E. coli in cookie dough: a primer on the bug

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Another month, another food-borne outbreak. In the most recent E.coli news, the bug has shown up in cookie dough and, possibly, beef.

In case you’ve wondered what it is that makes E. coliO157:H7 so very much more dangerous than the E. coli that resides in great numbers in our bowels (I have) here’s a primer.

But first, a look at it under the electron microscope. Pretty, eh? (In reality it’s not pink: that’s a bit of artistic license.)

According to the Centers for Disease Control and Prevention, there are many E. coli strains, but only some of them produce a shiga toxin. (O157:H7 is one of these.)

This toxin-producing trait doesn’t come from the E. coli genes -- it comes, instead, from a virus that invaded the bacterium some time in the past. There are a whole class of bacteria-invading viruses, known as bacteriophages, in nature -- in the Soviet Union, people used to drink bacteriophage tonics that they’d get from their doctors, in the hopes that these would fight bacterial infections. Scientists are still investigating whether some phages (as they’re known for short) may work effectively as antibiotic stand-ins.

The virus that directs formation of shiga toxin in E. coli isn’t the kind that invades a cell, then immediately turns it to tatters while making thousands of copies of itself. Instead, the virus can sit there happily inside the bug -- its DNA stitched neatly into the genome of the bacterium -- steadily directing formation of the shiga toxin protein courtesy of two genes it carries.

What does shiga toxin do? Nothing nice. According to Todar’s Online Textbook of Bacteriology (here’s the link, in case you don’t have it set as your home page), shiga toxin is 100,000 times more toxic than snake venom. It acts by inactivating a key part of a cell’s machinery, stopping the cell’s ability to make proteins. And thus the cell dies.

The toxin, as it’s formed, leaks out of E. coli O157:H7. Then it binds to human cells and enters them: it so happens that it binds especially well to kidney cells, helping explain why renal failure can occur in E. coli O157:H7 food-poisoning cases. Cattle aren’t bothered by the bug -- because their kidneys lack the receptor that allows the toxin to get into human kidneys and wreak such havoc.

So there you have it. Granted, none of this is the kind of thing that anyone in the throes of a dangerous food-poisoning episode would be particularly interested in knowing about.

-- Rosie Mestel