Clinical Microbiology
(MLCM- 201)
Prof. Dr. Ebtisam.F. El GhazzawiMedical Research Institute (MRI)
Alexandria University
OUTCOMESBy the end of this lecture the Student should be able to understand Gram Negative Rods Related to Enteric Tract (Types, pathogens and laboratory Diagnostic tests)
The Enterobacteriaceae are a large heterogeneous
group of gram negative rods whose natural habitat
is the intestinal tract of humans and animals.
The family includes many genera (Escherichia,
Shigella, Salmonella, Enterobacter, Klebsiella,
Serratia, Proteus and others).
Some enteric organisms, e.g. Escherichia
coli are part of the normal flora and
incidentally cause disease, while others, the
Salmonellae and Shigellae are regularly
pathogenic for humans.
The Enterobacteriaceae are facultative
anaerobes or aerobes, ferment glucose and
a wide range of carbohydrates, oxidase
negative and reduce nitrate to nitrite and
produce a variety of toxics and other
virulence factors.
Enterobacteriaceae, enteric gram-negative
rods may also be called coliforms.
They cause a variety of diseases with
different pathogenic mechanisms.
The clinical manifestations of infections
with E. coli and other enteric bacteria
depend on the site of infection and cannot
be differentiated by symptoms or signs from
processes caused by other bacteria.
Urinary tract infection: E. coli is the most common
cause of urinary tract infection and account for 90%
of first urinary tract infections in young women. The
symptoms and signs include urinary frequency,
dysuria, hematuria, and pyuria. Frank pain is
associated with upper tract infection. U.T.I can
result in bacteremia with clinical signs and sepsis.
E. coli-associated diarrheal diseases: E. coli that cause
diarrhea are extremely common worldwide.
Enteropathogenic E. coli (EPEC) is an important cause of
diarrhea in infants especially in developing countries. The
result of EPEC infection is watery diarrhea. Enterotoxigenic
E. coli (ETEC) is a common cause of “traveler’s” diarrhea
and a very important cause of diarrhea in infants in
developing countries.
Care in the selection and consumption of foods
potentially contaminated with ETEC is highly
recommended to help prevent traveler’s diarrhea.
Antimicrobial prophylaxis can be effective but may
result in increased antibiotic resistance in the
bacteria and probably should not be uniformly
recommended.
Once diarrhea develops, antibiotic treatment
effectively shortens the duration of disease.
Enterohemorrhagic E. coli (EHEC) has been
associated with hemorrhagic colitis, a severe form of
diarrhea, and with hemolytic uremic syndrome, a
disease resulting in acute renal failure, hemolytic
anemia and thrombocytopenia.
Enteroinvasive E. coli (EIEC) strains are
nonlactose or late lactose fermenters and are
nonmotile. EIEC produce disease by invading
intestinal mucosal epithelial cells. The disease
occurs most commonly in children in developing
countries and in travelers to these countries.
Enteroaggregative E.coli (EAEC) causes acute and
chronic diarrhea (> 14 days in duration) in persons
in developing countries. These organisms are also
the cause of food-born illnesses in industrialized
countries. They are characterized by their
characteristic pattern of adherence to human cells.
Sepsis: when normal host defenses are inadequate, E.
coli may reach the blood stream and cause sepsis. New
borns may be highly susceptible to E. coli sepsis
because they lack IgM antibodies. Sepsis may occur
secondary to urinary tract infection.
Meningitis: E. coli is one of the leading causes of
meningitis in infants.
Specimens suspected of containing E. coli are grown on
blood agar plate and on differential media such as Mac-
Conkey’s agar or Ethylene methylene blue agar (EMB).
E. coli which ferments lactose forms pink colonies.
On EMB agar E. coli colonies have a characteristic
green sheen.
E. coli can be distinguished from other lactose
fermenting gram negative rods by:
a. Its production of indole from Tryptophan.
b. It is motile.
c. It uses acetate as the only source of carbon.
d. It decarboxylates lysine.
AssignmentStaphylococcus Aureous
Study QuestionsComplete the following questions:
•The Enterobacteriaceae are a large
heterogeneous group of gram negative rods
whose natural habitat is ………….of humans and
animals.
•The family includes many genera (………, …….,
………,…………, Klebsiella,…………)
E. coli can be distinguished from other lactose fermenting gram negative rods by :
a)b)c)d)
Recommended TextbooksManual of Clinical Microbiology, Vols. 1 and 2: Eighth Edition Patrick R. Murray