e coli : what are they, where did they come from, and why are some so dangerous?

Escherichia coli (E. coli) are members of a large group of bacterial germs that inhabit the intestinal tract of humans and other warm-blooded animals (mammals, birds). Newborns have a sterile alimentary tract, which within two days becomes colonized with E. coli. 

More than 700 serotypes of E. coli have been identified.  The “O” and “H” antigens on their bodies and flagella distinguish the different E. coli serotypes, respectively.  The E. coli serotypes that are responsible for the numerous reports of outbreaks traced to the consumption of contaminated foods and beverages are those that produce Shiga toxin (Stx), so called because the toxin is virtually identical to that produced by another bacteria known as Shigella dysenteria type 1 (that also causes bloody diarrhea and hemolytic uremic syndrome [HUS] in emerging countries like Bangladesh) (Griffin & Tauxe, 1991, p. 60, 73).  The best-known and most notorious Stx-producing E. coli is E. coli O157:H7.  It is important to remember that most kinds of E. coli bacteria do not cause disease in humans, indeed, some are beneficial, and some cause infections other than gastrointestinal infections, such urinary tract infections.  This section deals specifically with Stx-producing E. coli, including specifically E. coli O157:H7.

Shiga toxin is one of the most potent toxins known to man, so much so that the Centers for Disease Control and Prevention (CDC) lists it as a potential bioterrorist agent (CDC, n.d.).  It seems likely that DNA from Shiga toxin-producing Shigella bacteria was transferred by a bacteriophage (a virus that infects bacteria) to otherwise harmless E. coli bacteria, thereby providing them with the genetic material to produce Shiga toxin. 

Although E. coli O157:H7 is responsible for the majority of human illnesses attributed to E. coli, there are additional Stx-producing E. coli (e.g., E. coli O121:H19) that can also cause hemorrhagic colitis and post-diarrheal Hemolytic Uremic Syndrome (D+HUS) HUS is a syndrome that is defined by the trilogy of hemolytic anemia (destruction of red blood cells), thrombocytopenia (low platelet count), and acute kidney failure.

Stx-producing E. coli organisms have several characteristics that make them so dangerous.  They are hardy organisms that can survive several weeks on surfaces such as counter tops, and up to a year in some materials like compost.  They have a very low infectious dose meaning that only a relatively small number of bacteria (fewer than 50) are needed “to set-up housekeeping” in a victim’s intestinal tract and cause infection.

The Centers for Disease Control and Prevention (CDC) estimates that every year at least 2000 Americans are hospitalized, and about 60 die as a direct result of E. coli infection and its complications. A recent study estimated the annual cost of E. coli O157:H7 illnesses to be $405 million (in 2003 dollars), which included $370 million for premature deaths, $30 million for medical care, and $5 million for lost productivity (Frenzen, Drake, and Angulo, 2005).