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Infectious and Dangerous Escherichia coli Bacterial Strains

© Donald Reinhardt,2011
E. coli. photo credit: CDC
Escherichia coli is a common intestinal organism that is usually a non-pathogen and commensal  bacterium (commensal: a microbe that lives on or in another organism but does not harm the host organism). Unfortunately, not all E. coli are created equal and some E. coli strains are true and serious pathogens that cause illness and even death of the hosts which they enter.

Under certain circumstances and conditions – such as with immunocompromised or very ill patients or the elderly – pathogenic E. coli strains can be rapidly fatal. The 2011 European strain of E. coli is responsible currently for at least 18 deaths and almost 500 patients with serious systemic disease.

Important Questions and Answers about the Escherichia coli and E. coli Epidemics

1. What is Escherichia coli? Answer. E. coli is a Gram-negative (stains red in the Gram stain procedure) bacterium that is a single, simple-celled, microscopic organism (microorganism) that divides by binary fission as often as every 12 to 15 minutes under ideal conditions.

2. Where is E. coli found and how does it live? Answer. E. coli is a normal inhabitant of the intestinal tract of many animals including reptiles, amphibians, rodents and mammals. It lives and survives and multiplies by using simple food materials in the intestinal tract of these animals. Cattle, pigs and chickens harbor E. coli and these animals often also have strains of pathogenic Salmonella.

3. How does E. coli infect humans? Answer. These bacterial infections occur when intestinal contents and fecal material containing these bacteria contaminate water or foods such as meats, vegetables or fruits. Improperly treated water or inadequately washed or cooked materials often contain live, pathogenic microbes that enter the GI tract and begin to multiply there.

4. How do these bacteria make someone ill? Answer. There are different means by which infectious disease or  illness happens with E. coli. First, someone must get infected by eating or drinking bacterially-contaminated material. Then, the organism must be able to survive passage through the acid of the stomach and then begin to multiply in the small and large intestines. Some strains of intestinal pathogens enter cells of the host and multiply within those cells. Other strains produce toxins that kill surrounding protective and guardian host cells. Some E. coli may pass into the bloodstream and get carried to distant organs where exotoxins and endotoxins (the cell envelopes of the bacteria) injure or destroy host cells and tissues at those sites. The recent European strain is characterized as an organism that causes HUS or hemolytic uremic syndrome with damage to the kidneys and to red blood cells.

5. How is a patient's infection stopped or reversed? Answer. Sometimes the infected patient or host fights off the invasion and recovers after several days. In other cases, antibiotics are needed before a recovery is possible. Sometimes the bacterial strain is very virulent or pathogenic and resistant to antibiotics and the infected person or host dies due to organ damage.

6. What is an epidemic? Answer. An epidemic is an outbreak of disease and an increase in disease incidence above normal background levels, i.e. there is a spike and noticeable increase in the number of people who have a certain type of illness. In the case of the European situation the epidemic there was a sudden and dramatic number of E. colicases. This new outbreak of intestinal disease indicated a major source of contamination that many people were exposed to and large numbers of these people soon became diseased as a result.

7. Is this E. coli transmissible or infectious among people? Answer. If normal precautions are taken, such as thorough hand washing and avoidance of contaminated areas with feces or the organism, then the transmissibility is low. However, failure to wash hands and eating or drinking without doing so could lead to an infection of persons in contact will infected patients.

8. When will the epidemic stop or cease? Answer. Any epidemic stops when the source or sources are found and removed, destroyed or inactivated and the population is no longer exposed to the infectious agent in those sources. Sometimes, the source is as simple as a single water or food source and sometimes it is multiple infectious sources. In HIV epidemiology people to people transfer is the main means of disease transmission.

Important Features of Escherichia coli (E.coli) Account for Its Disease Effects

The E. coli bacteria are Gram-negative bacteria as explained above. This feature of being Gram-negative means that these bacterial cells contain lipopolysaccharide envelopes (outer structures that surround the inner cell membrane of Gram-negative bacterial cells). 

Lipopolysaccharide or LPS is endotoxic whenever this large envelope of LPS disintegrates or fragments into smaller pieces. These smaller molecules of LPS damage cells and tissues of those mammalian organisms. For example, endotoxic shock  caused by LPS involves a severe drop in central, venous blood pressure and disseminated intravascular coagulation (DIC). DIC causes blood stasis in the peripheral blood supply and capillaries. This blood pooling leads to oxygen starvation of tissues and cell injury and death.

Some E. coli also produce the Shiga-toxin – a toxic molecule that is characteristic of a related enteric bacterium calledShigella. The disease situation known as HUS or Hemolytic Uremic Syndrome, is a diseased condition of red blood cell lysis or breakdown (hemolysis) and kidney destruction. Kidney damage causes a rise in the levels of body urea due to failure of the kidneys to excrete the toxic urea byproduct of nitrogen metabolism.

O and H Serotypes and Strains of E. coli

The types and strains of E. coli are often characterized by their O and H antigens and also by genetic profiling known as PFGE pulsed-field gel electrophoresis.

H antigens are protein antigens of the motile flagella (see figure in top left corner showing red-stained strings of long flagellae). The O antigens are carbohydrate or sugar antigens of the lipopolysaccharide envelope. Typing labs like those at CDC and the FDA in the U.S. can determine the specific type of strain or serotype by serology. The O157:H4 has been the most common type of HUS and the more recent O104:H4 is the emerging European pathogen. 

Notes: The letter "H" designates the word "Hauch" in German which means "breath". "Hauch" first signified bacterial colonies that looked like breath condensate and appeared as swirls of swarming and flattened bacterial colonies on the nutrient agar surface inside the Petri plates. "Ohne Hauch" signified non-motile bacterial cell colonies that lacked H or were without "H" antigen (Ohne = without; Hauch = breath). Over time it bacame obvious that all Gram negative bacteria have O antigen and some have flagellar, protein appendages for swimming about whereas others lack them. 

Agglutination (Clumping) Reaction of E. coli with Specific Antibody. Note Negative, Non-Aggultination on Right Microscope Slide

Sources and References

CDC. What is PulseNet? Accessed 2 June, 2011 @

Greenberg, R.S. et al. 2001. Medical Epidemiology. Third Ed., Lange Medical Books/McGraw-Hill, New York. 215 pp.


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Donald Reinhardt is a Consultant in Medical and Industrial Microbiology and a Freelance Science writer. He is available for specific assignments for those who are interested – by contacting Other questions related to this teaching site should be directed to