Escherichia coli in fluorescent antibody (FA) stained section of intestine from an 8-month old child suffering from chronic diarrhea. In a small number of individuals (mostly children under 5 and the elderly), E. coli can cause hemolytic uremic syndrome, in which the red blood cells are destroyed and the kidneys fail. E. coli are Gram-negative rod-shaped bacteria that are part of the normal flora of the human gut. A direct fluorescent antibody (DFA or dFA) is an antibody that has been tagged in a direct fluorescent antibody test. Its name derives from the fact that it directly tests the presence of an antigen with the tagged antibody, unlike western blotting, which uses an indirect method of detection, where the primary antibody binds the target antigen, with a secondary antibody directed against the primary, and a tag attached to the secondary antibody.