Since You Asked: Listeria outbreak raises a plethora of issues

Dear Dr. Science: I thought listeria (bacteria) species were associated with mammals, not plants. Is this like E. coli infections of spinach by water contaminated from feedlot operations?

How do farmers prevent their plants from being contaminated by feedlot water? Do organic farmers take any special precautions? Thanks for your speedy investigation.

— Seriously addicted to melons

First of all, Ms./Mr. Seriously, we appreciate your recognition of our doctoral diploma, but regret to inform you that the print shop that was producing those diplomas for $1.99 has gone out of business, so the status of our title is somewhat in question.

Meanwhile, back at the ranch — er, melon field — you're right about the listeria bacteria being more related to fauna than flora.

In the case of the recent listeria outbreak, though, it's inconclusive whether the infected melons in Colorado had anything to do with feedlot contamination.

The bacteria lives in the intestinal tracts of numerous mammals.

"Fecal matter can contain listeria," said Kathleen Page, professor of cell biology, microbiology and immunology at Southern Oregon University. "That's how it gets in the soil, so you can find (it) just in soil or dirt."

That means fruits or vegetables can become contaminated on the exterior when they come up through the soil. The contamination does not grow in the fruit or vegetable, but can be transferred to their surfaces.

The recent listeria outbreak among Colorado-grown cantaloupes caused more than 20 deaths. Federal health officials said dirty processing equipment and unsanitary conditions in a packing plant likely contributed to the outbreak.

There also were pools of water on the floor throughout the plant and the cantaloupes were not adequately cooled before being refrigerated, according to a New York Times story.

How the bacteria got there is still inconclusive. A dump truck that transported melons to a cattle ranch could have been the culprit.

As for specific precautions, that's up to the individual farmer. There are no feedlots in Jackson County, but there are plenty of cattle. Irrigation water being delivered to specific sites has no quality assurance guaranteed with regard to bacteria.

Farmers have the option to switch their watering method to a drip system, in which water is delivered to the root system and not the above-ground flowers. Pathogens cannot be accumulated through roots.

And, of course, you can help ensure your own safety by washing produce before it's eaten.

Send questions to "Since You Asked," Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by email to

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