40 countries worldwide. For further information about the Broad Institute, go to www.broadinstitute.org.