German officials announced Monday that initial tests show sprouts from an organic farm in the country's north are not the cause of the E. coli outbreak as first suspected.

Lower-Saxony state's Agriculture Ministry said 23 of 40 samples from the sprout farm suspected of being behind the outbreak have tested negative for the highly aggressive, "super-toxic" strain of E. coli bacteria.

It said tests are underway on the other 17 sprout samples.

"The search for the outbreak's cause is very difficult as several weeks have passed since its suspected start," the ministry said in a statement, cautioning that further testing of the sprouts and their seeds was necessary to achieve full certainty.

Negative test results on sprout batches now, however, do not mean that previous sprout batches weren't contaminated.

The ministry statement about samples from the Gaertnerhof organic sprouts farm in the northern German village of Bienenbuettel left consumers across the Continent still puzzled as to what is safe to eat.

"A conclusion of the investigations and a clarification of the contamination's origin is not expected in the short term," the ministry said.

The outbreak has killed at least 22 people and made more than 2,300 sick across Europe, leaving customers uneasy about eating raw vegetables.

On Sunday, preliminary tests suggested that bean sprouts and other sprout varieties from the organic farm could be the likely source. Before that, Spanish cucumbers were thought to be the cause.

Since the outbreak began in early May in Germany, 10 other European nations have also reported cases and there are at least four in the U.S. linked to Germany.

The World Health Organization cautioned people against taking antibiotics if they fall ill from E. coli because they could make the condition worse.

Anti-diarrhea medication also should be avoided because it stops the bacteria from quickly leaving the body, WHO epidemiologist Andrea Ellis told reporters in Geneva earlier this week.

Aspects of outbreak unique: disease official

The outbreak is considered the third-largest involving E. coli in recent world history, and it is already the deadliest. Twelve people died in a 1996 Japanese outbreak that reportedly made more than 9,000 people sick, and seven died in the Walkerton, Ont., tainted water outbreak in 2000.

Several aspects of this outbreak are unique, according to Robert Tauxe of the U.S. Centers for Disease Control and Prevention, which has been working with German health officials.

"First of all, it's mostly adults that are being affected rather than children, and a very large [number] are developing a kidney failure complication called HUS [hemolytic uremic syndrome]," Tauxe said in an interview with CBC News on Friday. "Whether that is because this organism is particularly virulent or whether it's because a lot of people have been exposed to something like a food contaminated with the organism is not completely clear."

To avoid food-borne illnesses, WHO recommends that people:

• Wash their hands
• Keep raw meat separate from other foods
• Thoroughly cook their food
• Wash fruits and vegetables, especially if eaten raw
• Experts also recommend peeling raw fruits and vegetables if possible.

With files from CBC News