She added that, although commercial producers do some testing for E. coli on wash water and irrigation water, not every single product that makes it into the hands of a customer is tested.

'A very safe food supply' overall

The E. coli testing is based on the Food Safety Modernization Act, a set of regulations enacted in August 2015 that requires growers with a certain size farm to sample water associated with produce, Noble said.

"The goal was to set up a monitoring scheme to protect the public," she said. The regulations are still being phased in, so some growers have begun monitoring programs but others have not.

Though these monitoring programs measure the total volume of E. coli in the water, it might not take a high number of bacteria to make someone sick, since the Shiga toxin-producing strains can be potent, Noble said.

All told, Farber believes, "both the US and Canada have a very safe food supply."

Still, consumers have "a role to play," he said, by paying attention to food recalls and asking questions when they are unsure of quality or safety of a food product. They also need to know "that 'best before' or 'use by' dates are only based on quality and not safety."

With the growing season in the Yuma region at an end, Harrison Farms and others in the region are not growing any lettuce now, but the CDC still warns consumers against eating romaine lettuce at this time unless it isn't from the region.

Generally, Farber recommends washing your hands thoroughly with warm water and soap for at least 20 seconds before and after handling lettuce and then washing lettuce thoroughly under fresh, cool running water. Wilted or brown leaves should be discarded along with the outer lettuce layer, he said.

The CDC also offers recommendations for consumers to avoid becoming infected with a harmful strain of E. coli. Generally, the agency advises using proper handwashing and kitchen sanitation when preparing food; cooking meat at proper temperatures; avoiding raw milk, unpasteurized dairy products and juices; and not swallowing water when swimming.

"There is no need to use anything other than water to wash lettuce," Farber said. "Washing it gently with water is as effective as using produce cleansers."

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