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Pathogenic E. coli (page 1)
(This chapter has 4 pages)
© Kenneth Todar, PhD
E. coli O157:H7. Phase contrast image of cells
immobilized on an agar-coated slide. William Ghiorse, Department of
University, Ithaca, New York. Licensed for use by ASM Microbe Library http://www.microbelibrary.org
Theodor Escherich first described E. coli in 1885, as Bacterium
coli commune, which he isolated from the feces of newborns. It was
later renamed Escherichia coli, and for many years the
bacterium was simply considered to be a commensal organism of the large
intestine. It was not until 1935 that a strain of E. coli was
shown to be the cause of an outbreak of diarrhea among infants.
The GI tract of most warm-blooded animals is colonized by E. coli
within hours or a few days after birth. The bacterium is ingested
in foods or water or obtained directly from other individuals handling
the infant. The human bowel is usually colonized within 40 hours of
coli can adhere to the mucus overlying the large intestine. Once
an E. coli strain may persist for months or years. Resident
shift over a long period (weeks to months), and more rapidly after
infection or antimicrobial chemotherapy that perturbs the normal flora.
The basis for these shifts and the ecology of Escherichia coli
the intestine of humans are poorly understood despite the vast amount
information on almost every other aspect of the organism's existence.
entire DNA base sequence of the E. coli genome has been known
E. coli is the head of the large bacterial family, Enterobacteriaceae,
the enteric bacteria, which are facultatively anaerobic
rods that live in the intestinal tracts of animals in health and
The Enterobacteriaceae are among the most important bacteria
A number of genera within the family are human intestinal pathogens
Several others are normal colonists of the human gastrointestinal tract
these bacteria, as well, may occasionally be associated with diseases
Physiologically, E. coli is versatile and well-adapted to
characteristic habitats. It can grow in media with glucose as the sole
organic constituent. Wild-type E. coli has no growth factor
and metabolically it can transform glucose into all of the
components that make up the cell. The bacterium can grow in the
or absence of O2. Under anaerobic conditions it will grow by
means of fermentation, producing characteristic "mixed acids and gas"
end products. However, it can also grow by means of anaerobic
since it is able to utilize NO3, NO2 or fumarate
as final electron acceptors for respiratory electron transport
In part, this adapts E. coli to its intestinal (anaerobic) and
extraintestinal (aerobic or anaerobic) habitats.
E. coli can respond to environmental signals such as
pH, temperature, osmolarity, etc., in a number of very remarkable ways
considering it is a unicellular organism. For example, it can sense
presence or absence of chemicals and gases in its environment and swim
towards or away from them. Or it can stop swimming and grow fimbriae
will specifically attach it to a cell or surface receptor. In response
to change in temperature and osmolarity, it can vary the pore diameter
of its outer membrane porins to accommodate larger molecules
or to exclude inhibitory substances. With its complex mechanisms for
of metabolism the bacterium can survey the chemical contents in its
in advance of synthesizing any enzymes that metabolize these compounds.
It does not wastefully produce enzymes for degradation of carbon
unless they are available, and it does not produce enzymes for
of metabolites if they are available as nutrients in the environment.
E. coli is a consistent inhabitant of the human intestinal
and it is the predominant facultative organism in the human GI tract;
however, it makes up a very small proportion of the total bacterial
The anaerobic Bacteroides species in the bowel outnumber E.
by at least 20:1. however, the regular presence of E. coli in
human intestine and feces has led to tracking the bacterium in nature
an indicator of fecal pollution and water contamination. As such, it is
taken to mean that, wherever E. coli is found, there may be
contamination by intestinal parasites of humans.
Unstained cells of E. coli
viewed by phase microscopy. about 1000X magnification. CDC.