Everything Science Forum

Gene Sequence of Deadly E. Coli Reveals Surprisingly Dynamic Genome
http://www.nih.gov/news/pr/jan2001/niaid-24.htm

The just completed genome sequence of a deadly type of Escherichia coli bacteria suggests that the microbe frequently picks up new DNA from other bacteria and bacterial viruses, including genes that may help explain why this organism is exceptionally virulent and sometimes difficult to treat. The results of this sequencing project, supported by the National Institute of Allergy and Infectious Diseases (NIAID), are reported in the upcoming January 25 issue of Nature.

The type of foodborne E. coli that was sequenced, designated O157:H7, is a worldwide threat to public health and has triggered scores of recent outbreaks of hemorrhagic colitis (painful, bloody diarrhea) and many fatalities from kidney failure, according to project leaders at the Genome Center of the University of Wisconsin-Madison (UW-Madison). Close to 75,000 infections caused by O157:H7 transmitted through contaminated food occur annually in the United States, and such infections are most dangerous to children under the age of 10 and the elderly. One well-known U.S. outbreak in 1982, linked to contaminated hamburger meat, led to identification of O157:H7. An outbreak last summer in Milwaukee, Wisconsin, resulted in 60 cases and the death of a 3-year-old child.

"E. coli O157:H7 is one of the most dangerous pathogens threatening our food and water supplies," says Anthony S. Fauci, M.D., director of NIAID. "Better ways to diagnose, treat and prevent E. coli O157:H7 infections are badly needed. This new information will provide important leads to scientists working to reduce the human and economic burdens of this important pathogen."

When researchers compared the more than 5,000 genes of this harmful E. coli to those of a previously sequenced and harmless laboratory strain, they found O157:H7 possessed more than 1,000 genes the other strain lacked. Many of these new genes appear to have been transferred from other bacteria by way of bacterial viruses, indicating that over evolutionary time E. coli acquires foreign genes at a much higher rate than other organisms.

"We found a whole host of unexpected differences between the two types of E. coli," says lead author Nicole T. Perna, Ph.D., of UW-Madison, "things that have never been seen before, and things we hadn't thought to look for." The genetic variability of E. coli and its close relatives may help explain the diversity of human diseases they cause.

"This bacterium is loaded with interesting genes," says UW-Madison research team leader Frederick R. Blattner, Ph.D. E. coli can obtain new genes in several ways, he explains, but the new research especially points the finger at viruses called bacteriophages that infect only bacteria. Bacteriophages insert their genetic material into bacterial DNA. Some of these viral genes, originally acquired from other bacteria in E. coli's environment, may prove advantageous. The new genes can quickly spread through an E. coli population through a process called conjugation, whereby bacteria exchange DNA directly. "We have found that the genomic pieces are constantly shuffling around so that any particular strain contains a subset of the full range available," Dr. Blattner says. "We've termed this larger pool of available genes the pathosphere."