The genome sequence of the O157:H7 strain of Escherichia coli—the cause of a serious gastric illness called hemorrhagic colitis—has provided some clues as to the source of this pathogen's deadliness. Researchers led by Nicole Perna at the University of Wisconsin in Madison used the whole-genome “shotgun” approach to sequence the entire genome of the O157 strain, and compared this with sequence of a relatively harmless laboratory strain (Nature 409, 529–533, 2001). Surprisingly, O157 possessed 1,387 genes not present in the nonpathogenic E. coli. The authors suggest that these additional genes could have been transferred to O157 from other pathogenic bacteria by bacteriophages. Indeed, some of the extra genes could code for toxins or virulence factors, explaining the pathogenicity of the O157 strain. However, Perna says that the new genes could also be used as “targets for surveillance,” or to develop vaccines to clear O157 from domestic animals. Around 75,000 cases of O157 infection in humans occur each year in the United States, and there is currently no means of detection or treatment.