Everything about Escherichia Coli totally explained
Escherichia coli (
E. coli), is a
bacterium that's commonly found in the lower
intestine of warm-blooded animals. Most
E. coli strains are harmless, but some, such as
serotype, can cause serious
food poisoning in humans, and are occasionally responsible for costly
product recalls. The harmless
strains are part of the
normal flora of the
gut, and can benefit their hosts by producing
vitamin K2, or by preventing the establishment of
pathogenic bacteria within the intestine.
E. coli are not always confined to the intestine, and their ability to survive for brief periods outside the body makes them an ideal
indicator organism to test environmental samples for
fecal contamination. The bacteria can also be grown easily and its
genetics are comparatively simple and easily-manipulated, making it one of the best-studied prokaryotic
model organisms, and an important species in
biotechnology.
E. coli was discovered by German pediatrician and bacteriologist
Theodor Escherich in 1885, and is now classified as part of the
Enterobacteriaceae family of
gamma-proteobacteria.
Strains
A
strain of
E. coli is a sub-group within the species that has unique characteristics that distinguish it from other
E. coli strains. These differences are often detectable only on the molecular level; however, they may result in changes to the physiology or lifecycle of the bacterium. For example, a strain may gain
pathogenic capacity, the ability to use a unique carbon source, the ability to inhabit a particular ecological niche or the ability to resist antimicrobial agents. Different strains of
E. coli are often host-specific, making it possible to determine the source of fecal contamination in environmental samples. Depending on which
E. coli strains are present in a water sample, for example, assumptions can be made about whether the contamination originated from a human, other mammal or bird source.
New strains of
E. coli evolve through the natural biological process of
mutation, and some strains develop
traits that can be harmful to a host animal. Although virulent strains typically cause no more than a bout of diarrhea in healthy adult humans, particularly virulent strains, such as or O111:B4, can cause serious illness or death in the elderly, the very young or the
immunocompromised.
Biology and biochemistry
E. coli is
Gram-negative,
facultative anaerobic and
non-sporulating. It can live on a wide variety of substrates.
E. coli uses mixed-acid fermentation in anaerobic conditions, producing
lactate,
succinate,
ethanol,
acetate and carbon dioxide. Since many pathways in mixed-acid fermentation produce hydrogen gas, these pathways require the levels of hydrogen to be low, as is the case when
E. coli lives together with hydrogen-consuming organisms such as
methanogens or
sulfate-reducing bacteria.
Optimal growth of
E. coli occurs at 37°C, but some laboratory strains can multiply at temperatures of up to 49°C. Growth can be driven by aerobic or anaerobic respiration, using a large variety of
redox pairs, including the oxidation of
pyruvic acid,
formic acid,
hydrogen and
amino acids, and the reduction of substrates such as
oxygen,
nitrate,
dimethyl sulfoxide and
trimethylamine N-oxide.
Strains that possess
flagella can swim and are
motile, but other strains lack flagellum. The flagella of
E. coli have a
peritrichous arrangement.
E. coli and related bacteria possess the ability to transfer
DNA via
bacterial conjugation,
transduction or
transformation, which allows genetic material to
spread horizontally through an existing population. It is believed that this process led to the spread of
shiga toxin from
Shigella to
E. coli O157:H7.
Normal role
E. coli normally colonizes an infant's
gastrointestinal tract within 40 hours of birth, arriving with food or water or with the individuals handling the child.
In the bowel, it adheres to the
mucus of the
large intestine. It is the primary
facultative organism of the human gastrointestinal tract. Virotypes include:
- Enterotoxigenic E. coli (ETEC) – causative agent of diarrhea (without fever) in humans, pigs, sheep, goats, cattle, dogs, and horses. ETEC uses fimbrial adhesins (projections from the bacterial cell surface) to bind enterocyte cells in the small intestine. ETEC can produce two proteinaceous enterotoxins: the larger of the two proteins, LT enterotoxin, is similar to cholera toxin in structure and function, while the smaller protein, ST enterotoxin causes cGMP accumulation in the target cells and a subsequent secretion of fluid and electrolytes into the intestinal lumen. ETEC strains are non-invasive, and they don't leave the intestinal lumen.
