Escherichia 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

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

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

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.

Uropathogenic Escherichia coli

Uropathogenic Escherichia coli (UPEC) strains cause 70 to 90% of community-acquired urinary tract infections (UTIs) in an estimated 150 million individuals annually and about 40% of all nosocomial UTIs. Various virulence genes are associated with Escherichia coli-mediated urinary tract infections.

Uropathogenic Escherichia coli (UPEC)

Uropathogenic strains of Escherichia coli express distinctive bacterial properties, products, or structures (virulence factors) that help them to overcome host defenses and colonize or invade the urinary tract.

Compared to commensal E. coli, uropathogenic E.coli are better adapted to the urethra and cause a greater proportion of UTIs. The more virulence factors a strain expresses, the more severe infection it can cause.

Virulence factors of recognized importance in the pathogenesis of urinary tract infection (UTI) are:

1. Adhesins (P fimbriae, certain other mannose-resistant adhesins, and type 1 fimbriae):  By attaching to host structures, it avoids being swept along by the normal flow urine and eliminated. Attachment is a necessary first step in colonization and a precedent for invasive infection in many situations.
2. K antigen (capsular polysaccharide): They coat the cell, interfering with 0-antigen detection and protecting it from host defense mechanisms. The capsule has antiphagocytic and anticomplementary activities. The degree of impairment of phagocytosis is proportional to the amount of polysaccharides.
3. Resistance to serum killing: Acidic capsular polysaccharides and the Kl capsule contribute to virulence by shielding bacteria from phagocytosis and possibly from serum killing, partly by blocking the activation of the alternative complement pathway.
4. Hemolysin: Cytolytic protein toxin that lyses erythrocytes of all mammals
5. Aerobactin system: E.Coli uses aerobactin to sequester Iron.

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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