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Bad Burger Bacteria Genome

By Sean Henahan, Access Excellence

e.coli 0157Madison, WI (24 Jan '01)- Enterohaemorrhagic Escherichia coli (EHEC) O157:H7 is the full name of the toxic bacteria behind several highly publicized outbreaks of food contamination in North America in recent years. With the genome sequence of that killer bug now described, researchers expect to learn much more about the pathogenesis and prevention of this food-bourne infectious agent.

In 1997, University of Wisconsin researchers published the sequence of a common non-pathogenic form of E.coli found in lower intestine, the K-12 strain. Since that time they have been able to identify the DNA sequence of far more dangerous 0157:H7 variety, and have identified surprising and important differences between the two strains. This kind of comparative genomics helps researchers to better understand some of the basic processes of infection.

Left- Genome Map of E.coli O157:H7. Click for printable version.

A comparison of the genome of 0157:H7 with the harmless variety showed remarkable similarity in some of the genetic components of each, but marked discrepancies as well. Indeed, the investigators found 1,387 new genes in the infectious variety of the bug. These new genes are very similar to genes associated with infectious activity in other microorganisms such as Salmonella and Shigella. Of particular interest are genes found in O157:H7 that encode toxic proteins known to be involved in the earliest stages of gastrointestinal infection. Moreover, the sequence data suggests that at least some of the rogue genes may have come from viruses interacting with the bacterial genome.

Insights gained from comparing the evolution of different species of E. coli could lead to much need treatments for infection caused by O157:H7 and related toxic variants. On a larger scale, the research will help to elucidate how an organism becomes infectious. O157:H7 infection, associated with eating raw or undercooked beef, kills more than 60 people and sickens 70,000 more in the United States each year alone. Worldwide, infection rates number in the tens of millions.

The researchers instantly made the complete O157:H7 sequence available online. The raw data is available on Entrez Genome database and an annotated sequence has submitted to the public genome data site, GenBank. Their research appears in the January 25, 2001 issue of Nature.

A Quick Look at E. coli O157:H7
Known Routes of Infection  
  • Eating meat, especially undercooked ground beef
  • Consumption of sprouts, lettuce, salami, unpasteurized milk and juice
  • Swimming in or drinking sewage-contaminated water
  • Exposure to diarrheal stools of infected persons
Course of Infection  

Symptoms include severe bloody diarrhea and abdominal cramps with little or no fever. Most cases resolve in 5 to 10 days. Up to seven percent of cases lead to more serious problems including hemolytic uremic syndrome, kidney failure and death.



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