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