Escherichia coli O157:H7 is a serotype of the bacterial species Escherichia coli and is one of the Shiga-like toxin–producing types of E. coli. It is a cause of disease, typically foodborne illness, through consumption of contaminated and raw food, including raw milk and undercooked ground beef. Infection with this type of pathogenic bacteria may lead to hemorrhagic diarrhea, and to kidney failure; these have been reported to cause the deaths of children younger than five years of age, of elderly patients, and of patients whose immune systems are otherwise compromised. Transmission is via the fecal–oral route, and most illness has been through distribution of contaminated raw leaf green vegetables, undercooked meat and raw milk. Signs and symptoms E. coli O157:H7 infection often causes severe, acute hemorrhagic diarrhea (although nonhemorrhagic diarrhea is also possible) and abdominal cramps. Usually little or no fever is present, and the illness resolves in 5 to 10 days. It can also sometimes be asymptomatic. In some people, particularly children under five years of age, persons whose immunologies are otherwise compromised, and the elderly, the infection can cause hemolytic–uremic syndrome (HUS), in which the red blood cells are destroyed and the kidneys fail. About 2–7% of infections lead to this complication. In the United States, HUS is the principal cause of acute kidney failure in children, and most cases of HUS are caused by E. coli O157:H7. Bacteriology Like the other strains of the species, O157:H7 is gram-negative and oxidase-negative. Unlike many other strains, it does not ferment sorbitol, which provides a basis for clinical laboratory differentiation of the strain. Strains of E. coli that express Shiga and Shiga-like toxins gained that ability via infection with a prophage containing the structural gene coding for the toxin, and nonproducing strains may become infected and produce shiga-like toxins after incubation with shiga toxin positive strains. The prophage responsible seems to have infected the strain's ancestors fairly recently, as viral particles have been observed to replicate in the host if it is stressed in some way (e.g. antibiotics). All clinical isolates of E. coli O157:H7 possess the plasmid pO157. The periplasmic catalase is encoded on pO157 and may enhance the virulence of the bacterium by providing additional oxidative protection when infecting the host. E. coli O157:H7 non-hemorrhagic strains are converted to hemorrhagic strains by lysogenic conversion after bacteriophage infection of non-hemorrhagic cells. Natural habitat While it is relatively uncommon, the