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Spinach epidemic: Controlling E. coli

POSTED 5 OCTOBER 2006

Confronting the monster
Ever since 1982, when the first E. coli O157:H7 outbreak occurred, scientists and the meat industry have sought to keep the deadly bug out of the food supply. The meat industry is "very proactive in trying to eliminate this," says Todd Callaway of the U.S. Department of Agriculture, who adds that outbreaks caused by infected beef "have gone down pretty dramatically with the new science and technology that has come on line in the last 5 years. We have gone from 1905 scratch-and-sniff microbiology [where inspectors literally sniffed the meat], to modern swab-and-test" using advanced microbiology. These days, meat packers clean dung from the hide before the animal enters the slaughterhouse, and hold their product until microbial tests come back clean, Callaway says.

Many cattle milling around in a dirt lot, with grain elevator in background
Some cattle feedlots have already begun feeding animals a bacterial supplement, in an effort to reduce E. coli O157:H7 contamination in feces. Photo: USDA

With the obvious changes in meat production now under way, attention is shifting toward steps that could reduce E. coli at the source -- inside the cow's gut. Most of these improvements emerged from the beef-safety initiative, but some could also succor spinach-lovers in their night of despair.

Bleach that bug! Here's a toxic brainstorm: Force E. coli O157:H7 to poison itself from within. E. coli carries an enzyme that changes nitrate (NO3) to nitrite (NO2), but if you feed cattle sodium chlorate (NaClO3), the same enzyme will change chlorate (Cl03) into chlorite (Cl02). To us civilians, that is called "bleach," and it kills the bacteria that makes the enzyme dead.

Although Callaway admits that sodium chlorate is toxic, he puts it in the same league as sodium chloride -- table salt. So far, tests show that feeding sodium chlorate to cattle, pigs and sheep significantly reduces E. coli O157:H7 and also salmonella, another serious food-borne pathogen.

"The data are very promising," says Francisco Diez-Gonzalez, an O157 researcher at the University of Minnesota. "It's probably the single compound that causes the most dramatic killing of E. coli and salmonella in the intestine and rumen, but you can only use it a few hours before slaughter. You cannot feed it long-term because of toxicity. That doesn't help with spinach, but it's probably going to be very helpful with ground beef."

The antibiotic solution? If your problem is bacteria in cattle, why not just kill them the old-fashioned way, with a work-horse antibiotic like neomycin? "If you really want to reduce O157 in cattle," says Diez-Gonzalez, "neomycin would not have any regulatory hurdles, it's already approved for cattle."

Although only one study of neomycin's effect on E. coli O157:H7 has been published, Diez-Gonzales says, it would be "Very valid to give it at least to animals in those three counties around Salinas [California, where the contaminated spinach grew], and get this [current outbreak] over with." (Experts currently speculate that cattle manure contaminated surface water or irrigation water, which infected spinach in the field. Man in red shirt reads the nutrition info on back of a bag of greensThe bags identified so far have been conventional, not organic, spinach. In early October, California's department of public health found O157 in manure in the three counties at the center of the outbreak, but it's not yet clear whether they are the exact same strain that caused the outbreak).

Does this bag contain the secret ingredient: deadly E. coli O157:H7? When it comes to food safety, it pays to heed the fed's warnings. Photo: USDA

However, neomycin would need to be added to feed over the long-term, he says; a single dose would not work. Unfortunately, such antibiotic usage can breed antibiotic-resistant bacteria. "Resistance is not something you eventually want to cause," Diez-Gonzalez adds.

Bug versus bug? A third tactic against E. coli O157:H7 uses "probiotics:" harmless strains of bacteria that can out-compete the pathogenic strain (O157 is, of course, harmless to cattle but deadly to people). In 2003, for example, Michael Doyle of the University of Georgia and colleagues reported that they had fed O157 and several other disease-causing strains of E. coli to calves, then some of the animals got a cocktail containing five harmless strains of E. coli. Treated animals discharged less O157 in their feces than control animals eight to 30 days later. Only two of six treated animals still carried O157 at autopsy, compared to five of six untreated animals (see "Fecal Shedding of Enterohemorrhagic..." in the bibliography).

What is happening? It looks as if the various strains of E. coli actually kill one another by making proteins that "poke holes" in their cell membrane, Doyle says. This tactic seems common in E. coli, he adds. "We have tested 50 to 100 strains against the various pathogenic E. coli, and found most to be effective."

Although Doyle's experiments bear out the idea that benign microbes can outcompete harmful ones in the gut, they don't work as well against other pathogenic strains of E. coli, and they have yet to be tested at the feedlot scale. "We plan, ultimately, to get to that point, but we want to make sure we work out all the glitches in the lab first," Doyle says.

In evolutionary terms, such competition makes sense, says Diez-Gonzalez. Because various strains of E. coli eat essentially the same stuff, "They are in competition, and that's why they make anti-microbial proteins to kill other strains of E. coli. It's a wild world out there."

