Since its discovery in 1805, E. coli was considered a harmless, Gram negative, motile, nonsporulating;
rod shaped, facultative
anaerobic bacterium,
a normal
inhabitant
of Gastro-intestinal
Tract
(GIT) of humans
and warm
blooded
animals
and
birds. As it is present in high number
in
intestine,
it has been used
as an
indicator
organism
of possible
fecal contamination
of enteric
pathogens
in foods and water. Since
in mid
1940s,
evidence
has accumulated
that certain E.
coli
strains
cause diarrhea, particularly
in infants, and they were then
designated as pathogens. The
four
groups of pathogenic E.
coli
are