The source of the deadly E. coli outbreak in Europe that has infected more than 2,100 and killed at least 22, is still a mystery. First, German authorities said it could be cucumbers. Then, sprouts. But as of Monday, preliminary tests from a sprout farm in Northern Germany had come back negative, and they were still searching.


In the meantime, Germans have also been warned against eating tomatoes and lettuce, until more is known.


Foodborne illness experts who have studied sprouts were not surprised to see them identified as a possible culprit.  Sprout farms, says William Marler, a foodborne illness attorney, are perfect incubators for bacteria. In fact, more than 30 outbreaks of E. coli and salmonella have occurred at sprout farms in the past 20 years. This includes the 1996 E. coli outbreak in Japan, considered one of the worst ever,  in which 10,000 people fell ill and 12 died.


Sprouts grow in balmy weather, ideally at about 100 degree Fahrenheit. These are also prime conditions for bacterial growth.  E. coli can easily contaminate the water in which sprouts are grown, and it can remain dormant on the seeds. 


"One of the scary things about E. coli is that about 50 bacterium are enough to sicken and kill you," Marler said. "One hundred thousand of them would fit on the head of a pin."


In other words, it doesn't take a lot of E. coli to sicken 2,000 people. Plus, some studies have shown that E. coli and salmonella can live in dry environments for as long as nine months, Marler said. These are hardy bacteria.  

E. coli, or Escherichia coli , is a rod-shaped bacterium that is often found lurking in the guts of warm-blooded animals, particularly in the intestines or colon. There are more than 700 variations, most of which are harmless, but some can be highly toxic, and cause severe illness and sometimes death.


The latest offending strain of E. coli - 0104:H4 - has proved uniquely virulent and rare. It has led to a staggering rate of hemolytic uremic syndrome, a sometimes-fatal complication linked to kidney failure.

Phillip Tarr, an E. coli expert and director of the gastroenterology and nutrition division and University of Washington at St. Louis says it's believed to be a result of different organisms exchanging genetic material.


"The old adage is, if you don't kill them, you make them stronger," Tarr said.


As few as 50 bacteria can colonize in the gut and cause problems. If the E. coli can make it past the stomach acid, and into the large intestines, then it can grow, eating away at the intestinal wall as it does so, which can cause severe bloody diarrhea and painful cramping.

Certain strains of E. coli , including this one, produce something called Shiga toxins. These are extremely potent toxins, which split the red blood cells, and contribute to kidney failure.


 "This is a toxin that essentially becomes lodged in the intestine and from there it can attack different organs in the body," said Caroline Smith De Waal of the Center for Science in the Public Interest.  "The kidneys are particularly vulnerable as they try to eliminate the toxin from the blood and can become overwhelmed."


Traditionally, E. coli infections will lead to hemolytic uremic syndrome, or HUS, in about 5 to 10 percent of cases. In this case the numbers are as high as 36 percent.  "It's a really significant injury, and there's no cure," Marler said. "Just supportive care, dialysis and keeping fluid levels correct."


The sheer number of people with HES and the number of deaths is really unprecedented in any recent outbreak, De Waal said.


E. coli traditionally affects children and seniors, but this outbreak has disproportionately affected women, as well as people in their twenties, thirties, forties and fifties. Marler suggested two possible explanations.

"It may be that people consuming the product were all women in that age group, and they like to eat salads.  It may be something about this particular bacteria, that it just simply may be a more virulent strain and attacking people you would not expect."

This article is reproduced with permission from PBS NewsHour. It was first published on June 6, 2011.