Almost two weeks after the Robert Koch Institute (RKI) announced that cucumbers, lettuce and tomatoes were the most likely sources of the deadly E. coli O104:H4 bacteria outbreak in Northern Germany, the list of suspect foods remains virtually the same, with the addition of sprouts this weekend.

Sunday night, Lower Saxony’s Minister of Agriculture said evidence pointed to a possible connection between sprouts grown at a farm in that state and the outbreak. And while lab results released Monday showed that 23 of 40 sprout samples collected from the grower were negative for the outbreak strain of E. coli (test results are pending on the other 17), officials said sprouts should by no means be ruled out as possible source of the epidemic.

People should avoid eating raw sprouts, along with cucumbers, tomatoes and lettuce “until we have clarity about the source of the disease,” Hamburg’s Minister of Health, Cornelia Prufer-Storcks, said Monday, according to a report in Der Spiegel.

And much more information is needed to get that clarity. Just because a food tests negative for an outbreak pathogen does not mean it was not the cause, says Dr. Bill Keene, senior epidemiologist at the Oregon Public Health Division. Bacteria that sickened people three or more weeks ago will not necessarily be found on food now available from that producer.

“If you get negative results,” he told Food Safety News, “they essentially mean next to nothing. They don’t rule it out, but they don’t say anything.”

This is especially true for sprouts, which are perishable and usually sold and eaten soon after being grown.

Lower Saxony officials insist they have evidence linking outbreak case patients to sprouts. “Our chain of causation is watertight and plausible,” said Lower Saxony’s Minister of Consumer Protection, Gert Hahne.

Authorities from that province conducted their research independent of the Robert Koch Institute (RKI), which is leading the search for the outbreak’s ground zero.  RKI announced Monday that it was reluctant to point the finger at sprouts, because only a small percentage of outbreak case patients reported having eaten them.

However, Dr. Keene warns that sprouts are such a high-risk category for foodborne illness that even a few reports of eating them should not be discounted. Sometimes people just don’t recall eating them, he says.

“The very first sprout outbreak that we worked on here, less than 40 percent of the [approximately 120] people remembered eating sprouts,” Keene said.

In Germany, a team of experts from RKI and the Federal Institute for Risk Assessment (BfR) have been sent to the Uelzen district, where the farm under investigation is located, to do more extensive testing on the sprouts.

Keene says he’s not sure whether sprouts will turn out to be the culprit, but that vegetables seem the most likely given the sector of the population hit hardest by the outbreak, namely adult women.

“That’s a marker [in the U.S.], and I would imagine in Germany as well,” he says, “for leafy greens, for salads, for sprouts. Those are classic vehicles with that demographic profile.”

While the source of the enterohemorrhagic E. coli (EHEC) bacteria remains elusive, the number of victims continues to climb, although at a somewhat slower rate.  So far, 2,330 have fallen ill, according to the Winnipeg Free Press. The death toll stands at 22.

And as the wait to identify a source stretches longer and longer, the political guns are coming out.  Germany’s left wing has begun criticizing right-of-center Health Minister Daniel Bahr, who has been in office just under a month, for lacking leadership during this crisis.

“I wonder what the Health Minister actually does,” said Renate Keunast, leader of Germany’s Green Party,” according to Der Spiegel.

The Hamburg Ministry of Health has been blamed for jumping to conclusions that the outbreak stemmed from Spanish cucumbers — cucumbers that were tainted but not with the outbreak strain of bacteria, a mistake that has led to Spain filing a suit with the EU.

“We want to express our displeasure at how the crisis was handled, damaging the interests of our country,” said Spanish Helath Minister Leire Pajin. “We want to ask for compensation for the serious and irreparable damages Spain has suffered and we will ask the European Commission to strengthen and improve the alert systems on food safety,” he said.

Annette Widmann-Mauz of the German Health Ministry, however, defended the decision to warn consumers about cucumbers from Spain.

“We truly owe that to the people, because preventative health protection requires following every clue,” she said, according to Der Spiegel International.

As for researchers from the Robert Koch Institute and the University Medical Centre Hamburg-Eppendorf, Dr. Alexander Friedrich, Head of the Department of Medical Microbiology and Infection Control at the University Hospital in Gronigen, Netherlands, is quick to applaud their diligence.

He lauds their work in narrowing the outbreak source to vegetables, and in publishing a file (in collaboration with Beijing Genomics Institute) that provides the polymerase chain reaction (PCR) primer sequence for E. coli O104:H4.. This can be used to create diagnostic kits so that other labs can quickly identify — ane perhaps help halt — this infectious bacterium.

“A lot is relying on what they’re doing,” he told Food Safety News. “They’re working day and night there.”