- Enteropathogenic E. coli (EPEC) – causative agent of diarrhea in humans, rabbits, dogs, cats and horses. Like ETEC, EPEC also causes diarrhea, but the molecular mechanisms of colonization and etiology are different. EPEC lack fimbriae, ST and LT toxins, but they utilize an adhesin known as intimin to bind host intestinal cells. This virotype has an array of virulence factors that are similar to those found in Shigella, and may possess a shiga toxin. Adherence to the intestinal mucosa causes a rearrangement of actin in the host cell, causing significant deformation. EPEC cells are moderately-invasive (for example they enter host cells) and elicit an inflammatory response. Changes in intestinal cell ultrastructure due to "attachment and effacement" is likely the prime cause of diarrhea in those afflicted with EPEC.
- Enteroinvasive E. coli (EIEC) – found only in humans. EIEC infection causes a syndrome that's identical to Shigellosis, with profuse diarrhea and high fever. EIEC are highly invasive, and they utilize adhesin proteins to bind to and enter intestinal cells. They produce no toxins, but severely damage the intestinal wall through mechanical cell destruction.
- Enterohemorrhagic E. coli (EHEC) – found in humans, cattle, and goats. The sole member of this virotype is strain, which causes bloody diarrhea and no fever. EHEC can cause hemolytic uremic syndrome and sudden kidney failure. It uses bacterial fimbriae for attachment, is moderately-invasive and possesses a phage-encoded Shiga toxin that can elicit an intense inflammatory response.
- Enteroaggregative E. coli (EAggEC) – found only in humans. So named because they've fimbriae which aggregate tissue culture cells, EAggEC bind to the intestinal mucosa to cause watery diarrhea without fever. EAggEC are non-invasive. They produce a hemolysin and an ST enterotoxin similar to that of ETEC.
Gastrointestinal infection
Certain strains of
E. coli, such as,
O121 and, produce
toxins.
Food poisoning caused by
E. coli are usually associated with eating unwashed vegetables and meat contaminated post-slaughter. O157:H7 is further notorious for causing serious and even life-threatening complications like
Hemolytic Uremic Syndrome (HUS). This particular strain is linked to the
2006 United States E. coli outbreak of fresh spinach. Severity of the illness varies considerably; it can be fatal, particularly to young children, the elderly or the immunocompromised, but is more often mild.
E. coli can harbor both heat-stable and heat-labile enterotoxins. The latter, termed LT, contains one 'A' subunit and five 'B' subunits arranged into one holotoxin, and is highly similar in structure and function to
Cholera toxins. The B subunits assist in adherence and entry of the toxin into host intestinal cells, while the A subunit is cleaved and prevents cells from absorbing water, causing
diarrhea. LT is secreted by the Type 2 secretion pathway.
If
E. coli bacteria escape the intestinal tract through a perforation (for example from an
ulcer, a
ruptured appendix, or a
surgical error) and enter the abdomen, they usually cause
peritonitis that can be fatal without prompt treatment. However,
E. coli are extremely sensitive to such
antibiotics as
streptomycin or
gentamicin, so treatment with antibiotics is usually effective. This could change since, as noted below,
E. coli quickly acquires drug resistance.
Intestinal mucosa-associated
E. coli are observed in increased numbers in the
inflammatory bowel diseases,
Crohn's disease and
ulcerative colitis. Invasive strains of
E. coli exist in high numbers in the inflamed tissue, and the number of bacteria in the inflamed regions correlates to the severity of the bowel inflammation.
Epidemiology of gastrointestinal infection
Transmission of pathogenic
E. coli often occurs via
fecal-oral transmission. Common routes of transmission include: unhygienic food preparation, irrigation of crops with contaminated
greywater
or raw
sewage,
feral pigs on cropland,
or direct consumption of sewage-contaminated water. Dairy and beef cattle are primary reservoirs of
E. coli O157:H7, Food products associated with
E. coli outbreaks include raw ground beef, raw seed sprouts or spinach, as well as direct contact with farm animals,
petting zoo animals, and airborne particles found in animal-rearing environments.
