Enteropathogenic E. coli subgroups that correlate
well with toxicoinfection belong to the
Enteropathogenic(EPEC) and enterotoxigenic(ETEC)
types.
They produce diarrheal diseases when ingested in
large numbers through contaminated foods and
water.
The symptoms are more like those in cholera.
The incidence is high in many developing countries
and is directly related to poor sanitation.
Characteristics
Many serotypes in both subgroups are involved in
human gastroenteritis.
They are Gram-negative small curved rods,
nonsporulating and motile(nonmotile strains can be
present). The strains are facultative anaerobes and
can grow effectively in both simple and complex
media and many foods.
Growth occurs between 10-50C, with optimum at
30-37C. Some strains can grow below 10C.
The cells are sensitive to pasteurization.
Habitat
All strains in both subgroups can establish in the
small intestine of humans without producing
symptoms.
The carriers can shed the organisms in feces and
can contaminate food and water directly or
indirectly.
Many animals, including domestic ones, can also
harbor different serotypes of both subgroups and
contaminate soil, water and food.
In animals, many serotypes may not produce
disease symptoms.
Toxins and Toxin Production
The strains in ETEC subgroup produce two types of
enterotoxins:
Heat labile(LT). And heat stable(ST).
A strain can produce either LT or ST or both.
LT toxin is an antigenic protein, similar to cholera
toxin that induces fluid secretion by epithelial cells
of small intestine.
ST is lower in M.Wt and is nonantigenic. It also
increases fluid secretion by intestinal cells, but
through a different mode of action.
The genetic determinants of the enterotoxins in
ETEC are plasmid linked and can be transferred to
other E. coli strains.
The production of enterotoxins by ETEC strains is
influenced by media composition, culture age, and
alteration of a culture during growth.
Optimum production occurs in a nutritionally rich
medium at pH 8.5. Aeration of broth facilitates
good toxin production.
The toxins are detected in growing culture within
24 h at 35C. However, the toxin can be produced by
the cells growing at 25-40C
Disease and Symptoms
EPEC strains were initially thought to be associated
with infant diarrhea in many tropical and developing
countries, causing high mortality.
In contast, ETEC strains are regarded as the cause of
traveler”s diarrhea, a nonfata diarrheal disease.
However, some consider that various EPEC strains can
cause traveler”s diarrhea. Ingestion of several millions
of viable cells is necessary for the symptoms to occur
within 24-72 h.
Symptoms include mild to severe diarrhea that
lastsfor24-30 h. In severe cases, dehydration and shock
may accompany diarrhea. Those who develop
symptoms may shed the organisms in feces after
recovery.
Food Association
Many types of foods, including meat products, fish,
milk and dairy products, vegetables, baked
products, and water have been associated with
gastroenteritis of E. coli origin in many countries.
These include serotypes from both EPEC and ETEC
subgroups.
Direct and indirect contamination of these foods
(and water) with fecal materials, along with
improper storage temperature and inadequate heat
treatment, were involved in these incidences.
Prevention
Prevention contamination of food and water, directly
or indirectly by fecal matter.
Effective sanitation in water supplies and treating and
disposing sewage.
Prevent contamination of food due to poor personal
hygiene by people who shed the pathogen.
If the pathogen is present in very small numbers in a
food, temperature abuse of the food can facilitate
multiplication of cells to high levels necessary for the
disease symptoms.
Thus, food should be refrigerated or eaten quickly ,
preferably after heating.
Detection Methods
Selective enrichment of sample(food, water
and feces).
Isolation of pathogens on selective agar
medium.
Biochemical characterization of suspected
isolates.
Confirmation tests to detect toxins involve
one or more serological tests e.g. ELISA.