E. coli: Disease, Properties, Lab Diagnosis

Last updated on June 25th, 2021

Source: microbiologyinpictures.com

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

  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).

Also read:

E. coli: Only Bacteria that wins record number of Nobel Prizes

Important properties of E. coli

LF and NLF colonies in MacConkey Agar
  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

 Scheme for Rapid Identification of E. coli.

Scheme for Rapid Identification of E. coli.
  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.

Biochemical Tests

  1. Indole positive: produces indole from tryptophan
  2. It is motile
  3. It decarboxylates lysine
  4. It uses acetate as the only source of carbon

Other important biochemical tests of E. coli are summarized in the table below.

Catalase testPositive
Oxidase testNegative
Nitrate reduction testPositive
Methyl-Red (MR) testPositive
Voges-Proskauer (VP) testNegative
Citrate utilization testNegative
Acetate utilization testPositive
Indole testPositive
Pyrrolidonyl-β-naphthylamide (PYR) testNegative
H2S production testNo
Urease testNegative
Oxidative-fermentative (OF) testFermentative
MUG testPositive
TSI reactionsAcid/Acid, Gas,
Phenyl Pyruvic acid (PPA) testNegative
Lysine decarboxylation test+
Arginine decarboxylation test-/+ (strain variability)
Ornithine decarboxylation test+/- (strain variability)
Sugar fermentation test
About Acharya Tankeshwar 474 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.


  1. What is the primary virulence factor for E. coli?
    a) Inflammatory response
    b) cAMP inducing toxin
    c) Toxin inhibits protein synthesis

    d) LPS
    e) Superantigen
    7) What is the primary virulence factor for P. aeruginosa?
    a) cAMP inducing toxin
    b) Capsule
    c) Spore
    d) Teichoic acid
    e) Type III secretion system

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