<P> 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 five to 10 days . It can also be asymptomatic . </P> <P> 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 . </P> <P> Strains of E. coli that express Shiga and Shiga - like toxins gained this ability due to infection with a prophage containing the structural 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). </P> <P> 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 a bacteriophage infection to non-hemorrhagic cells . </P>

What does the o in e. coli o157h7 refer to