Researchers have sequenced the genome of a deadly bacterium: a strain of Escherichia coli that causes severe food poisoning. Comparing the sequence to that of an innocuous variety has provided some insights into what makes some bacteria so nasty.
Hundreds of E. coli strains can inhabit the human gut, but only a handful are dangerous. The most infamous one, dubbed O157:H7, thrives in ground beef and undercooked hamburgers; it kills more than 60 people in the United States each year, sickening about 70,000 others. In the 25 January issue of Nature, a team led by Fred Blattner at the University of Wisconsin, Madison, presents O157:H7's genomic sequence and compares it to that of a harmless cousin called K-12.
The researchers found that the two strains share a "backbone" of similar genes, which occupy roughly the same positions. But scattered throughout O157:H7's genome are "islands" that contain a total of 1387 unique genes. Many of them look strikingly similar to genes that cause virulence in other gut-dwelling bacteria, such as Salmonella and Shigella. For instance, O157:H7 has genes that encode toxins, proteins involved in attachment to the gut wall, and enzymes that disarm parts of immune system.
The sequence suggests that some of those genes hitched a ride into E. coli on viruses that can hop in and out of bacterial DNA, sometimes taking bits with them. The authors found DNA remnants of at least 18 such viruses in O157:H7, likely accumulated since it and K-12 diverged from a common ancestor--about 4.5 million years ago, according to a study published last year. The data bolster the idea that uptake of foreign DNA is a "rampant process" driving E. coli evolution, Blattner says, "which means that there are constantly new varieties being created in the guts of humans and animals across the world."
The newly sequenced genes may help in the development of badly needed drugs to fight O157:H7 infection, Blattner says. Insights gained from the study may also apply to other pathogenic E. coli strains, he adds, which together cause about 10 million cases of diarrhea globally each year. James Kaper, a microbiologist at the University of Maryland, Baltimore, agrees. "This publication provides tremendous insights into the fundamental question of how a [bacterium] becomes a pathogen," he says.
E. coli Genome Project at the University of Wisconsin, Madison
E. coli Web site at the Centers for Disease Control and Prevention