Urinary tract infection
Uropathogenic
E. coli (
UPEC) is responsible for approximately 90% of
urinary tract infections (UTI) seen in individuals with ordinary anatomy. They also have the ability to form K antigen, capsular polysaccharides that contribute to
biofilm formation. Biofilm-producing
E. coli are recalcitrant to
immune factors and
antibiotic therapy and are often responsible for chronic urinary tract infections. K antigen-producing
E. coli infections are commonly found in the upper urinary tract. A study published in the journal
Science in August 2007 found that the rate of adaptative
mutations in
E. coli is "on the order of 10
–5 per
genome per generation, which is 1,000 times as high as previous estimates," a finding which may have significance for the study and management of bacterial antibiotic resistance.
Antibiotic-resistant
E. coli may also pass on the genes responsible for antibiotic resistance to other species of bacteria, such as
Staphylococcus aureus.
E. coli often carry multidrug resistant plasmids and under stress readily transfer those plasmids to other species. Indeed,
E. coli is a frequent member of
biofilms, where many species of bacteria exist in close proximity to each other. This mixing of species allows
E. coli strains that are piliated to accept and transfer
plasmids from and to other bacteria. Thus
E. coli and the other
enterobacteria are important reservoirs of transferable antibiotic resistance.
Beta-lactamase strains
Resistance to
beta-lactam antibiotics has become a particular problem in recent decades, as strains of bacteria that produce extended-spectrum
beta-lactamases have become more common. These beta-lactamase enzymes make many, if not all, of the
penicillins and
cephalosporins ineffective as therapy. Extended-spectrum beta-lactamase–producing
E. coli are highly resistant to an array of antibiotics and infections by these strains is difficult to treat. In many instances, only two oral antibiotics and a very limited group of intravenous antibiotics remain effective.
Increased concern about the prevalence of this form of "
superbug" in the
United Kingdom has led to calls for further monitoring and a UK-wide strategy to deal with infections and the deaths. Susceptibility testing should guide treatment in all infections in which the organism can be isolated for culture.
Phage therapy
Phage therapy—viruses that specifically target pathogenic bacteria—has been developed over the last 80 years, primarily in the former
Soviet Union, where it was used to prevent diarrhea caused by
E. coli. Presently, phage therapy for humans is available only at the Phage Therapy Center in the
Republic of Georgia and in
Poland. However, on January 2 2007, the United States FDA gave Omnilytics approval to apply its
E. coli O157:H7 killing phage in a mist, spray or wash on live animals that will be slaughtered for human consumption.
Vaccination
Researchers have actively been working to develop safe, effective
vaccines to lower the worldwide incidence of
E. coli infection. In March of 2006, a vaccine eliciting an immune response against the
E. coli O157:H7 O-specific polysaccharide conjugated to
recombinant exotoxin A of
Pseudomonas aeruginosa (O157-rEPA) was reported to be safe in children two to five years old. Previous work had already indicated that it safe for adults. A
phase III clinical trial to verify the large-scale
efficacy of the treatment is planned.
Role in biotechnology
Because of its long history of laboratory culture and ease of manipulation,
E. coli also plays an important role in modern
biological engineering and
industrial microbiology. The work of
Stanley Norman Cohen and
Herbert Boyer in
E. coli, using
plasmids and
restriction enzymes to create
recombinant DNA, became a foundation of biotechnology.
Considered a very versatile host for the production of
heterologous proteins, Modified
E. coli have been used in
vaccine development,
bioremediation, and production of immobilised
enzymes.
E. coli cannot, however, be used to produce some of the more large, complex proteins which contain multiple
disulfide bonds and, in particular, unpaired
thiols, or proteins that also require
post-translational modification for activity. These features protect wild type strains from
antibodies and other chemical attacks, but require a large expenditure of energy and material resources.
In 1946,
Joshua Lederberg and
Edward Tatum first described the phenomenon known as
bacterial conjugation using
E. coli as a model bacterium, and it remains the primary model to study conjugation.
E. coli was an integral part of the first experiments to understand
phage genetics, and early researchers, such as
Seymour Benzer, used
E. coli and phage T4 to understand the topography of gene structure. Prior to Benzer's research, it wasn't known whether the gene was a linear structure, or if it had a branching pattern.
Further Information
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