One is called O antigen and these are composed of complex carbohydrate structures on the surface of the bacterial cell. A second type of antigen is usually, but not always, present on E. coli cells. This is a protein structure termed a flagellum that is responsible for motility. These are termed H antigens. These are organisms that carry a toxin gene that has been acquired from a cousin of E. coli called Shigella.
Shigella are always pathogenic.
These organisms are common inhabitants of the intestinal tract of humans and animals. They can, however, also be found in soil, water, food or anything else that becomes contaminated with feces from these sources. The symptoms are those of an acute gastroenteritis that develops into a cramping, often grossly bloody diarrhea. Fever is low-grade or absent. Many cases are self-limiting, and resolve in eight to 10 days without specific treatment.