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DNA Testing

Pulsed-field gel electrophoresis (PFGE) is the DNA "fingerprinting" method that scientists use to determine the source of bacterial foodborne outbreaks.

How Does PFGE Work?

The DNA of the bacterial pathogen responsible for a foodborne illness is digested into pieces with enzymes that are able to specifically break the DNA molecule into individual pieces. The digested DNA is placed at one end of the gel. A pulsing electric field applied across the gel drives the DNA pieces into the gel over a period of hours. The smallest pieces slip through the pores of the gel more quickly, so the pieces are separated as distinct bands in the gel, based on size. The resulting pattern of 30 to 50 bands, which resembles a bar code is the DNA “fingerprint."  This DNA fingerprint is referred to as the PFGE pattern.

Read more about PFGE >>

Is E. coli O157:H7 Becoming More Virulent?

A phylogenetic analysis of E. coli O157:H7 has found that recombination or recurrent mutation has contributed to the evolution of E. coli O157:H7 into more virulent strains. The research was prompted by several recent outbreaks involving E. coli O157:H7 (e.g., 2006 spinach E. coli outbreak) that were “associated with more severe disease, as defined by higher hemolytic uremic syndrome and hospitalization frequencies, suggesting that increased virulence has evolved.”

500 E. coli O157:H7 clinical strains were analyzed and separated into 9 distinct clades:
Differences were observed between clades in the frequency and distribution of Shiga toxin genes and in the type of clinical disease reported. Patients with hemolytic uremic syndrome were significantly more likely to be infected with clade 8 strains, which have increased in frequency over the past 5 years. Genome sequencing of a spinach outbreak strain, a member of clade 8, also revealed substantial genomic differences. These findings suggest that an emergent subpopulation of the clade 8 lineage has acquired critical factors that contribute to more severe disease.
Read the results of this study published in Proceedings of the National Academy of Sciences of the United States of America.

As part of this study, Thomas Whittam, Hannah Distinguished Professor at the National Food Safety and Toxicology Center at Michigan State University and David Alland of the University of Medicine and Dentistry of New Jersey, two co-authors of the study, developed a new technique to test E. coli O157:H7 DNA.

Posted March 2008.

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New Technique to Test DNA of E. coli Bacteria

Michigan State University has developed a new technique to test the DNA of E. coli bacteria by examining very small genetic changes called single nucleotide polymorphisms or SNPs (pronounced snips). Using SNPs, scientists analyzed 96 markers, making genetic analysis of pathogenic bacteria possible at a rate never before accomplished.

“It used to take three months to score one gene individually,” said Thomas Whittam, Hannah Distinguished Professor at the National Food Safety and Toxicology Center at MSU. “Now, we are working on a new, more rapid system that can do thousands of genes per day.”

In a new study released in the Monday edition of the Proceedings of the National Academy of Sciences, “Variation in Virulence Among Clades of Escherichia coli O157:H7 Associated With Disease Outbreaks,” Whittam and his co-authors looked at the DNA of more than 500 strains of a particularly dangerous member of the E. coli family, O157:H7. In collaboration with David Alland of the University of Medicine and Dentistry of New Jersey, Whittam discovered that individual bacteria could be separated into nine major groups, called clades.

E coli makes people sick because they produce toxins, called Shiga toxins. These toxins block protein synthesis, an essential cellular function, particularly in the kidneys. What Whittam found was that the different clades produced different kinds of Shiga toxins in varying amounts based on their DNA.

“For the first time, we know why some outbreaks cause serious infections and diseases and others don’t,” Whittam said. “The different E. coli groups produce different toxins.”
Rapid genetic characterization also opens up a new world of possibilities for identifying the bacterial culprits in outbreaks and finding out where they originated.

“This is the first time anyone has been able to classify very closely related groups,” Whittam said. “This is also the first time we can tell the differences in how they cause disease.”

Whittam also has plans to use this methodology to study other bacterial strains, like Shigella, a major cause of diarrhea around the world. “This new equipment can be used to identify hundreds of thousands of pathogenic bacteria,” Whittam said.

Source: Michigan State University press release.

Posted March 2008.

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Food Poisoning Lawyer Fred Pritzker has appeared on national television and has been quoted by national publications, including The New York Times, The Wall Street Journal, Associated Press and Lawyers USA. He has been named a "Super Lawyer" by Law and Politics magazine. He is also listed in the current edition of The Best Lawyers in America. To contact Fred Pritzker about a food poisoning lawsuit or food safety advocacy, please call 1-888-377-8900 (toll free) or submit the firm's free consultation form.

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