updated 6/3/2011 1:19:24 PM ET 2011-06-03T17:19:24

The outbreak of a deadly form of E. coli bacteria in Germany has medical experts racing to pinpoint its source -- perhaps on a Spanish vegetable farm -- as well as how this new strain could have evolved.

Researchers now say the new bacteria is a hybrid of two existing ones that have combined to form a super-virulent strain.

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The World Health Organization reports the bacteria has killed 18 people and sickened more than 1,500 people in six nations, with the highest number of patients from Germany. U.S. authorities report two travelers returning from Germany contracted the disease and remain seriously ill, but have not spread the infection to others.

"You have an E. coli that's quite nasty and then it got genes from another toxic E. coli and it's become even nastier," said Paul Wigley, professor of food-borne diseases at the University of Liverpool.

Wigley said that the new strain is a combination of entero-aggregative E. coli (EAEC), which is an emerging microbe associated with diarrhea in developing nations, and entero-hemorrhagic e.coli (EHEC), which is similar to the one that caused the deadly hamburger outbreak of E. coli O157.

"It can hang around for a long time in the gut, invade pretty well and produce a toxin that damages the kidneys and can lead to bloody diarrhea," Wigley said.

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The bacteria likely came from human or animal waste that came in contact with food. A Beijing lab that sequenced the genome of the bacteria with the help of German health officials said Wednesday that it appears to be resistant to many antibiotics. In fact, using antibiotics, which break apart the bacteria's cells, can release more toxins into the body, Wigley said.

"All you can do is supportive therapy, give patients fluid replacement, blood transfusions, and if kidney failure, then kidney dialysis." Officials with the Centers for Disease Control in Atlanta caution against panic and are careful not to call it a "super-bug" that can jump borders at will.

"We are talking about a food-borne outbreak," said Christopher Braden, director of CDC's division of food-borne, water-borne and environmental diseases. "There's a potential for this to be transmitted person to person, but no indication that it's happened."

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Braden noted that so far, the disease has not affected children as much as adults, and that women are suffering more than men.

"Maybe it's coming from something kids don't normally eat, or something about this organism that doesn't affect kids as adults," Braden said. "We still don't know."

European health officials initially said they believed the source of the outbreak was contaminated vegetables grown in Spain, but have since backed off that claim. There are also reports that organically-grown vegetables like cucumbers were to blame since they rely on manure fertilizers.

In the meantime, several nations have banned food grown in Spain, while Russia has stopped all European produce imports.

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