Curly haired man points at computer screen as short-haired woman looks on
Veterinarian Bill Stoffregen shows microbiologist Evelyn Nystrom surface lesions and brown-stained E. coli O157:H7 bacteria in the gall bladder of an infected calf. Photo: USDA

Bovine nation vaccination Vaccines, which alert the immune system to prepare for an attack from a specific microbe, are one of the more promising preventatives for E. coli O157:H7. B. Brett Finlay of the University of British Columbia, who has studied E. coli O157:H7 for 16 years, says the bacterium has an unusual talent that may also prove to be a weakness: Creating its own "landing zone" in the intestine.

If pathogenic E. coli bacteria are to hang around in the fluid environment of the gastrointestinal system, they must attach to their host. Most bacteria, Finlay says, require particular attachment structures on the surface of host cells, but not E. coli O157:H7. This strain, he says, "harpoons a bacterial protein into the intestinal cell wall and hangs on, like a piton into rock. It injects its own receptor; this was unprecedented, inserting own landing path where it wants to land." (Don't believe us? View the videos).

Discovering the attachment mechanism got Finlay interested in ways to block the attachment through a vaccine. "I was initially thinking about a childhood vaccine, but I was running in the woods and thought, 'Forget the kids, it's the cows we should vaccinate.'" For one thing, the bacterium is much more common among cattle, and the legalities are much less daunting for veterinary vaccination than for human inoculation, so it should be possible to bring the vaccine to market much sooner.

Finlay found a way to spark an immune response to the harpoon, using a vaccine that has already been given to 40,000 cows. The vaccine cuts the level of E. coli O157:H7 in feces "to a level where it would not spread," Finlay says. "If the bacterium can not adhere, it should not be a problem."

For some reason, E. coli O157:H7 numbers in cattle wane in winter, then surge in summer. Finlay says he hopes the vaccine will be on the market, "hopefully by next hamburger season."

One final question: Who should pay the cost, which Finlay estimates at $5 to $10 per animal? "The producers are not keen, because the disease does not make the cows sick. There is lots of discussion about who is going to pay for it," he says.

Adding it up
Beyond trying to make E. coli O157:H7 scarcer in cattle and their manure, a second line of attack involves taking better care of cow dung. Carolyn Hovde Bohach, who studies O157 at the University of Idaho, told us "Cattle are the silent reservoir, they are the main source; we know it replicates in their [gastrointestinal] tracts."

That means we must mind our manure, which seems highly hospitable to E. coli O157:H7. These bugs "can survive longer in raw manure than in an individual [live] animal," Hovde Bohach says.

If manure is composted, heat will kill O157, she says, but only a small percentage of manure is composted before being spread on farmland. Much more of it stored as a liquid in giant lagoons, where competition from other microbes, temperature, and nutrient availability will kill O157. "Holding it in a lagoon will decrease O157," says Hovde Bohach, "but if the lagoon is not managed properly, O157 can survive, and the amount of decrease depends on how long it is held, the pH, and how much fresh manure is added." An error could produce "an issue with vegetables or anything eaten raw."

Giant ponds are separated by earthen dikesCattle manure is often stored in giant "lagoons" like this before being spread on land. If the lagoon is managed correctly, most E. coli O157:H7 will be killed. If not... Photo: USDA

More change ahead
The cattle industry has a vested interest in making meat safer, but Hovde Bohach says it is also aware of the incidental contamination problem. "In my interactions with the cattle industry, and with individual ranchers and farmers, I have always been really surprised at how concerned they are. ... If it is their animals that are now contaminating other foods, I think many would feel a responsibility."

Certainly, leafy-greens producers have joined the cattle industry in the cross-hairs of concern about O157. The current outbreak, after all, follows 19 E. coli outbreaks that were traced to leafy greens, according to MSNBC.com (Sept. 19, 2006) . MSNBC noted that the Food and Drug Administration had already cautioned about microbial contamination: "In 2004 and again in 2005, the FDA's top food safety official warned California farmers they needed to do more to increase the safety of the fresh leafy greens they grow. 'In light of continuing outbreaks, it is clear that more needs to be done,' the FDA's Robert Brackett wrote in a Nov. 4, 2005, letter."

On Sept. 28, Earthbound Farm, which packaged the contaminated spinach, announced a new food-safety initiative that will borrow some of the tactics that have helped the beef industry contain E. coli: "We will be testing all of the freshly harvested greens - spinach and everything else - that are brought to our facility before they enter our production stream. If pathogens are detected, the lot will be discarded. ...This 'firewall' will prevent anything like this E. coli-contaminated produce from ever entering our facilities."

As both the beef industry and Earthbound have learned, choices will have to be made. Expensive process changes may be needed. Our advice: keep your eye on E. coli O157:H7. Expect more bad news about this bloody bacterium.

Dive into our "bugs in cowgut" bibliography.

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Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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