HAMBURG, Germany, June 7 (Reuters)—The rate of infection in Germany's E. coli crisis is slowing, officials said, adding there was no conclusive proof of suspicions that organic bean sprouts were to blame for an outbreak that has killed 23.

Farmers across Europe have seen sales plummet after salad vegetables were first blamed, and the European Union was meeting to approve a package of aid for growers, which looked set to significantly top €150 million ($219.5 million).

In the north German port of Hamburg, center of the outbreak that has made more than 2,400 people in 12 countries ill, officials said one lead—a packet of bean sprouts in the fridge of one affected man—did not test positive for the E. coli bacteria.

Hamburg's state health minister, Cornelia Pruefer-Storcks, said clinics dealing with the outbreak "tell us the situation is gradually improving.

"We are seeing the first patients discharged, others are getting much better, so the first glimmers of hope are on the horizon."

But she also told a news conference that all test results so far on the bean sprouts, which Germans like to eat on their salads, have been inconclusive.

"We have strengthened our testing of bean sprouts, and they so far have been inconclusive," she said. "That applies also for the sprout package which was found in the refrigerator."

German Health Minister Daniel Bahr said there was reason to be cautiously optimistic but it was too early to give the all-clear. "There are some arguments suggesting the worst is behind us," he told reporters.

Affecting blood, kidneys

Health authorities had hoped the bean sprout tests would help end their struggle to cope with what appears to be the deadliest outbreak of E. coli ever seen, with one-third of patients developing the severe complication called hemolytic uremic syndrome (HUS) affecting the blood, kidneys and nervous system.

An unnamed Hamburg resident who recovered from the infection handed the bean sprouts over to local health authorities after discovering them in his refrigerator.

Other tests on bean sprout varieties from an organic farm in the village of Bienenbuettel, Germany, have so far proved inconclusive, despite links between it and suppliers of a number of restaurants in northern Germany where diners fell ill.

One clue linking the outbreak to the farm emerged when a local doctor said a worker at the farm had been taken severely ill with E. coli and had part of her intestine removed.

The 54-year-old woman, whose name was not given, developed bloody diarrhea followed by serious blood disorders. Dr. Anton Schafmayer, a physician who operated on her, said the woman had worked at the farm for 10 years and had eaten the sprouts.

"It went very fast. Such a pace is very rare," he told Reuters. "The surgery probably saved her. We removed a large part of the lower intestine."

Bean sprouts have been the source of serious E. coli outbreaks in the past, including in Japan and the United States, while some experts say organic farmers' preference for manure rather than chemical fertilizer makes them more vulnerable.

The original source of the contamination is most probably manure, since the Shiga toxin-producing Escherichia coli, or STEC, found in this outbreak are able to lurk in cattle guts.

Some scientists say the source may be the bean seeds, water used to grow them or a worker handling them.

"Bean sprouts are not an uncommon cause of food poisoning," said Paul Wigley of the University of Liverpool's School of Veterinary Science. "Both E. coli and salmonella outbreaks have been linked to sprouts in the United States and in Britain."

Human source

A researcher at Germany's Federal Institute for Risk Assessment (BfR) thinks the outbreak may have come from the guts of humans rather than cattle. "These pathogens have adapted themselves to people," Lothar Beutin told Der Tagesspiegel.

A large part of the bacteria's genetic sequence stems from the Enteroaggregative E. coli that is found in people as opposed to the Enterohemorrhagic E. coli bacteria found in animals, the newspaper said. Contacted by Reuters, Mr. Beutin declined comment.

Some scientists say the precise cause may never be known, though the geographical concentration of cases and the atypical patient profile, with a high preponderance of young women, could help narrow down the search.

"It's a bit like a crime investigation. It's very difficult to find the offending organisms because they can disappear as time goes on," said Brendan Wren, a microbiology professor at the London School of Hygiene and Tropical Medicine.

German officials have been criticized for their handling of the month-long crisis. Agriculture Minister Ilse Aigner rejected charges of excess bureaucracy and an unclear division of labor between federal and regional German authorities.

Mr. Bahr, just three weeks on the job, has been criticized for being out of the loop after saying on Sunday, when the trail had moved to sprouts, that the source of infection was still probably cucumbers, tomatoes and lettuce.

Spain has threatened legal action against German regional authorities for wrongly blaming Spanish cucumbers. The country's fruit and vegetable farmers say they lost €175 million ($256.1 million) in exports and €50 million ($73.2 million) in domestic sales in the first week after they were blamed for the outbreak.

The E.U.'s farm chief said he was prepared to increase substantially his initial offer of €150 million in financial compensation for farmers hit by the E. coli outbreak.