Escherichia coli (E. coli): Properties and Identification
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.
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
- 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.
- E.coli is the second most important cause of Gram-negative rod sepsis
- 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
- Enterotoxigenic Escherichia coli (ETEC) is responsible for the traveler’s diarrhea (watery diarrhea)
- Enterohemorrhagic strains of E.coli (i.e. Shiga toxin-STx producing E. coli ) cause bloody diarrhea and hemolytic uremic syndrome (HUS).
Important properties of E. coli
- Escherichia coli is a straight gram-negative short rod/bacilli, which is an important member of the family Enterobacteriaceae.
- E. coli is the most abundant facultative anaerobe in the colon and feces.
- 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.
- The generation (doubling) time of Escherichia coli is 20 minutes.
- 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)
- 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
- 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.
- Capsule: Interferes with phagocytosis, and plays the main role in systemic infections.
- Endotoxin (lipopolysaccharide): Responsible for several features of gram-negative sepsis such as fever, hypotension, and disseminated intravascular coagulation (DIC).
- 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
- 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)
- On EMB agar, E. coli produces characteristics green sheen.
- Indole positive: produces indole from tryptophan
- It is motile
- It decarboxylates lysine
- It uses acetate as the only source of carbon
Other important biochemical tests of E. coli are summarized in the table below.
|Nitrate reduction test||Positive|
|Methyl-Red (MR) test||Positive|
|Voges-Proskauer (VP) test||Negative|
|Citrate utilization test||Negative|
|Acetate utilization test||Positive|
|Pyrrolidonyl-β-naphthylamide (PYR) test||Negative|
|H2S production test||No|
|Oxidative-fermentative (OF) test||Fermentative|
|TSI reactions||Acid/Acid, Gas,|
|Phenyl Pyruvic acid (PPA) test||Negative|
|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 TankeshwarHello, 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.
Catalase test: Principle, Procedure, Results, Uses
Catalase test is used to distinguish among Gram-positive cocci. Staphylococci are catalase-positive but Streptococci are catalase-negative.