Escherichia coli

Escherichia coli

E.coli is the most common cause of urinary tract infection and gram negative rod sepsis. It is one of the two important causes of neonatal meningitis and the agent most frequently associated with ‘’traveler’s diarrhoea’’a watery diarrhea .some strains of E. coli are entero hemorrhagic and cause bloody diarrhea.

Morphology of Escherichia coli:

1. Gram negative rod.
2. Capsulated by slime layer.
3. Motile with peritrichous flagella
4. Facultative anaerobe and aerobe.
5. Lactose fomenters i.e. form pink colonies in Mac-Coney’s agar media.

Cultural Characters of Escherichia coli:

1. On blood agar: Large, grayish-white, opaque, moist ,circular colonies , some stains produce hemolytic colonies.
2. On Mac-Conkey agar: Colonies are rose pink which indicates that the organism is lactose fermenter.
3. DCA,CL D agar & SS  Agar: No or poor growth:
4. Agar streak: white, circular, smooth & white to yellowish white in colour.
5. Eosine-methylene blue agar: Circular moist colony.
6. Gelatin stab: Good growth along the line of inoculation % no liquefaction.
7. Litmus mile: Acid & gas produce by media coagulates.
8. Endomedium: Circular, smooth , red colonies with metallic sheen.
9. Broth medium: Rapid growth with the formation of pellicle %   a    slight slimy sediment.
10. Potato medium: Grayish to yellowish brown growth.
11. Wilson & Blair medium :No growth,

Biochemical reactions: 

E.coli is lactose fermented and produce acid & gas.

• Diseases caused by E.coli:

E.coli causes diseases both within and outside the enteric tract.

1. Urinary tract infections (UTI): 

Cystitis, pyelonephritis(by certain O –serotype).

2. Systemic infections :

=> Wound infection.

=> Pneumonia, septicemia, abscess,

=> Neonatal meningitis.

=> Disseminated intravascular coagulation(DIC)

=> Biliary tract infections

3. Intestinal tract infections : Diarrhoea,dysentery.

Classification of Escherichia coli:

Classification of E coli on the basis of diarrhea production:

1. Enteropathogenic E. coli (EPEC) : Dysentery characterized by bloody diarrhea.
2. Enterotoxigenic E . coli (ETEC) : Traveler’s diarrhea and infantile enteritis by LT& ST.
3. Vero toxin producing E.coli (VTEC) :

ð  Bloody diarrhea.

ð  Hemorrhagic colitis.

ð  Hemolytic uremic syndrome.

4. Enter invasive E.coli (EIEC) Infantile diarrhea by invasion.
5. Enteroaggregasive E.coli (EAggE): Persistent diarrhea.

Mechanism of diarrhea by E.coli:

Some species of E.  coli (ETEC) cause watery , non –bloody diarrhea and some species (EPEC,EHEC) cause bloody diarrhea.