The federal government sets standards for cleaning processed vegetables, like the shredded lettuce found in your taco. Co-director Shaun Kennedy at the National Center for Food Protection and Defense at the University of Minnesota says lettuce is cored at the farm, and then taken to a separate location where it is cleaned.
"That washing process is at least... three consecutive washes with water that has been treated not to sanitize the produce, but to reduce the possibility that the water is a potential source of contamination," he said.
Kennedy says the process takes care of about 90 to 95 percent of the E. coli bacteria found on produce. Sources at Bix Produce, where Taco John's got its lettuce, say the company does random analysis on the produce after its been washed. It also inspects visually as well as tests for contamination.
Kennedy says some produce is more likely to be a bacterial carrier than others. Lettuce, spinach and broccoli are all difficult to clean thoroughly.
In the last four months there have been four major E. coli outbreaks in the U.S., according to the Centers for Disease Control.
E. coli is a bacteria found in the guts of animals and humans. It's often spread through contact with fecal matter, including cattle manure. Kennedy says produce can get contaminated when manure reaches irrigation ditches, or through direct contact.
"Through accidental means or they were unable to restrict the access of wild animals to the fields," Kennedy said. "You also have the possibility, if they are using composted manure for the fertilizer, for the produce that if they didn't adequately compost it there would still be viable bacteria in the compost."
At least one outbreak has been linked to cow manure on the field. Kennedy doesn't believe we're seeing more poor food handling. Instead, he says, more of our food is produced by just a few big farms. So outbreaks can be bigger and spread across the country. He believes people are more aware of E. coli infections.
Dr. David Wallinga, of the Institute for Agriculture and Trade Policy, says the country is seeing an increase in E. coli outbreaks. He says feedlots and mega-dairies produce a lot of manure, and it's often used by the farms around them.
"If you put too much manure on a piece of land, you're simply going to overwhelm its capacity to use those nutrients, and then it becomes a pollutant," according to Wallinga.
Over-application, he says, is common. The manure that can't be absorbed goes into drainage ditches and aquifers or sticks to the plants.
Wallinga also says the type of E. coli bacteria showing up in outbreaks is particularly virulent.
"There have been some questions raised about whether, for instance, the way we're producing animals now -- farm animals -- might be, in fact, creating conditions that select for more virulent strains of E. coli," Wallinga said.
Wallinga says feeding animals antibiotics and keeping them in close confinement are two contributing factors. A group of University of Iowa toxicologists is calling for a moratorium on feedlots because of health concerns. Wallinga says in-field testing should be done on manure.
Kevin Elfering, is the director of dairy and food inspection at the Minnesota Department of Agriculture, says in the southeastern Minnesota outbreak manure likely wasn't the cause. He says it's more likely that a deer herd moved through the field and left droppings. He doesn't believe field testing would eliminate the problem.
"I think there are some new research that's been done, especially with things like electorlized water that would actually would be a much better intervention," Elfering said. "For years a chlorine bath solution has always been considered an acceptable means of eliminating this organism. And we're finding that that's not true."
A Bix Produce representative says the company will institute additional inspections of the produce as it arrives. The company says this is the first time it's been associated with an outbreak.