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.
Exotoxins producing strains of Escherichia coli can cause watery (nonbloody) 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. E.coli is the most common cause of urinary tract infections (UTI) and gram-negative rod sepsis.
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-producing E. coli ) cause bloody diarrhea and hemolytic uremic syndrome (HUS).
E. coli: Only Bacteria that wins record numbers of Nobel Prizes
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.
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