Tuesday, February 09, 2010

Shiga Toxin: One Of Man's Most Lethal Poisons (Part 1)

Part 2: E. coli O157:H7 Just One Among Many Shiga Toxin-Making Bacteria
Part 3: Shiga Travels To Kidney. Commits Lethal Deed.
Part 4: Marler Clark Holds USDA To Their Food Safety Mission, Legally

Shiga toxins are proteins. They're not alive. They don't reproduce. They are however manufactured and released from bacteria which are alive and do reproduce (e.g. the little fleeing bacteria on the right in the picture). One shiga-toxin-making bacteria is Shigella dysenteriae. Another is E. coli O157:H7, typically heard in association with illness caused by ground beef.

Other examples of protein toxins made by bacteria are tetanus toxin, diphtheria toxin, and botulinum toxin.

These toxins are some of the most powerful poisons known to man - more potent than snake venom or strychnine. This table ...

ToxinToxic Dose (mg)HostLethal toxicity compared with:
StrychnineSnake venom
Botulinum toxin0.8x10-8Mouse3x1063x105
Tetanus toxin4.0x10-8Mouse1x1061x105
Shiga toxin2.3x10-6Rabbit1x1061x105
Diphtheria toxin6.0x10-5Guinea pig2x1032x102

... shows you would need 1x106, or 1 million times more strychnine than shiga toxin to kill a rabbit.

But killing a rabbit isn't the same as killing a human. In fact, some animals, including cows, swine, and deer, are apparently immune to the effects of shiga. They lack receptors needed for the toxin to gain entrance into cells. Humans have those receptors. Shiga can kill us or maim us for life.

The CDC's best estimates, from 1999, are that 90 people die each year from coming into contact with E. coli that make shiga or shiga-like toxins (called STEC for Shiga Toxin-producing E. coli).1 Thousands are hospitalized.

You're exposed to shiga from animals or people who harbor the bacteria that make it, to use the words of the CDC, "when you get tiny amounts of human or animal feces in your mouth." That's it in a nutshell. Bacteria colonize intestines, consuming food that animals fail to digest. About a third of our solid excrement is bacteria, mostly dead bacteria. It only takes about 10 or 20 of some STEC bacteria to infect and in some cases to kill - what would fit on the head of a pin. The toxins these bacteria make - shiga - are that virulent.

Foods that have caused illness from shiga-making E. coli:1,2
  • Ground beef
  • Mechanically tenderized cuts of beef
  • Venison
  • Sausage
  • Dried (uncooked) salami
  • Unpasteurized juice and cider
  • Raw milk
  • Raw cheese
  • Lettuce
  • Spinach
  • Alfalfa sprouts
  • Radish sprouts
  • Parsley
  • Cabbage
  • Berries and other fruit
  • Nuts
  • ... and the latest: Cookie dough
You can also become infected by drinking contaminated water, or by touching contaminated animals or people or the things they touched (petting zoos, child-care settings, caring for an infected person). Our contaminated fingers must find their way into our mouths.

E. coli is a common and mostly harmless inhabitant of many large animals' intestines, including our own. At some point, some strains met up with and incorporated bits of DNA that manufacture shiga toxins - making some strains of E. coli more deadly than others. (E. coli that make shiga toxin have an evolutionary advantage since the toxin is lethal against predators. See graphic and footnotes.)

When did that happen? That's difficult to say. Shiga toxin was first discovered in E. coli in 1977, or thereabouts.3 Factory farming took off in America in the 1960s. The close quarters and high sanitation needs of industrial livestock farms present an ideal environment for the viral infection and gene transfer that can turn an innocuous bacterium into a deadly one. Large confined animal feeding operations are also excellent breeding grounds for coliform (intestine-living) bacteria like E. coli. Just saying.

Drinking raw milk and eating just-picked lettuce may not have been as risky in years past, at least in relation to shiga-making E. coli.

Part 2: E. coli O157:H7 Just One Among Many Shiga Toxin-Making Bacteria

1 Recommendations For Diagnosis Of Shiga Toxin-Producing Escherichia Coli Infections By Clinical Laboratories, Morbidity and Mortality Weekly Report, October 2009
2E. coli O157:H7, My Constant Companion For 17 Years, Bill Marler, January 2010
3Vero Response To A Cytotoxin Of Escherichia Coli, Infection and Immunity, December 1977.

Graphic of E. coli and one of its predators, Tetrahymena (the big guy on the left) from University at Buffalo. Caption:
"Predator or prey? The Tetrahymena is approximately fifty times the size of the E. coli bacteria it's trying to capture but it's entirely vulnerable to the Shiga toxin the bacteria carry in their DNA."

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