Investigators still don't know how the greens became contaminated with E. coli 0157, but they have descended on the Salinas Valley, which local farmers proudly call the Salad Bowl to the World. Because E. coli normally originates from the feces of people or animals, a team from the FDA is inspecting sanitation procedures used both in the fields and in processing plants, and looking into water-quality logs and even weather patterns, to determine if flooding or poor drainage caused contaminated runoff to bring the bacteria into contact with produce.
So far, flooding seems to be a reasonable explanation for the contamination, and earlier studies have found that it doesn't take much to taint growing produce. In 2004, Michael Doyle, director of the Center for Food Safety at University of Georgia, documented how long E. coli 0157 remained on produce such as carrots and green onions when these vegetables were planted in composted manure that his team had intentionally contaminated with E. coli. "We found that you only have to apply contaminated irrigation water once, and the E. coli can still get into the soil and then contaminate that produce as it is growing," he says. Depending on how long the produce remained in the ground, says Doyle, the bug stuck to the food for as long as 140 days. That's well beyond the period at which it would be harvested meaning that if the produce isn't treated and cleaned properly, the bacteria have a good chance of hitching a ride all the way to a salad plate.
Even more disturbing are recent studies showing that the bacteria may not be content to just live on the surface of produce, and may actually set up shop inside plant tissues, making them impossible to eliminate with a simple dousing in a chlorine bath, the current way that most fresh produce is cleaned. Eric Triplett, chair of microbiology and cell science at University of Florida, has published two studies documenting the ability of bacteria like salmonella to travel into a plant through its root system. "We just inoculate the roots and up they go, they fully colonize all over the plant," he told TIME.
If bacteria can routinely burrow into produce this way, that means that standards regulating ready-to-eat produce need to get even stricter. Potential sources of bacterial contamination, from animal droppings to improperly drained fields or unclean irrigation systems, should be monitored more tightly if the $2.6 billion prewashed salad industry is to survive. Already, some spinach farmers in California have plowed their spinach fields under, convinced that for the time being at least, no one will be eager to eat their greens.