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Escherichia Coli Infections: E. coli O157:H7

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  • 0:05 A Burger with a Side…
  • 1:18 E. coli 0157:H7
  • 3:57 Disease
  • 6:07 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

E. coli O157:H7 is one of the most infamous causes of foodborne illness. In this lesson, we will discuss how E. coli O157:H7 causes disease in the human host.

A Burger With a Side of E. coli

Good evening everyone. I am coming to you tonight from Burger Paradise. We have just learned that hundreds of thousands of hamburgers have been recalled after testing positive for E. coli. So far, there are hundreds ill with 45 children experiencing hemolytic anemia and kidney failure. Already, we can report three deaths due to this incredibly dangerous pathogen.

If you have recently indulged at your local Burger Paradise and are now experiencing nausea or diarrhea, report to your doctor or the nearest hospital immediately. As we speak, public health officials are working diligently to fully assess the extent of this massive outbreak. Until we learn more, the message is clear, people: Put those burgers down!

You might think this sounds like a dramatic scene from a medical thriller, but in Washington State in 1993, this was exactly what news reports were saying. The E. coli fast food hamburger outbreak was, and still is, one of the largest foodborne illness outbreaks in the United States. By the time it was over, it would forever change the way Americans processed and cooked their hamburgers and gave many parents serious pause before treating their kids to a quick burger meal.

E. coli O157:H7

The culprit in this massive outbreak was Escherichia coli O157:H7. Most people just know this bug as E. coli, but the O157:H7 designation is very important. This designation distinguishes this serotype from the many other serotypes of E. coli. A serotype is an antigenically distinct variety within a bacterial species. Something is antigenic if, when it enters the human body, it causes the body to produce an antibody that is compatible with it. In bacteria, there are many different antigens, which can be things like proteins or lipids.

For the species E. coli, the serotypes are distinguished by differences in the structure of the antigenic lipopolysaccharide found in the cell wall (this is the O part) and in the structure of the antigenic flagella (this is the H part). It just so happens that E. coli O157:H7 represents one of the most dangerous and infamous serotypes of E. coli.

Escherichia coli is a Gram-negative, rod-shaped, facultative aerobic, motile bacteria. Gram-negative refers to the structure of the cell wall, which contains a thin layer of peptidoglycan and an outer membrane composed of lipopolysaccharides. E. coli cells appear red in the classic Gram stain that distinguishes major bacterial types by the structure of the cell wall. E. coli is a facultative aerobe, meaning that it can grow in the presence of oxygen (called aerobic) and absence of oxygen (called anaerobic).

This means that when E. coli is living in the environment, it can use oxygen for growth, and then when it ends up in your anaerobic intestine, it can use anaerobic processes to grow. E. coli is also well known for being a motile organism. It has flagella, which are long, whip-like protein chains attached to the cell that act as tiny motors, allowing it to swim through the intestinal fluids until it reaches the intestinal walls where it likes to reside.

There are both friendly and pathogenic strains of E. coli, which are covered in other lessons. The pathogenic varieties of E. coli are commonly grouped in pathotypes, or strains that carry the same set of virulence genes and cause a similar set of symptoms. E. coli O157:H7 is the major member of enterohemorraghic E. coli pathotype, or EHEC for short. The name of this pathotype, which includes the prefix 'entero-' referring to the intestine and the suffix '-hemorrhagic' referring to rupture of blood vessels, gives some clue to the disease symptoms caused by O157:H7.

Disease

Let's talk about what O157:H7 does when it invades the human host. After consumption, E. coli cells pass through the stomach and into the intestinal tract. Fewer than 100 cells are enough to cause infection. Once in the intestine, the cells are very good at embedding themselves in the mucus lining the intestinal tract. The cells have surface structures, including fimbriae, pili and proteins, which bind tightly to the intestinal cells, preventing the E. coli from being flushed through the intestine.

Once attached, the mucus helps to protect the cells from the host immune response, allowing them to begin rapidly growing and dividing. The E. coli cells stay outside of the host intestinal cells, but they begin to produce large amounts of toxins, including the Shiga toxin, that is released into the host cells where it can be distributed throughout the body. The Shiga toxin is an example of an exotoxin, or a toxin that is released by actively growing bacterial pathogens.

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