Escherichia coli:Disease, Properties, Pathogenesis, and Laboratory diagnosis

Escherichia coli (abbreviated as E. coli) is a commensal (normal flora) of gut of humans and warm-blooded animals.

Most strains of E.coli are harmless, some even benefit the hosts by producing vitamin K in the gut. Some strains, however, can cause severe foodborne disease. E.coli  is the most common cause of urinary tract infections (UTI) and gram-negative rod sepsis.

Diseases caused by E. coli are: 

1. Urinary tract infections (UTI)
2. Gram-negative rod sepsis
3. Neonatal meningitis
4. Traveler’s diarrhea (watery diarrhea)
5. Enterohemorrhagic strains of E.coli (i.e. Shiga toxin-producing E. coli ) cause bloody diarrhea and hemolytic uremic syndrome (HUS).

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Important properties of E. coli

1. Gram staining test: Gram-negative short rods/bacilli
   
2. Most abundant facultative anaerobe in colon and faeces
3. Lactose fermenter (this property distinguishes it from Salmonella and Shigella-two most common intestinal pathogen)
4. Antigenic properties: There are more than 1000 antigenic types of Escherichia coli. 
   a. O-cell wall antigens (>150 types)
   b. H- flagellar antigen (>50 types)
   c. K- capsular antigen (>90 types)

Virulence factors of E.coli

1. Pili: Helps in adherence of organisms to the cells of jejunum and ileum in case of intestinal tract infection; urinary tract epithelium in case of urinary tract infections. 
2. Capsule: Interferes with phagocytosis, plays the main role in systemic infections.
3. Endotoxin (lipopolysaccharide): Responsible for several features of gram-negative sepsis such as fever, hypotension, and disseminated intravascular coagulation (DIC).
4. Exotoxins e.g. enterotoxin which act on the cells of jejunum and ileum to cause diarrhea. Other exotoxins are verotoxin, Shiga like toxin, etc.

Features of E.coli which are used for laboratory diagnosis

1. E.coli, ferments lactose and produces pink colonies on MacConkey Agar.  (E.coli O157: H7 does not ferment sorbitol, which serves as an important criterion that distinguishes it from other strains of E.coli)
2. On EMB agar, E. coli produces characteristics green sheen
3. Indole positive: produces indole from tryptophan
4. It is motile
5. It decarboxylates lysine
6. It uses acetate as the only source of carbon