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Foodborne pathogens hard to remove from produce, research is ongoing

October 03, 2006

URBANA - Will you ever feel comfortable eating fresh spinach again? All raw agricultural products carry a minimal risk of contamination, said a University of Illinois scientist whose research focuses on keeping foodborne pathogens, including the strain of E. coli found recently on spinach, out of the food supply.

That won't keep Scott Martin, a U of I food science and human nutrition professor, from eating bagged greens or other produce although he can see why it gives consumers pause.

"I definitely wouldn't eat spinach from the three California counties implicated in this latest outbreak of E. coli H0157:H7, but there have been no problems with spinach grown in other parts of the country," Martin said.

Martin said that food companies have recalled the particular products implicated in the outbreak, and that the contaminated spinach had a sell-by date of September 20, so none should remain on the shelves at this time.

If his reassuring tone makes the scientist sound less than aggressive toward E. coli 0157:H7 and other foodborne pathogens, you're mistaken. Martin and fellow U of I professor Hao Feng are dedicated to discovering ways to keep these microorganisms out of the food supply.

Martin's research is focused on finding ways to eliminate the biofilms that attach to produce and cause illness. "Once the pathogenic organism gets on the product, no amount of washing will remove it. The microbes attach to the surface of produce in a sticky biofilm, and washing just isn't very effective," he said.

"Another problem with this pathogen is that it has a very low infective dose. It only takes between 10 and 100 cells to cause an infection, so it's impossible to achieve a safe level of the pathogen once it gets on the product. At this point, we need to concentrate on avoiding a crop's exposure to the pathogen as the produce is being grown," he said.

Martin said the California spinach outbreak appears to have been caused by contaminated cow manure used by organic producers. "A very low percentage of cattle are always infected by this strain of E. coli. If fresh manure from those cattle is used as fertilizer, there's an outbreak in the making."

Growers should also be careful about the water they use on the plants. "If farmers irrigate with water from a lake close to a dairy farm, that can also be a potential source of infection," Martin said.

Another technique that has excellent potential in the fight against E. coli 0157:H7 is being developed in the lab of Martin's colleague Hao Feng. Feng is developing a process that uses ultrasound and low temperatures to kill pathogenic organisms in liquid products, such as cider and apple juice. A previous outbreak of E. coli 0157:H7 occurred in these products, Martin said.

"Before that outbreak, small producers could sell cider or apple juice without pasteurizing it. Now all growers are required to pasteurize these products," he added.

The scientist said normal, wild-type strains of E. coli live in the human intestinal tract as a beneficial organism, aiding in digestion and absorption of nutrients.

"Only a few strains of E. coli are pathogenic, and E. coli 0157:H7 is a really virulent strain. In most cases, it causes bloody diarrhea and abdominal pain, and in a small percentage of victims, it colonizes the intestinal tract and produces a toxin that can cause kidney failure. It's certainly an unpleasant and potentially fatal illness," he said.

"But, if you consider the amount of produce that's grown in this country and the number of reported cases we see, your risk of contracting the illness is actually very small," he noted.

In the meantime, Martin continues to study the biofilms that pathogens use to adhere to produce, and Feng experiments with ultrasound treatments that are yielding encouraging results. The scientists believe their work will soon make the food supply safer.

University of Illinois at Urbana-Champaign