Pathogenic Escherichia Coli
The GI tract of most warm-blooded animals
is colonized by E. coli within a few hours or days after birth by the bacterium
ingested in foods or water or directly from other individuals. The human
bowel is usually colonized within 40 hours of birth. E.coli can adhere
to the mucus overlying the large intestine. Once established, an E. coli
strain may persist for months or years. Resident strains shift over a long
period (weeks to months), and more rapidly after enteric infection or antimicrobial
chemotherapy that perturbs the normal flora. The basis for these shifts
and the ecology of Escherichia coli in the intestine of humans are poorly
understood despite the vast amount of information on almost every other
aspect of the organism's existence. In fact, the entire DNA base sequence
of the E. coli genome is known. E. coli is the head of the large bacterial
family, Enterobacteriaceae, the enteric bacteria, which are faculatively
anaerobic Gram-negative rods that live in the intestinal tracts of animals
in health and disease. The Enterobacteriaceae are among the most
important bacteria medically. A
number of genera within the family are human intestinal pathogens (e.g.
Salmonella, Shigella,Yersinia). Several others are normal colonists of
the human gastrointestinal tract (e.g. Escherichia, Enterobacter, Klebsiella),
but these bacteria, as well, may be associated with diseases of humans.The
Enterobacteriaceae are distinguished from the Pseudomonadaceae in a number
of ways which must be recalled readily by competent microbiologists. The
pseudomonads are respiratory, never fermentative, oxidase-positive, and
motile by means of polar flagella. The enterics ferment glucose producing
acid and gas, are typically oxidase-negative, and when motile, produce
peritrichous flagella. Physiologically, E. coli is versatile and well-adapted
to its characteristic habitats. It can grow in media with glucose as the
sole
organic constituent. Wild-type
E. coli has no growth factor requirements, and metabolically it can transform
glucose into all of the macromolecular components that make up the cell.
The bacterium can grow in the presence or absence of O2. Under anaerobic
conditions it will grow by means of fermentation, producing characteristic
"mixed acids and gas" as end products.
However, it can also grow by means
of anaerobic respiration, since it is able to utilize NO3, NO2 or fumarate
as final electron acceptors for respiratory electron transport processes.
In part, this adapts E. coli to its intestinal (anaerobic) and its extraintestinal
(aerobic or anaerobic) habitats. E. coli can respond to environmental signals
such as chemicals, pH, temperature, osmolarity, etc., in a number of very
remarkable ways considering it is a single-celled organism. For example,
it can sense the 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
that 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 (nutrients) or
to exclude inhibitory substances. With its complex mechanisms for regulation
of metabolism the bacterium can survey the chemical contents its environment
in advance of synthesizing any enzymes necessary to use these compounds.
It does not wastefully produce enzymes for degradation of carbon sources
unless they are available, and it does not produce enzymes for synthesis
of metabolites if they are available as nutrients in the environment. E.
coli is a consistent inhabitant of the human intestinal tract, and it is
the predominant facultative organism in the human GI tract; however, it
makes up a very small proportion of the total bacterial content. The number
of anaerobic Bacteroides in the bowel outnumber E. coli by at least 20:1.
The regular presence of E. coli in the human intestine and feces has led
to tracking the bacterium in nature as an indicator of fecal pollution
and water contamination. As such, it is taken to mean that, wherever E.
coli is found, there may be fecal contamination by intestinal parasites
of humans.
