The food-poisoning outbreak in Germany, which has so far sickened more than 1500 people and killed 18, involves a novel kind of Escherichia coli bacterium never seen before. Researchers in Münster, Germany, announced yesterday that the new E. coli is a hybrid of two other disease-causing types.
Now they plan to see whether the hybrid genome is what makes this strain so aggressive. They are also creating a custom testing kit for medical labs to find whether it has spread more widely, and possibly where it originated.
The outbreak so far appears to be limited to Germany. People have been taken ill as far afield as the US and the UK, and one person in Sweden has died, but all had recently visited Germany. Many say they have recently eaten raw tomatoes, cucumbers and salad vegetables.
Why so virulent?
The bacteria were initially identified as a strain of E. coli called STEC, or Shiga-like toxin-producing E. coli, which secrete toxins that cause bloody diarrhoea. Normally about 5 to 10 per cent of STEC infections progress to a severe complication called haemolytic-uraemic syndrome (HUS), in which toxin-induced blood clots in capillaries damage major organs, especially the kidneys.
Infections in the current outbreak seem to be more virulent, however. The Robert Koch Institute in Berlin, Germany's federal centre for disease control, reports that 470 patients – nearly a third of the 1500 known cases in the current outbreak – have developed HUS. That said, greater virulence may not be the only factor at play: differences in how cases are defined may also be responsible for the apparently high rate of HUS.
The German national consulting lab for HUS at the University Hospital in Münster yesterday announced that it had sequenced the outbreak bacteria's genome and in addition to STEC genes, they found genes from another kind of E. coli called entero-aggregative E. coli. These latter bacteria normally adhere to the gut wall, causing watery, persistent diarrhoea. While STEC lurks mainly in cattle guts, entero-aggregative E. coli are known only in humans.
"The rapid whole-genome sequencing results enabled us to discover within days a unique combination of virulence traits," says Alexander Mellmann, one of the researchers in Münster. The team plans to study whether this has caused a true increase in virulence in the hybrid.
Paul Wigley, a specialist in gut bacteria at the University of Liverpool, UK, says entero-aggregative strains are "tougher" than other E. coli because they can clump together to resist stresses. So an aggregative strain with the genes for Shiga-like toxin might well be more virulent than ordinary STEC.
Testing kit
Earlier this week the Münster team distributed a test for the new hybrid E. coli to allow doctors at German hospitals, which have been overwhelmed with diarrhoea cases, to identify people with the infection. The test, called a multiplex PCR, detects DNA sequences from four of the bacterium's genes, and "can analyse not only samples from people, but also, for example, isolates from food", says Helge Karch, head of the HUS lab at Münster.
The company that made the sequencing machine the Münster team used, Life Technologies of Carlsbad, California, now plans to use the whole-genome sequence to create a test that is even more specific for this hybrid strain.
The discovery underscores the limitations in testing for E. coli that have already led to economic disaster for Spain's vegetable industry, Europe's largest. Early in the outbreak German authorities revealed that they had found STEC on cucumbers imported from Spain. They reported the finding, as European Union rules require for any discovery of STEC, unleashing a wave of bans on Spanish produce.
On Wednesday, German authorities announced that the STEC they had found was not the same as the one causing the outbreak. Spanish officials quickly declared themselves vindicated, even though the bacteria that were found on the Spanish cucumbers were also pathogenic.
