Escherichia coli (E. coli): Properties and Identification

By Acharya Tankeshwar •  Updated: 06/06/22 •  4 min read

Escherichia coli (abbreviated as E. coli) is a commensal (normal flora) in the 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 diseases.

Different strains of Escherichia coli and diseases caused by them (Image source: Ref-1)

some strains of E. coli have evolved into pathogenic E. coli by acquiring virulence factors through plasmids, transposons, bacteriophages, and/or pathogenicity islands. Exotoxins producing strains of Escherichia coli can cause watery (non-bloody) diarrhea and bloody diarrhea depending on the exotoxin they produce.  For example, enterotoxigenic strains of Escherichia coli are a common cause of watery diarrhea (also known as traveler’s diarrhea) in developing countries.

Diseases caused by E. coli

  1. Urinary tract infections (UTI): E.coli is the most common cause of urinary tract infections; more than 75% of cases of UTI are caused by E.coli.
  2. E.coli is the second most important cause of Gram-negative rod sepsis
  3. Perinatal infection with E.coli (exposure of newborn to E.coli colonized in the birth canal of the mother during natural birth) is the predominant cause of neonatal meningitis
  4. Enterotoxigenic Escherichia coli (ETEC) is responsible for the traveler’s diarrhea (watery diarrhea)
  5. Enterohemorrhagic strains of E.coli (i.e. Shiga toxin-STx producing E. coli ) cause bloody diarrhea and hemolytic uremic syndrome (HUS).

Also read:

E. coli: Only bacterium that wins record numbers of Nobel Prizes

Important properties of E. coli

LF and NLF colonies in MacConkey Agar
  1. Escherichia coli is a straight gram-negative short rod/bacilli, which is an important member of the family Enterobacteriaceae.
  2. E. coli is the most abundant facultative anaerobe in the colon and feces.
  3. Escherichia coli cells are small rods 1.0-2.0 micrometers long, with a radius of about 0.5 micrometers. However, the size varies with the medium, and faster-growing cells are larger.
  4. The generation (doubling) time of Escherichia coli is 20 minutes.
  5. E.coli are lactose fermenters that give pink colonies in MacConkey agar (this property distinguishes E.coli from Salmonella and Shigella-two most common intestinal pathogens)
  6. 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, and 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 act on the cells of the jejunum and ileum to cause diarrhea. Other exotoxins are verotoxin, Shiga-like toxin, etc.

Diagnostic Features of E.coli

 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

References and further readings

  • Authors. Levinson W, & Chin-Hong P, & Joyce E.A., & Nussbaum J, & Schwartz B(Eds.), (2020). Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 16e. McGraw Hill. 
  • Michael A Noble, Bailey and Scott’s Diagnostic Microbiology, Eleventh Edition. Betty Forbes, Daniel F. Sahm, and Alice S. Weissfeld. St. Louis, MO: Mosby, 2002

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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