Bacteriology
E. coli serotype O157:H7 is a Gram-negative, rod-shaped bacterium. The "O" in the name refers to the cell wall (somatic) antigen number, whereas the "H" refers to the flagella antigen. Other serotypes may cause (usually less severe) illness, but only those with the specific O157:H7 combination are reviewed here. Other bacteria may be classified by "K" or capsular antigens. (The "O" stands for ohne Hauch ; "H" for Hauch; and "K" for Kapsel.)
This is one of hundreds of serotypes of the bacterium Escherichia coli. While most strains are harmless and normally found in the intestines of mammals, this strain may produce shiga-like toxins and cause severe illness, and is a member of a class of pathogenic E. coli known as enterohemorrhagic Escherichia coli or EHEC. Often they are referred to by their toxin producing capabilities, verocytotoxin producing E. coli (VTEC) or Shiga-like toxin producing E. coli (STEC).
E. coli O157:H7 was first recognized as a pathogen as a result of an outbreak of unusual gastrointestinal illness in 1982. The outbreak was traced to contaminated hamburgers, and the illness was similar to other incidents in the United States and Japan. The etiologic agent of the illness was identified as a rare O157:H7 serotype of E. coli in 1983. This serotype had only been isolated once before, from a sick patient in 1975.
E. coli O157:H7 is markedly different from other pathogenic E. coli, as well. In particular, the O157:H7 serotype is negative for invasiveness (Sereny test), adheres through the E. coli common pilus (ECP), and does not produce heat stable or heat labile toxins. In addition, E. coli O157:H7 is usually sorbitol negative, whereas 93% of all E. coli ferment sorbitol. E. coli O157:H7 also lacks the ability to hydrolyze 4-methylumbelliferyl-β-D-glucuronide (MUG) and does not grow at 45 °C in the presence of 0.15% bile salts. Because of the latter characteristic, this serotype cannot be isolated by using standard fecal coliform methods that include incubation at 45 °C.
E. coli O157:H7 serotypes are closely related, descended from a common ancestor, divergent in plasmid content more than chromosomal content, and are no more related to other shiga toxin producing strains than any other randomly chosen E. coli serotype. E. coli O55:H7 and E. coli O157:H7 are most closely related and diverged from a common pathogenic ancestor that possessed the ability to form attaching and effacing lesions. E. coli O157:H7 serotypes apparently arose as a result of horizontal gene transfer of virulence factors.
Among these virulence factors are a periplasmic catalase and shiga-like toxins. Shiga-like toxins are iron-regulated toxins that catalytically inactivate 60S ribosomal subunits of eukaryotic cells, blocking mRNA translation and causing cell death. Shiga-like toxins are functionally identical to toxins produced by virulent Shigella species.
Strains of E. coli that express 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). The periplasmic catalase is encoded on the pO157 plasmid, and is believed to be involved in virulence by providing additional oxidative protection when infecting the host.
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