FARMERS and food safety officials still have much to figure out about the recent spate of E. coli infections linked to raw spinach. So far, no particular stomachache has been traced to any particular farm irrigated by any particular river.

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First, some basic facts about this usually harmless bacterium: E. coli is abundant in the digestive systems of healthy cattle and humans, and if your potato salad happened to be carrying the average E. coli, the acid in your gut is usually enough to kill it.

But the villain in this outbreak, E. coli O157:H7, is far scarier, at least for humans. Your stomach juices are not strong enough to kill this acid-loving bacterium, which is why it's more likely than other members of the E. coli family to produce abdominal cramps, diarrhea, fever and, in rare cases, fatal kidney failure.

Where does this particularly virulent strain come from? It's not found in the intestinal tracts of cattle raised on their natural diet of grass, hay and other fibrous forage. No, O157 thrives in a new -- that is, recent in the history of animal diets -- biological niche: the unnaturally acidic stomachs of beef and dairy cattle fed on grain, the typical ration on most industrial farms. It's the infected manure from these grain-fed cattle that contaminates the groundwater and spreads the bacteria to produce, like spinach, growing on neighboring farms.

In 2003, The Journal of Dairy Science noted that up to 80 percent of dairy cattle carry O157. (Fortunately, food safety measures prevent contaminated fecal matter from getting into most of our food most of the time.) Happily, the journal also provided a remedy based on a simple experiment. When cows were switched from a grain diet to hay for only five days, O157 declined 1,000-fold.