A mix of two dangerous E. coli strains caused the recent deadly food poisoning outbreak in Germany, according to a new study of the bacteria's DNA.
Scientists said the E. coli outbreak strain combined one that makes a toxin and another that sticks to the gut in a way that potentially speeds up the body's absorption of the toxin. They described it as "unprecedented" in its lethality.
"The two strains are in themselves quite nasty," said Hugh Pennington, an emeritus professor of microbiology at the University of Aberdeen, who wrote an accompanying commentary on the research.
"It may be that more of the bugs are sticking to the intestines, and that may result in more toxin being produced," he said.
Experts had earlier suggested, based on an early DNA sequence of the bacteria, that the new strain was particularly aggressive because it reproduced quickly and released more toxin than similar bacteria.
Unlike previous E. coli outbreaks, the German strain caused three times as many cases of a severe complication that can lead to kidney failureIt was the deadliest E. coli outbreak in history, infecting 3,601 people and killing 39 across Europe, with most cases in Germany. More than 800 people developed kidney problems from the outbreak that peaked in late May.
German officials identified sprouts from an organic farm in northern Germany as the outbreak's cause, though they don't know how the sprouts got infected.
There are hundreds of E. coli strains in the environment, and many strains dangerous to humans come from animals like cows and sheep. People naturally carry E. coli in their gut but few strains cause illness. But the bacteria is constantly evolving and swapping genes with other strains, giving it countless opportunities to mutate into a killer version.
Pennington said it is crucial to find out whether this new E. coli strain is widely circulating in animals. "If it turns out to be very common in cattle, that would ring alarm bells that this kind of (outbreak) might happen again in the future," he said.
In the new study, researchers at the University of Muenster and the Robert Koch Institute analyzed samples from 80 patients sent to a national laboratory between May 23 and June 2. The analysis was paid for by the German government and Network Zoonoses. It was published Thursday in the journal, Lancet Infectious Diseases.
In separate research published Wednesday in the New England Journal of Medicine, German doctors released an early description of the outbreak, including the cases of 59 people hospitalized with the illness at the Hamburg University Medical Center. They found about 20 percent of patients developed the kidney failure complication linked to E. coli.
Unlike in previous E. coli crises, few children were affected. "The pathogen seems to have a special affinity for adults," said Dr. Gerard Krause, a director at the Robert Koch Institute, and one of the paper's authors. He said it could be that even if children picked up the bacteria, they didn't fall sick.
Krause said scientists are conducting studies to find where the bacteria naturally lives in the environment, but that it could take years to get an answer. He said control measures might help prevent future outbreaks, and that Germany's outbreak response system could be streamlined. Under their current reporting system, hospitals' notifications of serious illness still often wind their way to the national disease control center by mail.
Critics slammed Germany's bungled response to the outbreak and officials mistakenly blamed imported Spanish cucumbers for the crisis before pinpointing local sprouts as the cause. An editorial published earlier this month in the Lancet concluded that "coordination of the German public health response seems to have been utterly absent."